AD-A089 974I COLD REGIONS RESEARCH AND ENGINEERING LAS HANOVER M4 F/6 8/13REVIEW OF TECIHdIGUES FOR MEASURING SOIL NOIST~ltE IN SITU. (UlAUG 80 H L NCKIM, .J E WALSH, 0 N ARION

UNCLASSIFIED CRREL-SR-8O-31 H

ED

li~t ~ 1.4 BI1.8

MICRO(TPY R~lM)LUIION II SI CIAI1

Special Report 80-31 2

August 1980 A A ~ '"

REVIEW OF TECHNIQUES FOR MEASURINGSOIL MOISTURE IN SITU

H.L. McKim, i.E. Walsh and D.N. Arion

Prepared for5.,.DIRECTORATE OF CIVIL WORKS

jQOFFICE OF THE CHIEF OF ENGINEERSC'By

(~s UNITED STATES ARMY477~~~ COLDS RONREACHNDN(NF ERNGINEERRSORCOLR OFNG RNEEAC ANRSII ABRTRHANOVER, NEW HAMPSHIRE, U.S.A

~pprovw d for public reica t distribut ion unlimilcd

- La

Cover: Tensiometer/transducer system and ther-mistors interfaced to the GOES data col-lection system, Sleepers River ResearchWatershed, Danville, Vermont.

SECURITY CLASSIFICATION OF THIS PAGE ("en Data Entered)

READ INSTRUCTIONSREPORT DOCUMENTATION PAGE BEFORE COMPLETING FORMI. RPOR NUBEROVT CCESIO No 3.RECIPIENT'S CATALOG NUMBER

S PcidI ej; 0-31-,, e, S-'-r"" S. TYPE OF REPORT & PERIOD COVERED

RVIEW OFJECHNIQUES FORMEASURINGSOIL MOIST;RE IN SITU.

6. PERFORMING ORG. REPORT NUMBER

7. AUTHOR(.) S. CONTRACT OR GRANT NUMBER(*)

/) H. L/MCK irnJ_!jjalh.D.N. Arion ' 1 19. PERFORMING ORGANIZATION NAME AND ADDRESS 10. PROGRAM ELEMENT. PROJECT, TASK

U.S. Army Cold Regions Research and Engineering Laboratory AREA & WORK UNIT NUMBERS

Hanover, New Hampshire 03755 CWIS 31587

I. CONTROLLING OFFICE NAME AND ADDRESS TE

Directorate of Civil Works .,g/"

Office of the Chief of EngineersahintonD.C. 20314 30

14. MONITORING AGq l.._ & AOORESSJI different from Controlilng Office) IS. SECURITY CLASS. (of thl report)

/4L F4 -7 .5 g7 P/ UnclassifiedS.. IS&. DECL ASSI FIC ATION/ DOWNGRADING

.. SCHEDULE

16. DISTRIBUTION STATEMENT (ol this Report)

Approved for public release; distribution unlimited.

17. DISTRIBUTION STATEMENT (of the abstract entered In Block 20, It different from Report)

IS. SUPPLEMENTARY NOTES

IS. KEY WORDS (Continue on reverse side If necesary nd Identify by block numbr)

In-situ measurementSoil moistureSoil tension

R. ABITACT (CaWftue m feee OM i soam, d iden/tiy by block number)

Recently there has been an increased interest in the in-situ measurement of soil moisture content in the areas ofhydrology, meteorology, agriculture and environmental studies. Current methods generally have limitations, depend- Iing upon the use of the data, that greatly influence acquisition and reliability of the soil moisture determination.This report discusses gravimetric, nuclear, electromagnetic, tensiometric and hygroscopic techniques and the advantageland disadvantages of using the technique. Emphasis is placed on the tensiometric and electromagnetic techniques.These two measurements when coupled together would supply information on the wetting and drying soil moisturecharacteristic curves and thereby provide a means of tracing moisture movement under field conditions in coldclimates.

DD Fo 143 iDTraIon OF 0 NOV 60 IS OSSOMLETE UcasfeJATWS Unclassified DaSECURITY CLASSlIFICATION OF THIS PAQRI ("a DMS Itniered)

PREFACE

This report was prepared by Dr. H.L. McKim, Research Soil Scientist, EarthSciences Branch, Research Division, U.S. Army Cold Regions Research andEngineering Laboratory , Dr. I.E. Walsh, Associate Professor of Physics, Dart-mouth College, and D.N. Arion, formerly a Computer Technician at CRREL. Thisstudy was sponsored as part of the U.S. Army Corps of Engineers Civil Works UnitCWIS 31587, Inventory Techniques for Ground Beneath Snow Cover.

The authors would like to thank Allen Tice, Dr. Richard Berg and others fortheir assistance in technically reviewing this report.

The contents of this report are not to be used for advertising or promotionalpurposes. Citation of brand names does not constitute an official endorsement orapproval of the use of such commercial products.

ii

CONTENTSPage

A b stract ....................P reface . ...... . .......

Introduction .1

Gravimetric techniques 1

Nuclear techniques ............ . ... ... 2

Neutron scattering 2............ . . ... .. 2

Gam m a ray attenuation 4. . ........ . .. .. .... . 4

Nuclear magnetic resonance ............. 5

Electromagnetic techniques 6........... . . 6

Tensiometric techniques ................ 8

Hygrom etric techniques ................. .... ..... . 11

W hite hydrocal m ethod ................ .... ........ ..... 11

Capacitance m ethod ....... .... ... .. ........ 11

Dew or frost point method .... .. .... .. ... .. .. 11

Psychrometric method ............ 11

Electrolysis m ethod ............... . .......... 12

Summary .............. 12

Literature cited .. .............. .. ......... ... . 13

iii

P--b~.c.77.

-- mambo".."...

FI

REVIEW OF TECHNIQUES FORMEASURING SOIL MOISTURE IN SITU

H.L. McKim, J.E. Walsh and D.N. Arion

INTRODUCTION supply adequate information while costing lessand taking less time.

The ability to measure soil moisture in situ isimportant in all soil science disciplines. It is criti-cal in the planning, design, construction, opera- GRAVIMETRIC TECHNIQUEStion and management of any system where soilis a major component. The problems involved in The oven-drying technique is probably thethe measurement of soil moisture are both in- most widely used of all the gravimetric methodsstrumental and spatial due to the complexity of for soil moisture measurement, and it is oftenthe soil environment. The major problem seems used to calibrate other soil moisture procedures.to be the spatial inhomogeneity of a soil system, The method consists of oven-drying a soil sam-both horizontal and vertical, pIe at 1050C until a constant weight is obtained.

Many field procedures have been employed Usually this occurs within 12 hours, but withto measure soil moisture in situ. This report will large samples the drying time increases. The wetreview some of these procedures and evaluate weight of the soil sample is taken prior to oven-advantages, disadvantages and the accuracy of drying. The amount of water in the sample canthe most commonly used methods. The soil be determined and the moisture content calcu-moisture measurement techniques covered in lated and expressed on a percentage basis. If thethis report are: gravimetric, nuclear, electromag- volumetric water content is required, the gravi-netic, tensiometric and hygrometric. metric value is multiplied by the bulk density of

The primary problem in measuring soil mois- the soil:ture is determining how many measurementsmust be made to obtain a representative 0 = (WW/Wd)(ydyw) X 100 (1)moisture value and where these measurementsshould be made. The problem that must be where 0 = volumetric water content (%)faced no matter which procedure is used is the W w = weight of water (g)horizontal and vertical variability of the soil Wd = dry weight of soil (g)characteristics. Each method has advantages Yd = oven-dry bulk density (g/cm 3 )and disadvantages because of the complexity of yw = density of water (g/cm 3)instrumentation, difficulty in instrument calibra- There are advantages and disadvantages totion and cost. The selection of a procedure the oven-drying gravimetric procedure. The ad-should be based on how the value for soil mois- vantages are:ture content is to be used. If very detailed infor- 1. Samples can be taken with an auger or tubemation is needed for research modeling, one sampler.may choose a sophisticated measurement tech- 2. Sample acquisition is fast and inexpensive.nique. But in many engineering, design and oper- 3. Soil moisture content is easily calculated.ational programs a less detailed procedure can The disadvantages include:

:- i. , ,, -' . , :,... , , , ',:, :, : .....................................-.-.-. ,, .

1. It is difficult to obtain representative soil soil by measurement of the hydrogen particlemoisture values in an inhomogeneous soil density. Initial development of the neutronprofile. probe began in 1950 (Belcher et al. 1950, 1952).

2. Many samples are required to provide an Gardner and Kirkham (1952) defined the princi-adequate estimate of soil moisture content. . pies on which the method is actually based. In

3. Many samples are required over long peri- this method, high energy neutrons emitted by aods of time to monitor moisture movement radioactive source are injected into the soil. Col-or amount of moisture over time and space. lisions with soil components thermalize the in-This is very destructive to the site. jected neutrons. The energy lost in these colli-Even though gravimetric methods are the sions is much greater when they take place with

most widely used, there are inherent problems in atoms of low atomic weight than when they oc-interpreting the data. Hewlett and Douglass cur with heavier atoms. In most soils, hydrogen(1961) determined the number of sample clusters is the only element of low atomic weight presentrequired at two sites to maintain the estimated in large quantities, and hydrogen can decreasestandard error of moisture at 1 % by volume (a the speed of fast neutrons much more effec-sample cluster contained one soil density and tively than any other element present in the soil.moisture determination). The required number The density of the resultant cloud of slow neu-of clusters ranged from 11 to 22 and was depen- trons is a function of the soil moisture content indent on site-specific soil characteristics. A low the liquid, solid or vapor state. The number ofcorrelation (r = -0.40) between density and slow neutrons returning to the detector per unitmoisture percentage resulted from this study. A time over a known volume of influence or soilhigher correlation (r = -0.80) was obtained when volume is counted and the soil moisture contentthe number of clusters was reduced from 5 to 13. determined from a standard curve of counts vsThey concluded that the use of gravimetric sam- volumetric water content.piing in evapotranspiration studies at these two Two types of probes have been developed.sites had doubtful possibilities. One is a depth probe that is lowered through an

Studies conducted at the Deer Creek Lake, access tube to the depth at which the moistureOhio, land treatment site, where gravimetric content is desired. The other is a surface probemoisture determinations were obtained at that gives the moisture content of the top fewknown time intervals during wastewater applica- centimeters of soil.tion to three 3-acre test sites, showed great vari- Several sources of high energy neutrons haveability in the results (Abele et al. 1979). In these been used. The americium-beryllium (Am-Be)studies a 1 % error in the measured or computed source seems to be the one most used (Bell andgravimetric water content resulted in a 1.6% er- McCulloch 1966). Van Bavel and Stirk (1967)ror in computing the volumetric water content. found that this source eliminated gamma radia-The 1.6% variation corrcsponds to 49% of the tion, decreased the probe weight, increased thetotal water applied, count rate and possibly increased the depth re-

Other gravimetric methods are discussed in solution of the soil moisture measurement.National Cooperative Highway Research Pro- The strength of the source varies with the typegram (1973); these include freeze drying, distilla- and manufacturer. Van Bavel (1962) found that 1tion, desiccant weight gain, alcohol burning, or 2 millicuries (mc) of a radium-beryllium (Ra-alcohol extraction, use of calcium carbide (hy- Be) source were adequate. The strength of thedritel or Karl Fisher reagent, and immersion (py- source of Am-Be that Van Bavel and Stirk (1967)crometer). Of these techniques the calcium car- used was 150 mc. Others (Bell and McCullochbide method is the most frequently used (Antrim 1966, Long and French 1967) reported use ofet al. 1970, Blystone et al. 1961). Am-Be sources of 10, 30, 50 and 300 mc.

If subsurface measurements are required, theneutron probe must be placed in an access tube

NUCLEAR TECHNIQUES that may be closed at the bottom. The size andcomposition of the tube can affect the resultant

Neutron scattering neutron density (Stolzy and Cahoon 1957). Place-The neutron scattering method is an indirect ment of the tube in the field has been discussed

way of determining soil moisture content. The by many authors (Kozachyn and McHenry 1964,method estimates the moisture content of the Bowman and King 1965, Koshi 1966, Long and

2

. .i

VFrench 1967). The method most used is to drill a (1960) and Olgaard (1965). They suggest that theslightly undersized hole and tamp the access "sphere of importance" is the one which, if alltube into the drilled hole to ensure a tight fit. the soil and water outside the sphere were re-

The accuracy of the neutron probe can be moved, would yield a neutron flux at the sourcefound from the deviation calculated from the re- that is 95% of the flux obtained in an infinitegression analysis in which neutron counts are medium, The volume of soil over which theconverted to volumetric moisture content. Wil- measurement is made becomes very importantson (1971) reported that the calibration depends when measuring soil moisture with depth. Inupon the source strength, the nature of the many studies however, the diameter of thedetector, the geometry of the source and detec- sphere of importance or influence cannot betor in the probe, the materials used to construct easily related to resolution. This is due to thethe probe, the size and composition of the ac- heterogeneity that occurs with soil depth. Thecess tube, and the physical and chemical proper- vertical resolution is critical to many studies, es-ties of the soil. In addition, Visvalingam and Tan- pecially those dealing with soil moisture moni-dy (1972) found that vehicle ignition noise great- toring over time and space.ly influenced the neutron probe readings. The advantages of the neutron probe are:

In laboratory calibration the volume of soil 1. Moisture can be measured regardless ofused should be large enough to be considered ef- its physical state.fectively infinite relative to the neutron flux. 2. Average moisture contents can beManufacturers of probes supply a generalized determined with depth.calibration curve with each unit; however, the 3. The system can be interfaced to accom-probe should be calibrated for each soil type if modate automatic recording.an accurate moisture content determination is 4. Soil moisture can be monitored on a sea-desired. Procedures have been developed for sonal basis.laboratory and field calibration (Douglass 1966, 5. Rapid changes in soil moisture can beKing 1967, Luebs et al. 1968). The moisture con- detected.tent value represents an average over a known 6. Readings are directly related to soilvolume of soil. Therefore, in laboratory calibra- moisture.tion the soil used should be homogeneous in re- The disadvantages are:gard to texture, structure, density and moisture 1. Inadequate depth resolution makescontent (Belcher et al. 1950, Van Bavel 1971, measurement of absolute moisture content1962, Douglass 1966). Field calibration of the impossible (reduces the use of the procedureneutron probe is reported to be extremely diffi- for determining the exact moisture profile incult (Stewart and Taylor 1957, Lawless et al. the study of evaporation, infiltration, percola-1963). tion and placement of the phreatic water

No matter what type of calibration is used, all surface).electrical equipment has the potential to drift. 2. The moisture measurement is dependentTherefore, primary standards should be em- on many physical and chemical properties ofployed to enable periodic recalibration of the the soil which are, in themselves, difficult toprobe. Various recalibration procedures have measure.been reported (Stewart and Taylor 1957, Marais 3. Care must be taken to minimize healthand Smit 1958, 1962, Bowman and King 1965, risks.Churayev and Rode 1966, Holmes 1966, Stone et 4. The sphere of influence of the depthal. 1966, Bell and Eeles 1967, Long and French probe does not allow for an accurate1967, Ursic 1967, Luebs et al. 1968, Olgaard and measurement of soil water at or near the soilHaahr 1968). surface.

The sphere of influence of the neutron probe Stone et al. (1966) stated that the accuracy ofmeasurement, the volume over which the aver- neutron probe measurements exceeds that ofage moisture content is calculated, depends on other techniques, but Stewart and Taylor (1957)the amount of moisture in the soil. Van Bavel et argued that it is slightly inferior. If the neutronal. (1956) and Glasstone and Edlund (1957) de- probe is used, the purchaser should look for afined the "sphere of influence" as that volume portable, durable model with stable electronicswhich contains 95% of all the thermal neutrons. and power components that are compatible withThis concept has been criticized by Mortier et al. available equipment (Bell and McCulloch 1966,

3

Zuber and Cameron 1966, Bell 1969). Gurr (1962), Ferguson and Gardner (1962), David-Prudhoe (1970) used the neutron probe to son et al. (1963), and Dmitriyev (1966) was instru-

study the water balance of a natural catchment. mental in developing the theoretical basis andHe found that the use of the neutron scattering procedure for its use.moisture meter in conjunction with permanent The basic equipment includes a gammaboreholes proved a convenient and speedy source surrounded by a collimator, a detectormethod for obtaining soil moisture at various with a collimator, and a scaler. Gurr (1962) useddepths, but that the method of measuring soil a 25-inc cesium 137 source with a lead colli-moisture at the surface requires improvement. mator, the beam emerging from a circular holeCalibration of the instrument was also a problem 4.8 mm in diameter. A scintillation counter wasbecause of the large number of samples re- used as a detector, shielded by a lead collimatorquired. The maintenance and repair costs should containing a 12.5-mm-diam hole. Mansell et al.be balanced against the advantages of the neu- (1973) stated that collimated radiation from 300tron probe method to determine its use in hydro- mc each of "'1Am and 137Cs provided a high-logic studies. intensity beam consisting of 60 and 662 key

In a study conducted by Gear et al. (1977), a photons. Count rates measured by a single de-simple, accurate technique using the neutron tector and a two-channel gamma spectrometerprobe to initiate an irrigation schedule was de- were corrected for coincidence losses due toveloped using a graphic display of the measure- pulse-resolving time It was concluded that errorments made with the neutron probe. It was in soil water content measurement by the dualfound that consistent timing alone could im- energy gamma attenuation method will prob-prove water use efficiency by more than 10%. ably not exceed a standard deviation of 1 %.They concluded that a coupling of neutron The gamma attentuation technique has theprobe irrigation schedules with system delivery same advantages of items 2, 3 and 4 listed undercapacity can lead to a coordinated delivery of neutron meters as well as the following:water to make the most efficient use of both irri- 1. Data can be obtained over very smallgation and drainage systems while improving horizontal or vertical distances.water use efficiency and reducing farm irrigation 2. The measurement is nondestructive.cost. The disadvantages are:

Soil moisture studies conducted at CRREL 1. Large variations in bulk density andhave shown that the neutron probe can be used moisture content can occur in highly stratifiedto monitor moisture content changes below a soils and lead to a limitation in spatialdepth of 30 cm. Above 30-cm depth the accu- resolution.racy of the soil moisture value decreased, prob- 2. Field instrumentation is costly and dif-ably due to the area of influence over which the ficult to use.determination was made. It is difficult if not im- 3. Extreme care must be taken to ensurepossible to calibrate the probe in the field to en- that the radioactive source is not a healthsure that the moisture value near the surface is hazard.correct. Sloane (1967) and Corey et al. (1971) also used

dual energy, collimated beam gamma-rays toGamma ray attenuation make simultaneous measurements of wet bulk

The gamma ray attenuation method is a radio- density and moisture content in moist soil col-active technique applicable when a soil mois- umns. Others who have investigated the tech-ture content value is required in layers 1-2 cm nique include Gardner and Roberts (1967) andthick. This method assumes that scattering and Gardner et al. (1972). In their study they usedabsorption of gamma rays are related to the den- two collimated beams of monoenergeticsity of matter in their path and that the specific gamma-rays from "'1Am and "Cs, but movedgravity of a soil remains relatively constant, The the soil column from one beam to another. Inchange in wet density is measured by the gamma this study the error in Yd and 6 resulted from 1)transmission technique and the moisture con- randomness of the emission from the sources, 2)tent determined from this change. random error in attenuation coefficients and soil

Gamma rays may be collimated to a narrow column thickness measurements, 3) presence ofbeam, which permits a representative reading to a smail higher energy peak in the "'Am spec-be obtained at dnv position in the soil. Work by toui,. and 4) counting dead time.

4

Goit et al. (1976) experimentally showed that NMR technique for measuring water in the liquidthe variability due to differences in Yd and 9 of a or solid state under laboratory conditions hassoil within the beam of a dual-energy system can been demonstrated by various authors (Ducrosresult in large measurement errors. Nofziger and Dupont 1962, Graham et al. 1964, Wu 1964,(1978) concluded from his studies that, indeed, Hecht et al. 1966, Touillaux et al. 1968, Prebblelarge errors in the measurement of Yd and 0 can and Curries 1970, Pearson and Derbyshire 1973).occur in highly stratified materials when using Woessner and Snowden (1969) indicated thatthe dual gamma beam technique. Generally, pulsed NMR techniques offer a number of ad-small errors occur if Yd and 6 change linearly in vantages over the wide-line NMR absorption pro-the collimated beam. Nofziger also confirmed cedures. This technique had not been developedthat both the single and dual gamma systems ac- previously because of capital costs, but small,curately measure the average water content in relatively inexpensive pulsed NMR systems arethe collimated beam if the bulk density of the now available.soil is constant. However, the average water con- Tice et al. (1978) developed a simple, rapidtent in the beam may not represent the water method for determining the unfrozen water con-content at the middle of the collimated beam tent in frozen soils. The method employs the useand in the middle of the present time period, of the amplitude of the first NMR pulse. OneFrom this study, graphs were prepared to esti- way to obtain this information is through themate the error due to inhomogeneity of the soil. PRAXIS Model PR 7103 Pulsed NMR analyzer.

A major problem in many areas of cold re- The analyzei consists of two parts: 1) a samplegions is the inability to measure in-situ water probe which contains a permanent magnet (2.51conditions in the freezing, thawing or frozen kilogauss), and 2) a sample coil and a radio fre-state. Goit et al. (1976) conducted studies to quency (10.72-MHz) pulser.evaluate attenuation of a dual gamma beam and The console portion of the analyzer containsfound that it was a powerful technique for inves- the electronics required to provide radio fre-

tigating the swelling phenomena associated with quency pulses, signal detection and signal aver-freezing soil. They found that errors resulted aging. The system is tuned to analyze hydrogenwhen attenuation equations developed for (protons) in the sample. The protons accepthomogeneous mixtures were applied to strati- energy from the radio frequency field when in a

fied media. Nofziger (1979) determined that the strong, fixed magnetic field, and the protons

errors in 0 and Yd due to nonuniform soil systems release this energy and return to equilibrium

must be considered to establish the overall ac- through a series of relaxation processes which

curacy of gamma ray measurements. can be easily measured. The measured differ-Since attenuation of gamma rays is indepen- ences in the relaxation processes are related to

dent of the state of the water in the material physical and chemical soil properties associated

tested, the measurement of attenuation is unaf- with the hydrogen in the sample. The analyzer

fected by the transition of liquid water to ice. uses the NMR technique to obtain signals from

Therefore, the use of gamma attenuation has an the hydrogen in both the liquid and solid states.

advantage in that measurements of dry bulk Therefore, quantitative information can be ob-

density and total water content (including ice) tained without weighing the sample.

can be made simultaneously. Tice et al. (1979) used the instrument to meas-ure unfrozen water in frozen soil. A comparison

Nuclear magnetic resonance is made between the first pulse amplitude in the

Placement of a soil/water mixture in a fixed thawed condition and the signal obtained at

magnetic field and a varying magnetic field re- temperatures below freezing. With this tech-

suits in an increased absorption of energy at a nique the unfrozen water content at various

specific frequency of the varying magnetic field. temperatures below freezing can be obtained,

This is referred to as nuclear magnetic resonance and a phase comparison curve can be generated.(NMR). The varying nuclear magnetization is Tice et al. (1979) were able to complete a phase

converted into a voltage by using either the sin- comparison curve for four soils in about 48

gle coil absorption technique or the quadrature- hours. To obtain the same information using iso-

coil induction technique. Geary (1956) noted thermal calorimetry techniques would have

that the NMR spectrum is directly related to the taken months.

water content of the soil. The reliability of the Laboradtry studies are continuing at CRREL

5

A2"

using the NMR technique. Currently the system Table 1. Spectral regions.is not well-adapted to field measurement of soilwater. In the future the NMR could provide a Frequency or

means of monitoring soil water in the field under Region wavelength range

freezing, thawing and frozen conditions.1 DC to lkHz

2 1 kHz to 1MHz3 1 MHz to 100 MHz

ELECTROMAGNETIC TECHNIQUES 4 100 MHz to 1 GHz

5 1 GHz to 30 GHzElectromagnetic techniques include all 6 30 GHz to 300 GHz

methods which depend upon the effects of mois- 7 300 GHz to 3 X 10' GHz (10pm)

ture on the electrical properties of soil. Excluded 8 10 Yrm to 1 im

but treated elsewhere in this document are thosetechniques which are essentially electrical meth- quency is greater than (

0R the dipoles can no

ods of estimating proton density, such as nuclear longer follow the field and the ability of the me-magnetic resonance or gamma ray attenuation. dium to store electric field energy decreases.The magnetic permeability of soils is very nearly The function i(w) is a measure of the energythat of free space and hence the categorization dissipation rate in the medium. Viewed as achosen reduces the problem to a discussion of function of frequency, and starting from a lowmethods of exploiting the moisture dependence value of cu, it rises to a peak at wR and thereafterof the dielectric properties of soil. decreases. The behavior described is due to the

The range of frequencies involved in this anal- permanent dipoles in the soil medium. In com-ysis is very great and discussion will be facili- plicated heterogeneous media there may betated if the spectrum is broken up into the more than one relaxation mechanism and moreranges specified in Table 1. The first seven of than one absorption peak. Furthermore, at fre-these categories are given in frequency while the quencies above cuR the medium may show fur-last is more commonly given in wavelength. Be- ther dispersion and absorption regions due tofore proceeding to the discussion of available direct molecular excitations. The frequency w Rtechniques in the ranges listed, we will mention will generally lie in the microwave range (18some general considerations of the problem. GHz in water) while the latter molecular excita-

The dielectric properties of the moist soil may tions will be in the submillimeter or infraredbe characterized by a frequency-dependent regions of the spectrum (Bottcher 1952, Hastedcomplex dielectric response function (Bottcher 1974).1952): The preceding description applies in a general

way to all dispersive media. In soils the value of4(W) = EW() + fEf(w) (2) E, typically lies within ranges 3 through 5 while

the value of Fr for water is about 80. Hence rela-where w) = dielectric response function tively small amounts of free water in a soil will

r(ow) = the real part of E greatly affect its electromagnetic properties.-= 1_- Discussion of the way this is used to monitor soil

ti(w) = the imaginary part of E moisture will be divided according to the basic

= the angular frequency measurement technique involved. There arethree such techniques: 1) use of implanted sen-

The function £,(w) is approximately constant sors, 2) monitoring of the radiation emitted by afrom co = 0 out to the neighborhood of the re- moist soil (radiometric method), and 3) monitor-

laxation frequency oR of dipoles in the medium. .ing of reflected electromagnetic waves (radarThe time w-1 is the time constant for the decay method).of polarization, when the electric field is re- A variety of implantable sensors, responsive

moved. Beyond wR the function Fr decreases un- either to resistivity (£i), polarization (Er), or totil it is equal to the index of refraction squared in both have been constructed (DePlater 1955,the visible region of the spectrum. The real part Gagne and Outwater 1961, Thomas 1963, Wexlerof the dielectric response function is a measure 1965, Roth 1966, Silva et al. 1974). Traditionally,of the energy stored by the dipoles aligned in an and because of engineering limitations, theseapplied electromagnetic field. When the fre- have been designed for operation in the regions

6

of the spectrum labeled 1 and 2 in Table 1. of both E, and E, These can be used separately asRecently, however, due to a steady decrease in moisture indicators (Layman 1979, Walsh et al.the physical size of high quality, high frequency 1979).components, implantable sensors in region 3 As was the case for resistive sensors, thehave become a practical reality (Selig et al. 1975, capacitive sensors can be very compact. TheyLayman 1979, Walsh et al. 1979), and implant- can be implanted at any desired depth and they

able sensors in regions 4 and 5 are possible. can be connected to data collection platform-;.i The resistivity ot soil is sensitive to moisture The present restriction to regions 2 and 3 ofcontent and, hence, it can serve as the basis for a Table 1 is dictated on the lower end of the spe( -

sensor. It is possible either to measure the resis- trum to escape the large ioni( contributions andtivity between electrodes in a soil or to measure on the higher end by convenience. If modernthe resistivity of a material in equilibrium with microwave solid-state oscillator technology isthe soil. Sensors of either kind can be very com- used, however, there is no reason to assume thatpact and an array of them can be connected to implantable sensors could not he made tostandard data collection platforms. The difficul- operate in regions 4 and S. Such a system mightty with resistive sensors is that the absolute consist of a Gunn diode oscillator and an anen-value of soil resistivity is dependent on ion con- na loaded by the soil. A second antenn,i ( ould(centration as well as on moisture concentration be placed on the surfa(v for data (olle( tion(Bouyoucos and Mick 1948). Therefore, careful purposes.calibration is required for this technique. Even In addition to the present speculative possibil-with careful calibration, the instrument may re- ity of implantable sensors operating in the mi-quire frequent recalibration due to changes in crowave range of the spectrum (region 5 and theorganic or salt concentrations. The calibration lower part of 6) there are two other ways to useproblem becomes less severe as the operating microwave radiation in moisture sensing. One,frequency is increased, since the relative contri- involving a measurement of the brightness tem-bution of ion motion decreases. perature of a soil in the microwave range, is a

Implantable sensors which are sensitive to passive radiometric technique (Poe and Edger-polarization 1

R essentially measure capacitance ton 1971, Eagleman and Lin 1976, Choudhury et(DePlater 1955, Gagne and Outwater 1961, al. 1978, Schanda et al. 1978, Schmugge 1978).Thomas 1963, Selig et al. 1975, Layman 1979, The other makes use of the reflection of a trans-Walsh et al. 1979). This parameter is the electri- mitted signal from the soil surface and is an ac-cal quantity which is the most direct indicator Gf tive radar technique (Battivala and Ulaby 1977).moisture concentration. When the moisture held Both are sensitive to moisture in the upper fewin the soil can be regarded as free, as .it is in most centimeters of soil and are affected by surfacesandy soils, the relationship between Fr and mois- roughness.ture is linear. Furthermore, even in more compli- In the case of radiometry, knowledge of thecated materials where the water is relatively effective temperature of the incident radiation,tightly bound, such as in montmorillonitic clay, the temperature of the surface, and the reflec-it is possible to obtain moisture content data by tion and emission coefficients permits a deter-measuring the capacitance of an implanted sen- mination of the brightness temperature. Thus,sor. Because of this, a number of capacitive sen- both Lr and t, affect the brightness temperature.sors have been constructed. The majority of The theoretical dependence on moisture con-these sensors have been designed for operation tent is very complicated, but a good correlationin spectral regions I and 2 (see Table 1) although between soil moisture in the surface layer andmore recently some work has also been done in microwave brightness temperature has beenregion 3 (Silva et al. 1974, Selig et al. 1975, established in the lower part of region 5. In theL~uian 1979, Walsh et al. 1979). The motivation upper part of the microwave spectrum the corre-for increasing the operating frequency is again lation is not as good. As the radiometry methodto minimize the contribution of ionic conductivi- is still in a research phase, the reason for thisty, since if this is large, it can make accurate discrepancy may become clear in the future.measurement of the capacitance difficult. One The microwave radar approach utilizes the re-other promising technique is to work at an in- flection of an electromagnetic wave from thetermediate frequency (region 3), and to utilize a soil surface to characterize the moisture con-bridge technique which allows a determination tent. This reflection approach has been studied

7

extensively by the group at the University of planted sensor, but results indicate that it is po-Kansas (Battivala and Ulaby 1977). This is a very tentially sufficient for some applications. Finallycomplicated problem but the results indicate there is the fact that this method is most sensi-that the overall scattering coefficient does tive to the upper few centimeters of soil, andcorrelate with soil moisture. There is an ap- hence it is probably not the method of choiceparent variation of reflectance with soil type when moisture profile data are required.which may be due to the relatively greater bind- The conclusions about infrared methods areing for water in a nmaterial with a higher clay con- similar to those given for the microwave ap-tent. The infrared emissioii of a moist soil (~an proach. The main difference is that the infraredalso be used to measure the relative moisture technique does not work well in the presence ofcontent. In frequency region 8, corresponding to a crop canopy. This disadvantage for some ap-infrared emission, it is the thermal inertia of the plications can sometimes be bypassed, however,soil water that is used as an indicator. The diur- by making use of the radiation changes relatednal variation of the infrared brightness, when all to crop moisture uptake. The radiative methodsother factors are the samne, is less when the soil are sufficiently accurate for survey work. Thernois4ure is increased. implantable techniques seem to offer greater ac-

The-relative advantages and disadvantages of curacy, precision and profile capability with thethe implanted sensor techniques are not unex- disadvantage of a somewhat more cumbersomepected. The main advantages are that they are technology.capable in principle of providing absolutevalues for soil moisture, and they can be im-planted at any depth. The latter point means TENSIOMETRIC TECHNIQUESthat moisture profile data, which are importantin some applications, can be obtained by this The term "tensiometer" was used by Richardsmethod. A wide variety of sensor configurations and Gardner (1936) to unambiguously refer tovarying from very small to quite large are possi- the porous cup and vacuum gauge combinationble, and hence there is some control over the for measuring capillary tension, i.e. the energysensor volume of influence. The precision of with which water is held by the soil. Tensio-both the resistive and capacitive sensors is high. meters were first used to measure soil water ten-The first of these is also relatively accurate when sion in unsaturated soils as early as 1922 (Gard-adequate control over other parameters is main- ner et al. 1922). Richards (1949) and others havetained, while the second has a relatively high in- made extensive developments and improve-trinsic accuracy which is more nearly indepen- ments in the tensiometers used in the field anddent of parameters other than moisture. This fol- laboratory soil water studies.lows from the fact that the capacitive sensors The energy term can be expressed as pF,are directly responsive to the amount of polar- which is defined as the common logarithm of theized energy stored in the region of the sensor, height of a water column in centimeters equiva-and this quantity is normally dominated by the lent to the soil moisture tension, but is usuallywater present. expressed as a suction (negative pressure) or a

The moisture sensor must be implanted prop- potential (energy per unit mass). Elrick (1967)erly to minimize disturbances to the soil. In addi- recognized six components of the total energytion, there are questions of long-term reliability, of soil water, of which matrix suction is one. Hemaintenance of the calibration, and interface defines matrix suction as the pressure differencewith remote data collection platforms. Never- across a boundary permeable only to water andtheless, it would seem that the overall relative solutes which separates bulk water and soiladvantages in some applications would warrant water in hydraulic, chemical and thermalserious consideration of the implantable equilibiium. Dissolved salts or chemicals in thesensors. soil water contribute to solute suction. Bayer et

The radiometric and active microwave ap- al. (1972) suggested that the term "capillaryproaches have the advantage that they are both potential" be used to denote the total potential,remote sensing techniques. Although they in- which includes not only surface tension forcevolve expensive instrumentation, very large but also the osmotic and adhesion forces.areas can be covered. The accuracy and preci- The most widely known method tor measuringsion will probably be less than those of an im- the capillary or moisture potential is based upon

8

WA -A A&M

-~ ~ ~ ~ ~~, -

/I

the so-called suction force of the soil for water or manometer can be inserted at the perforation(Bayer et al. 1972, Richards 1965). Tensiometers to measure the soil water tension. Cope andare used to measure suction and consist of a por- Trickett (1965) present information on a tensio-ous ceramic cup filled with a liquid (usually meter design that has worked well in their stud-water), connected by a continuous liquid col- ies. In addition, tensiometers with a Bourdon-umn to a manometer or vacuum gauge. In our re- type vacuum gauge, reading in centibars to in-cent designs, the liquid is an ethylene glycol- dicate the soil water tension, are commerciallywater solution and the measuring gauge a trans- available. Richards (1949) and Reeve (1965) haveducer with a millivolt output. The output of the outlined procedures for setting the zero scale fortransducer can be interfaced to near real time the manometer or Bourdon vacuum gauge.data acquisition systems. The use of an ethylene Many procedures have been used to installglycol-water solution as a replacement for conventional tensiometers (Reeve 1965, Rich-water in the tensiometer allows the use of a ten- ards 1965). Ingersoll (1979) used a technique thatsiometer/transducer system in cold climates required making an oversized hole in the soil to(McKim et al. 1976). Since the tensiometer/ trans- a depth about 8 cm above the point at which theducer system has a millivolt output and re- soil moisture content or tension data are re-sponds rapidly to changes in soil tension,it is quired. Another soil probe is then inserted,well suited for automatic (including satellite which produces a hole 8 cm deep and of therelay) recording systems (Elzeftawy and Mansell same diameter as the ceramic cup. The ceramic1975, McKim et al. 1975, Gillham et al. 1976). cup/plastic tube tensiometer system is inserted

The essential steps in the technique include firmly into the 8-cm-deep hole, care being takende-airing the water or solution in the tensio- not to turn the plastic tube and break the cer-meter, placing the tensiometer system in the amic cup. The area around the tube is backfilledsoil, and allowing it to come to equilibrium with with the soil removed from the augered hole andthe soil water. The ceramic cup is porous to montmorillonite pellets are placed around thewater and solutes but not to air, so that water tube at the ground surface.can flow, and soil water conditions or the Various types of transducers and many sizes ofchange in moisture content can be determined, ceramic cups have been used to design a tensi-As the soil water content increases, it is held at a ometerftransducer system that will measure soillower tension; when the tensiometer reads zero, water potential which can be converted to soilthe soil is saturated, and there is zero water ten- water content. Studies are in progress at CRRI-Lsion. The highest tension reading that can be ob- to evaluate not only the costs of various trans-tained with a tensiometer is about 1 bar (1 at- ducers, but also the size of the ceramic cup andmosphere). In most instances, data cannot be ob- the type of liquid required for laboratory andtained beyond 0.8-0.9 bar because the air entry field tests.value of the ceramic cup is exceeded. Therefore, One study recently accomplished was thethe moisture content range over which the tensi- placement of a tensiometer/transducer and tem-ometer can be used is limited. Richards (1949) perature sensor system in the field under thestated that for coarse, sandy soils the tensio- snowpack. The liquid used in the tensiometermeter may cover more than 90% of the avail- was a 50/50 mixture of ethlylene glycol andable moisture content range. Clay soils pose a water. The millivolt output of the strain gaugedifferent problem. For example, soils containing type transducer was passed through an inter-over about 42% montmorillonite clay can exper- face, developed at CRREL, to the GOES geosta-ience a change in tension from 200 to 800 cm tionary satellite. The data were telemetered to awater with a 1% change in volumetric water downlink station located at the National Ocean-content (Abele et al. 1979). ographic and Atmospheric Administration

Many techniques have been used to design a (NOAA) in Suitland, Maryland. It was possible to ttensiometer system (Cope and Tricket 1965, In- obtain the data from NOAA on a daily basis viagersoll 1979). In most cases tensiometers are telephone. The tensiometer and temperaturemade by gluing a length of rigid plastic tubing to data were compiled at six-hour increments soa porous ceramic cup about 20 mm in diameter. that four data points for each of two sensors,A perforation is made in the plastic tube a few placed at depths of 30 cm and 60 cm, could becentimeters below the top, and the top is obtained. This method of data acquisition couldcovered with a rubber stopper. A vacuum gauge be a powerful tool in management of irrigation

9

systems, not only for conventional agricultural 3. Field installations drift electronically.but also for slow infiltration land treatment The ability to measure soil tension and volu-systems. metric water content under field conditions in

Klute and Peters (1966) have described the use time and space is necessary for the calculationof diaphragm type pressure transducers using of soil infiltration rate which is required in thestrain gauge resistance elements to detect min- design of land treatment systems (McKim et al.ute deflections under applied pressure. They 1975). In addition, these field data can providealso developed an operational technique for required inputs for validation of one- and two-their use. This tensiometer/transducer system dimensional flow models (Nakano et al. 1978),can be installed for field use by utilizing a Rice (1969) has described two approaches tobattery-operated amplifier (and recorder, if ne- recording pressure from a number of tensio-cessary) and mercury cells for exciting the bridge meters. The first, a hydraulic scanning system,circuit in the transducer. consists of a number of tensiometer cups con-

Soil moisture measurement procedures using nected to one transducer through a hydraulictensiometer/transducer systems are rapidly be- scanning valve. In the second, an electrical scan-coming the predominant means of monitoring ning system is used (with each tensiometer hav-soil tension in the field. Recent studies by Bian- ing its own transducer, usually located near thechi (1962), Klute and Peters (1962), Watson porous cup), and the signals are electronically(1967), Rice (1969), and Anderson and Burt (1977) scanned. The results from this field studyhave shown the advantages of using pressure showed that the hydraulic scanning systemtransducers to produce a fast response, low vol- could be used satisfactorily for 10 months. In theume displacement tensiometer system. These loam soil used in Rice's study, the response timetypes of systems are capable of monitoring soil of the electrical scanning-tensiometer systemtension changes that occur in infiltration, irriga- was about two minutes.tion, groundwater recharge and evapotranspira- Williams (1978) also developed a rapid re-tion studies. sponse automatic tensiometer system that pro-

Tensiometers have been used for years to vided on-site recording of soil moisture condi-measure soil tension, and during recent years tions. He used a 24-way fluid wafer switch toadvancements in system design and perfor- connect 22 tensiometer units sequentially to amance have made possible the implementation pressure transducer. The data were recorded onof soil moisture field monitoring programs. a chart recorder. The advantage of Williams' sys-However, care still needs to be taken in monitor- tem is that it is relatively insensitive to air tem-ing the use of the system. Listed below are some perature fluctuation, which Rice (1969) found toof the advantages of using tensiometers: be a problem.

1. Systems are easy to design and con- Richards et al. (1974) used tensiometers instruct. shallow groundwater studies. They concluded

2. The cost of a system is relatively low. that a tensiometer installed to read the total3. Information on moisture conditions hydraulic head with a ground surface reference

under saturated and unsaturated conditions in gives water table information roughly compar-near real time can be obtained, able to that obtained from water observation

4. The tensiometer can usually be placed in wells. The tensiometer reading also allowed de-the soil easily and with minimal disturbance. termination of equipotential lines above and

5. The system can operate over long time below the water table.periods if properly maintained. Tensiometer experiments to evaluate un-

6. The response time of the tensiom- steady moisture flow through soils have beeneterltransducer system is very rapid. performed by many workers (Geisel et al. 1970,

7. Different types of liquid can be used, e.g. Fitzsimmons and Young 1972). Oster et al. (1976)an ethylene glycol solution to obtain data dur- also used tensiometers and salinity measure-ing freezing and thawing conditions. ments in irrigation control. They found that a

Disadvantages include the following: control system for high frequency irrigation,1. The tensiometer can be broken easily based on in-situ measurement of soil salinity and

during installation. water potential, has the capability of controlling2. The range of information obtainable is irrigation so that a relatively stable and continu-

limited to 0-800 cm of water tension. ous leaching fraction is maintained.

10

HYGROMETRIC TECHNIQUES tively small change in capacitance of the sensorrelated to the change in humidity. Charlson and

The general nature of the relationship be- Buettner (1963, 1964) and Charlson et al. (1966)tween moisture content in porous materials and used a hygroscopic liquid to improve this techni-the relative humidity (RH) of the immediate at- que. However, there are still many problems evi-mosphere is reasonably well known (National dent in all the Lapacitance systems.Cooperative Highway Research Report 138). The piezoelectric sorption, infrared absorp-Therefore, a number of relatively simple appar- tion and transmission, and dimensionally varyingatus for measurement of RH have been de- element hygrometer techniques are all very wellsigned. Basically, the sensors can be classified documented in National Cooperative Highwayinto seven types of hygrometers: electrical resis- Research Program (1973) and will not be dis-tance, capacitance, piezoelectric sorption, infra- cussed further.red absorption and transmission, dimensionallyvarying element, dew point, and psychrometric. Dew or frost point methodIn evaluating hygrometric methods, the follow- The dew or frost point hygrometer techniqueing characteristics are extremely important: depends on the measurement of temperatures

1. Range in soil moisture content, and is relatively simple and inexpensive. Wexler2. Hysteresis in material tested and sensor (1965) showed that modern improvements in the

calibration, technique, such as using of Peltier devices for3. Size of the sensitive element, cooling and photometric detection of condensa-4. Kind of water to be measored, and tion, have added greatly to the cost. Systems5. Durability of the sensor. have been designed that are very small

(Brousaides and Morrissey 1967).White hydrocal method

The various types of electrical resistance Psychrometric methodhygrometers include chemical salts and acids, Psychrometric instruments form another ma-aluminum oxide, electrolysis, thermal and white jor hygrometer grouping. The greatest advan-hydrocal. Bouyoucos and Cook (1965) consid- tage of the psychrometer is its simplicity. If theered the white hydroc:d hygrometer to be the ambient air has a velocity of more than 3 m/s,best one available. The measured resistance of two thermometers, one covered with a moistthe resistive element is a function of the relative wick, are all that is required. Miniature thermo-humidity. They state that casting of the stainless couple psychrometers are now commonly usedsteel electrodes in white hydrocal (a form of for measuring water potential (matrix suctionplaster of Paris) leads to greater accuracy be- and osmotic suction) in soil systems. Rawlinscause the cement sets hard, is pure, has a low (1966) has presented the theory of this system.solubility and contains no added salts. Zollinger et al. (1966), Rawlins and Dalton (1967),

and Rawlins et al. (1968) have described theCapacitance method equipment and method to use. The procedure

Two types of capacitance hygrometers are ca- has been used in the laboratory but not in thepacitive transducers and microwave refractom- field. The problems in field use are the necessityeters. A typical capacitive transducer is con- of controlling the temperature of the psychrom-structed with a thin plastic film on acetyl resin eter chamber, which in some cases must be con-which is a crystalline form of highly polymerized stant to within 0.0010 C, and the time requiredformaldehyde. The capacitor plates are formed for the chamber to reach temperature and hu-with evaporated gold electrodes that are thin midity equilibrium. One method which may beenough to be pervious to water vapor and still be applicable in field use has been described byelectrically conductive. Webb and Neugebauer Rawlins and Dalton (1967). This method is inde-(1954) developed a capacitive system using two pendent of temperature fluctuations of a fewconcentric cylinders for the capacitor plates. degrees Celsius and permits measurement overSensors of this type can be used in the range of an entire range of soil suction. However, the pre-10 to 100% RH over a temperature range of 35 cision is only a few tenths of a bar.to 800 C (Nelson and Anbur 1965). Nelson and An- Phene et al. (1971, 1973) developed a heat dis-bur (1965) made improvements in the capacitor sipation sensor that was used to measure the soilmethod, but the main disadvantage is the rela- moisture potential. The accuracy of the matrix

11

- i .'."-- . :, .-.C ..

potential sensor proved to be as good as or bet- Anderson et al. (1976) proposed the use of anter than that of the thermocouple psychrometer electrolysis system to evaluate the occurrenceor salinity measurements. The sensor, which had of water on Mars. A Beckman P,05 integratinghigh sensitivity in the 0 to 2-bar matrix potential water detector is being configured for use in therange, had an accuracy of ± 0.2 bar. The ac- Mars penetrometer soil sampler. Similar in de-curacy decreased progressively to ± 1 bar at a sign to the detector discussed above, it consistsmatrix potential of -10 bar. of two parallel, unconnected electrical conduc-

The primary advantages in using the afore- tors bathed in a solution of phosphorus pent-mentioned hygrometers are the simplicity of the oxide-phosphoric acid which functions as anapparatus and the low cost. The basic disadvan- electrolyte. The end of each conductor is con-tages in using the method include deterioration nected to a source of EMF sufficient to electro-effects of the soil components on the sensing lyze water. The increased current is exactly pro-element and the requirement for special calibra- portional to the number of dissociated watertion for each material to be tested. The main use molecules.for this technology seems to be in applications Tests of five P,05 sensors were performed in awhere RH in the material is directly related to simulated Mars environment. The resultsother properties. One example would be drying showed nearly flawless performance of the P,05and shrinkage of cements (National Cooperative hygrometer under Mars-like conditions. ThisHighway Research Program Report 138). work is continuing at CRREL.

Electrolysis methodElectrolysis systems employ a continuous SUMMARY

flow of a gas mixture over a thin layer of partial-ly hydrated phosphorus pentoxide (P205). The The measurement of water, wherever it occurs,moisture in the gas is absorbed by the P,05. A continues and will continue to be a researchsensor can be built with two platinum wires area where advances in science and technologyplaced helically inside an insulating tube. The will have a great impact. We have covered manytube is then coated with a layer of P,05 (Na- of the techniques used to measure soil and plan-tional Cooperative Highway Research Program etary water but not all that have been tried. InReport 138). A DC voltage is applied across the field use, where low costs must be maintained,two wires, dissociating the water molecules into the gravimetric method will, at present, give rea-gaseous hydrogen and oxygen. At equilibrium, sonable moisture content values. However, ifthe measured current is proportional to the monitoring of the soil moisture is the importantmoisture absorbed. The DC voltage must be problem, many samples are required using thelarger than the polarization voltage (-p2 V). gravimetric technique, and nuclear, tensio-

Many people have used the electrolysis tech- metric, electromagnetic or hygrometric tech-nique to measure moisture in materials (Fraade niques may be more cost-effective.1963, MacCready 1962, Roth 1966, Thacker 1967, Capital costs associated with the nuclearBarton and Maffei 1968, Honnell and Hibbitts methods can be quite large. Of the three nuclear1968, Kreider 1968). methods discussed in this report only the neu-

The advantages of the electrolysis method are tron scattering system is portable and can bethe following: used effectively in the field. This technique is

1. The modular design can permit signal nondestructive to the samples, has an accuracytransmission to a remote central location, greater than 0.1 %, is relatively rapid in obtain-

2. The sensitivity of the measurement can ing soil moisture data (1 to 5 min per moisturebe up to 1 ppm. value) and can be set up as an automatic record-

3. The response time for the measurement ing system. The major problems are in calibra-is relatively short. tion and resolution, which can limit its use.The disadvantages include: Electromagnetic techniques to measure soil

1. There is a need for a constant mass flow water have had limited use because of the inabil-and constant temperature. ity of the sensors to measure very tightly bound

2. There is a possibility of interference from water. This is especially true in high clay contentgases that would interact with PO,. soil/water systems. Recently, a variety of sensors

3. The cost of the system is very high. have been configured that work very well under12

laboratory conditions. Many new techniques, including NMR, differ-A number of capacitive sensors have recently ential scanning calorimetry, capacitance sensor-

been developed and tested at CRREL. Their main ing, use of the PO, detector and others, mayadvantage is that they are durable, low in cost, lead the way to new methods for measuring soilmeasure in all tension ranges, and provide an ab- moisture. As technology in electronics, corn-solute value for soil moisture. A wide variety of puter science, and other related fields advances,sensor configurations varying from very large to the use of this technology in hydrology willvery small are possible and hence there is some greatly improve the ability to measure soil mois-control over the area of influence of the sensor. ture conditions under field conditions. The otherResistance and capacitance are independently problem, relating these values to the spatial dis-measured and the precision of both is high. The tribution of soil water, will continue for manyoutput from the sensor is in millivolts and there- years.fore can be easily interfaced to near real timedata acquisition systems.

The tensiometric techniques are primarily LITERATURE CITEDused in agriculture, but studies have been ac-complished recently using tensiometer/ Abele, G., H.L. McKim and B.E. Brockett (1979) Mass watertransducer systems for automating irrigation balance during spray irrigation with wastewater atsystems, operating land treatment systems Deer Creek Lake land treatment site CRREL Special

Report 79-29measuring soil tension with depth at specific Anderson, M.B and T.P. Burt (1977) Automatic monitoring ofsites for water routing, and monitoring fluctua- soil moisture conditions in a hillslope spur and hollowtions of groundwater. Tensiometers can supply lournal ot Hydrology, vol. 33. p. 27-36information on tension and soil water close to Anderson, D.M., I.B. Stephens, F.P. Fanale and A.R Ticesaturation (tension range from 0-800 cm water), (1976) Mars soil water analyzer Instrument description

and status. Proceedings, Colloquiirm on Water inbut beyond a tension of 800 cm water the air en- Planetary Regoliths, p. 149-156try value of the porous ceramic cup is exceeded, Antrim, ).D, F B. Brown, HW. Busching, J.A Chisman, I.Hair can enter the system, and the water column Moore, J.P Roston and A.E Schwartz (1970) Rapid testmay separate. A loss of calibration will result. methods for field control of highway construction. Na-The response time of the tensiometer/transducer tional Cooperative Highway Research Program Report103.system is relatively short, with the lifetime vary- Barton. G.B and H.P. Maffei (1968) Cover gas impurities anding from 1 to 12 months. their interactions with sodium Battelle-Northwest.

One important aspect of the tensiometer/trans- NASA File No. N69-25968.ducer system is that it can be used to monitor Battivala, P. and F.T. Ulaby (1977) Estimation of soil moisturesoil water conditions under the snow. The re- with radar remote sensing. Proceedings, 11th Interna-

tional Symposium, Remote Sensing of the tnviron-quirement itn ment, Environmental Research Institute of Mu higanfreeze when temperatures go below O°C. The Report 1557data obtained under winter field conditions can Bayer, L D, W H Gardner and W R (,ardner (19721 Sbe obtained in near real time using available physics. New York. lohn Wiley and Son%

Belcher, D.J.. T.R. Cuykendall and H S Sack (1950) The meas-urement of soil moisture and density by neutron and

The primary advantage of using hygrometric gamma-ray scattering. Technical Development andtechniques is that the apparatus for measuring Evaluation Center Technical Development Reportrelative humidity is simple and the cost of con- No. 127, p 1-20.struction is low. Inherent problems lie in the del- Belcher, D.J., T.R. Cuykendall and H.S. Sack (1952) Nucleareterious effects of the soil components on the meters for measuring soil density and moisture in sur-

face layers. Civil Aeronautics Administration let hni-sensing element and the site-specific calibration cal Development and Evaluation Center. lethnicalfor each material. The accuracy of the hygro- Development Report No. 161, p 1-8metric resistance technique can be very poor Bell, IP (1969) A new design principle for neutron soilbecause the sensor measures the relationship be- moisture gauges. The "Wallingford" neutron probetween Soil Science, vol. 108, p. 164-169rien the pressure potential in the porous mate Bell. J.P and C,W 0 Eeles (1967) Neutron random countingrials and the relative humidity of the immediate error in terms of soil moisture for nonlinear calibrationatmosphere under equilibrium conditions, In curves Soil Science. vol 103. p. 1-3.many instances the equilibrium conditions are Bell, I.P and I.S.G. McCulloch (1966) Soil moisture estima-difficult to achieve. tion by the neutron scattering method in Britain lout-

nal of Hydrology, vol 4, p 254-263

13

-. .,. 1

Bianchi, W.8 (1%2) Measuring soil moisture tension Eagleman, I and W Lin (1976) Remote sensing of soil mois-changes Agricultural Engineering, vol. 43, p. 398-399. ture by a 21 cm passive radiometer. Journal of

tilystone, IR., A. Pelzner and G.P. Steffens (1961) Moisture Geophysical Research. vol. 81, p. 3660.content determination by the calcium carbide gas Elrick, DE. (1967) Soil water movement: theory and applicd-pressure method. Public Roads Magazine, vol 31, p tions. Proceedings, First Canadian Conference177-181 Micrometeorology. Part II. Department of Transpor-

Both her, C I.F. (1952) Theory of electric polarization. Amster- tation, Canada, p. 477-487.dam. Elsevier Elzeftawy, A. and R.S. Mansell (1975) Hydraulic conductivity

Biouyoucos. GI. and R L. Cook (1965) Humidity sensor perma- calculations for unsaturated steady-state andnent electric hygrometer for continuous measurement transient-state flow in sand. Soil Science Society ofof the relative humidity of the air (1). RILEM/CIB Sym- America Proceedings, vol. 39, p. 599-603posium on Moisture Problems in Building, vol. II, sec. Ferguson, H. and W.H. Gardner (1962) Water content6 measurement in soil columns by gamma ray absorp-

Bouyoucos. G.I and A Mick (1948) A comparison of elec- tion. Soil Science Society of America Proceedings, voltrical resistance units for making continuous measure- 26, p 11-18ments of soil moisture under field conditions Plant Fitzsimmons, D.W and N.C. Young (1972) Tensiometer-Physiology, vol 23, p 532. pressure transducer system for studying unsteady flow

Bowman, D H and K M. King(l%S) Determination of evapo- through soils. Transactions of the American Society oftranspiration using the neutron scattering method. Agricultural Engineers. vol. 15, no. 2.Canadian Journal of Soil Science, vol. 45, p. 117-126. Fraade, D.I (1963) Measuring moisture in gases. Instruments

Brousaides, F.1 and I F Morrissey (1967) Stratospheric hu- and Control Systems, vol. 36. p. 100-105,miditv sensing with the alpha radiation hygrometer. Gagne, G. and 1. Outwater (1961) A portable electronic mois-Air force Cambridge Research Laboratories In- ture detector for reinforced plastics. University of Ver-strumentation Papers. No 133. mont Report 3219 (01) (X).

Charlson, R I and K.I K Buettner (1963) The investigation of Gardner, W., OW. Israelsen, N.E. Edlefsen and H. Conradsome techniques for measurement of humidity at high (1922] The capillary potential function and its relationaltitudes. International Symposium on Humidity and to irrigation practice. Physical Review, series 2. vol. 20,Moisture, Washington, D.C. Report 1, Contract AF 19 p. 196.(628)-303. Gardner W. and D. Kirkham (1952) Determination of soil

Charlson, R.I and K 1K. Buettner (1964) Liquid film hygrom- moisture by neutron scattering. Soil Science, vol. 73, petry University of Washington Report 2, Contract No. 391-401.AF 19 (628)-303 Gardner, R.P. and K.F. Roberts (1967) Density and moisture

Crharlson, R I., K I.X Buettner and GA. Maylrsut (tl96) Liquid content measurement by nuclear methods. Nationalfilm hygrometry University of Washington Final Re- Cooperative Hydrology Research Program Report 43.port, Contract No AF 19 (628)-303. Gardner, W.H., G.S. Campbell and C. Calissendorff (1972)

Choudbury, B., T. Schmugge, R W. Newton and A. Chang Systematic and random errors in dual gamma energy(1978) ffe( t of surface roughness on the microwave soil bulk density and water content measurementsemission from soils NASA Technical Memorandum Soil Science Society of America Proceedings, vol. 36, p79606, 393-398.

Churayev. N V and L G. Rode (1966) Measuring the moisture Gear, R.D., AS. Dransfield and M.D. Campbell (1977) Irriga-(ontent of organic peat soils by the neutron method, tion scheduling with neutron probe. Journal IrrigationPochvovedenie, vol 1, p. 96-100. Drainage Division, American Society of Civil Engin-

Cope, F and E S Trickett (1965) Measuring soil moisture. eers. September, p. 291.298.Soils and Fertilizers, vol 28, p. 201-208 (c-are, P I (1956) Determination of moisture in solids Britain

Corey, I C , S F Peterson and M.A Wakat (1971) Measure- S( win(e Instrument Resear( h Assoiation. Rei-arhment of attenuation of '11Cs and 1

"Am gamma rays Report M 24, p 1-52for soil density and water content determinations, Soil Giesel, W,, S. Lorch and M. Renger (1970) Water-flow calcula-Science Society of America Proceedings, vol. 35. p. lions by means of gamma-absorption and tensiometer215-219 field measurements in the unsaturated soil profile.

Davidson, I M, M W. Biggar and D.R. Nielsen (1963) Gamma Proceedings of the Symposium on Isotope Hydrology,radiation attenuation for measuring bulk density and International Engineering Agency.transient water flow in porous media. Journal of Geo- Gillham, R.S.. A. Klute and D.F. Heerman (1976) Hydraulicphysical Research, vol. 68, p 4777-4783. properties of a porous-medium -Measurement and

DePlater, C V (1955) A portable capacitance type soil mois- empirical representation. Soil Science Society of Amer.ture meter Soil Science, vol. 80, p. 391. ica Proceedings, vol. 40, p. 203-207

Dmitriyev, M T (1966) Gammascopic measurement of soil Glasstone, . and M C. Edlund (1957) The elements of nuclearmoisture Pochvovedenie, vol 2, p. 208-217 theory. New York: Van Nostrand.

Douglass, I E (1966) Volumetric calibration of neutron mois- Goit. 1.B., P.H Groenevelt, B.D. Kay and I.G.P. Loch (1976)lure probes Soil Science Society of America Pro- The applicability of dual gamma scanning to freezingceedings, vol, 30, p 541-544 soils and the problem of stratification. Soil Science of

Ducros, P. and M Dupont (1962) A nuclear magnetic res- America Proceedings, vol 42, p. 858-863.onance study of water in clays in magnetic and elec Graham, I.. G.F. Walker and G.W. West (1964) Nuclear mag-trical resonance and relaxation C R Xl' Colloque netic resonance study of interlayer water in hydratedAmpere, Eindhoven. layer silicates Journal of Chemical Physics, vol 40. p.

540-SS0.

14

"-->7

Gurr. CG. (1%2) Use of gamma rays in measuring water con- Marais, P C and W B. De V Smit (1%2) Effet t of bulk densitytent and permeability in unsaturated columns of :iI and of hydrogen in forms other than free water on theSoil Science, vol 94, p 224-229 calibration curve of the neutron moisture meter South

Hasted, T (1974) Aqueous dielectrics. Londot" hi African Journal of Agricultural Science. vol 5. pHall 225-238

Hecht. M, M. Dupont and P. Ducros (1966) A stuOc MacCready. P B (1%2) A moisture analyzer for martian at-transport phenomena of water absorbed by certairn mosphere Final Report. JPL Contract No 950207,clay minerals by nuclear magnetic resonance Bulletin NASA Contract NAS7-100de la Societe Francaise de Mineralogie et de Cristal- McKim, HL, R L Berg. R W McCaw, R T Atkins and Ilographie, vol 89, p 6-13. Ingersoll (1976) Development of a remote-reading ten-

Hewlett, ID0 and I.E Douglass (1%1) A method for calcu- siometer/transducer system for use in subfreezing tern-lating error of soil moisture volumes in gravimetric peratures Proceedings. Second Conference on So /-sampling. Forest Science, vol. 7, p. 265-272. Water Problems in Cold Regions (American Geophvs-

Holmes. J.W. (1966) Influence of bulk density of the soil on ical Union), Edmonton, Alberta. Canada, 1-2 Septem-neutron moisture meter calibration. Soil Science, vol. bet. p 31-45102, p. 355-360 McKim, H.L, D.M. Anderson. R L Berg and R Tuinstra (1975)

Honnell, R.E and I0 Hibbits (1968) An evaluation of the ac- Near real time hydrologic data acquisition utilizingcuracy of Beckman hygrometers. General Electric the Landsat system Proceedings of the Conference onCompany, Missile and Space 'ision. Cincinnati. Soil-Water Problems in Cold Regions (American Ceo-Ohio, Pb-18210, CEMP-638 physical Union), Calgary, Alberta, Canada, 6-7 May. p

Ingersoll, I. (in press) Soil tensiometers ior use at temper- 200-211atures below freezing. CRREL Special Report. Mortier, P., M DeBoodt, W Donsercoer and L DeLeenheer

King, K.M. (1%7) Soil moisture-instrumentation, measure- (1960) The resolution of the neutron scattering methodment and general principles of network design. In Soil for soil moisture determination Transactions. 7th In-Moisture, Proceedings rf the Hydrology Symposium ternational Congress of Soil Science. vol 1. p 321-329No 6, p. 269-285. Canada Department of Energy, Nakano, Y., R.A Khalid and W H Patrick, Ir (1978) WaterMines and Resources, Ottawa. movement in a land treatment system of wastewater

Klute, A. and D B Peters (1966) Hydraulic and pressure head by overland flow Proceedings of the Internationalmeasurements with strain gauge pressure tran Jucers Conference on Developments in Land Methods ofProceedings UNESCO International Hydrological De- Wastewater Treatment and Utilization. p 141 -14/21cade Symposium on Water in the Unsaturated Zone, National Cooperative Highway Research Program Report 138Wageningen, The Netherlands, 19-25 June (1973) Instrumentation for measurement of moisture

Klute, A. and D.B. Peters (1%2) A recording tensiometer with Literature review and recommended research. Highway

a short response time Soil Science Society of America Research Board, National Resear h CouncilProceedings, vol 26, p. 87-88 Nelson, D.E and El. Anbur (1%5) A relative humidity sensor

Koshi, P.R. (1966) Soil-moisture measurement of neutron based on the capacitance variations of a plastics tilmmethod in rocky wildland soils. Soil Science Society of condenser In Humidity and moisture (A Wesler andAmerica Proceedings, vol. 30, p. 282-294. R.E Ruskin, eds ), Rheinhold, p 597-601

Kozachyn, 1. and I.R. McHenry (1964) A method for installa- Nofziger, D1 (1978) Errors in gamma-ray measurements oftion of access tubes and the development of field water content and bulk density in nonuniform soisequipment for measuring soil moisture by neutron Soil Science Society of America journal, vol 42. pscattering Agronomy Journal, vol 56, p. 443-444. 845-850.

Kreider, H.B., Jr. (1968) Modified moisture analyzer for solid Olgaard, P.L. (1%5) On the theory of the neutronic methodradioactive materials. Mound Laboratory, AEC Con- for measuring the water content in soil Risco Reporttract AT(33-1)-EGN-53 No. 97, Danish Atomic Energy Commission. p 1-44

Lawless, G P, N A MacCillivray and P.R Nixon (1%3) Soil Olgaard, P.L and V Haar (1968) On the sensitivity of sub-moisture interface effects upon readings of neutron surface neutron moisture gauges to variations in bulkmoisture probes Soil Science Society of America Pro- density Soil Science, vol 105. p 62-64ceedings, vol 27, p. 502 Oster, I.D., LS. Willardson, I van Schilfgaarde and I 0

Layman, R. (1979) Soil moisture sensor. Dartmouth College Goertzen (1976) Irrigation control using tensiometersPlasma Physics Laboratory, DCPPL-SM3 and salinity sensors Transactions of the American

Long, I F and B.K French (1967) Measurement of soil mois- Society of Agricultural Engineers, vol 2ture in the field by neutron moderation. Journal of Soil Pearson, R T and L Derbyshire (1973) NMR studies of waterScience, vol 18, p 149-166. adsorbed on a number of silica surfaces journal of

Luebs, R.E., M I Brown and A.E. Laag (1968) Determining Colloid Interface Science, vol 46. p 232-248water content of different soils by the neutron Phene. C., G I Hoffman and S L Rawlins (1971) Measuringmethod. Soil Science, vol. 106, p. 207-212. soil matric potential in situ by sensing heat dissipation

Mansell, R.S., L.C Hammond, R.M McCurdy (1973) Coin- within a porous body I Theory and sensor (onstrut-cidence and interference corrections for dual-energy tion Soil Science Society of America Proceedings, volgamma ray measurements of soil density and water 35. p. 27-33content. Soil Science Society of America Proceedings, Phene, C, G J Hoffman and R S Austin (1973) Controllingvol 37. p. 500-4. automated irrigation with soil matric potential sensor

Marais, PC. and W.B. De V Smit (1958) Determination of soil Transactions of the American Society of Agriculturalmoisture with a neutron meter Industrial Review of Engineers, vol. 16, p 773-776Africa. p 78.

-- * ... . . ... .. ..... . .. .. ... .. .. .. .. ... . , , :.,,. '1 5

Poe, G.A. and A.1 Edgerton (1971) Determination of soil Stone, I.F., R.H Shaw and D. Kirkham (1966) Statistical par-moisture content with airborne microwave ameters and reproducibility of the neutron method ofradiometry. Summary Report 4006R-2, Microwave measuring soil moisture. Soil Science Society of Amer-Division, Aerojet General Corporation, El Monte, Cal. ica Proceedings, vol. 24, p. 435-438.

Prebble, R.E and l.A. Curries (1970) Soil water measurement Thacker, CC (1967) Moisture content determination in rigidby a low-resolution nuclear magnetic resonance tech- polyurethane foams. Sandia Corporation Contractnique. Journal of Soil Science, vol. 21, p. 273-288 AT(29-1)-789.

Prudhoe, ) (1970) A study of the water balance of a natural Thomas, A. (1963) In situ measurement of moisture in soil andcatchment using a neutron-scattering moisture meter. similar substances by fringe capacitance. Journal ofRoad Research Organization Laboratory Report LR Scientific Instruments, vol. 43, p 996.361. lice, A R., CM. Burrous and D M Anderson (1978) Deter-

Rawlins, S L and F N. Dalton (1967) Theory for thermotouple mination of unfrozen water in frozen soil by pulsed

psy hrometers used to measure water potential in soil nut lear magnetic resonance Third International Con-and plant samples Agricultral Meteorology. vol 3. p ference on Permafost. Edmonton, Alberta, Canada24 -t) Tice, AR., C.M. Burrous and D.M. Anderson (1979) Phase

Rawlins. !) L andi F N. Dalton (1967) Psychrometric measure- composition measurements at very high water con-nient of Soil water potential without precise tempera- tents by the pulsed nuclear magnetic resonance tech-ture control. Soil Science Society of America Pro- nique. Transportation Research Record No. 675, p.ceedings, vol. 31, p. 297-301. 11-15.

Reeve. R.C. (1965) Hydraulic head in methods of soil analysis. Touillaux, R., P. Salvador. C. Vandermeersche and J.J. FripiatAgronomy, vol. 9, p. 180-196. (1968) Study of water layers adsorbed on Na- and Ca-

Rice. R. (1969) A fast-response, field tensiometer system montmorillonite by the pulsed nuclear magnetic reso-Transactions of the American Society of Agricultural nance technique. Israel journal of Chemistry, vol. 6, p.Engineers. vol. 12, p. 48-50. 337-348.

Richards, L.A. (1949) Methods of measuring soil moisture ten- Ursic, Sj. (1967) Improved standards for neutron soil watersion. Soil Science, vol. 68. p. 95-112. meters. Soil Science, vol. 104, p. 323-325.

Richards, S.j. (1965) Soil suction measurements with tensiom- Van Bavel, C.H.M. (1962) Accuracy and source strength in soileters in methods of soil analysis. Agronomy, vol. 9, p. moisture neutron probes. Soil Science Society of Amer.153-163. ica Proceedings, vol. 26, p. 405.

Richards, L.A. and W. Gardner (1936) Tensiometers for meas- Van Bavel, C.H.M. and G.B. Stirk (1967) Soil water measure-uring the capillary tension of soil water. Journal of the ment with an Am"'-Be neutron source and an appli-American Society of Agronomy, vol. 28, p. 352-358. cation to evaporimetry, Journal of Hydrology, vol. 5, p.

Richards, S.., L.S. Willardson, S. Davis and J.R. Spencer 40-60.(1974) Tensiometer use in shallow ground-water Van Bavel, C.H.M., N. Underwood and R.W. Swanson (1956)studies. Journal of the Irrigation Drainage Division, Soil moisture measurement by neutron moderation.American Society of Civil Engineers, vol. 99, p. 457-464. Soil Science, vol. 82, p. 29-41.

Roth, M. (1966) How to measure moisture in soilds. Chemical Van Bavel, C.H.M. (1961) Calibration and characteristics ofEngineering, p. 83-88. two neutron moisture probes. Soil Science Society of

Schanda, E., R. Hofer, D. Wyssen, A. Musy, D. Meylan, C. America Proceedings, vol. 25, p. 329-334.Morzier and W. Good (1978) Soil moisture determina- Visvalingam, M. and J.D. Tandy (1972) The neutron methodtion and snow classification with microwave for measuring soil moisture content-A review, jour-radiometry. Proceedings, 12th International Sym- nal of Soil Science, vol. 243, No. 4, p. 499-511.posium on Remote Sensing of Environment. Walsh, J., D. McQueeney, R. Layman and H. McKim (1979)

Schmugge, T. (1978) Remote sensing of surface soil moisture. Development of a simplified method for field monitor-journal of Applied Meteorology, vol. 17, p. 1549. ing of soil moisture. Proceedings of 2nd Colloquium on

Selig, E.T., D.C. Wobschall, S. Mansukhani and A. Motiwala Planetary Water and Polar Processes.(1975) Capacitance sensor for soil moisture measure- Watson, K.K. (1967) A recording field tensiometer with rapidment. Transportation Research Board Record 532, p. response characteristics, Water Resources Research,64. vol. 5, p. 33-39.

Silva, L.F.. F.V. Schultz and I.T. Zalusky (1974) Electrical Webb, P. and M.J. Neugebauer (1954) Recording dielectric hy-methods of determining soil moisture content. Labora- grometer for expired air. Review of Scientific Instru- -,

tory for Applied Remote Sensing Information Note ments, vol. 25, p. 1212-1217.112174, Purdue University. Wexler, A. (1965) Humidity and moisture. Vol. 5, p. 1-14, New

Soane, B.D. (1967) Double energy gamma transmission for York: Reinhold.moisture and density measurement in soil tillage stud- Williams, T.H. Lee (1978) An automatic scanning and record-ies. International Soil-Water Symposium, Prague, Dis- ing tensiometer system. Journal of Hydrology, vol. 39.

cussion 197. p. 175-183.Stewart, G.L. and S.A. Taylor (1957) Field experiments with Wilson, R.G. (1971) Methods of measuring soil moisture The

the neutron scattering method of measuring soil mois- Secretariat, Canadian National Committee for the In-ture. Soil Science, vol 83, p. 151-158. ternational Hydrological Decade. Ottawa. Canada

Stolzy. LIH. and G.A. Cahoon (1957) A field calibrated port- Woessner, D.E. (1974) Proton exchange effects on pulsedable neutron rate meter for measuring soil moisture in NMR signals from preferentially oriented water mole-citrus orchards. Soil Science Society of America Pro- cules. Journal of Magnetic Resonance, vol. 16, p. 19.ceedings, vol. 21. p. 571-575. Woessner. D.E. and B.S Snowden, Jr. (1969) A study of the

16

orientation of adsorbed water molecules on montmor-illonite clays by pulsed NMR. Journal of Colloid andInterface Science, vol. 30. p. 54-68.

Wu. T.H. (1964) A nuclear magnetic resonance study of waterin clay, Journal of Ceophysical Research. vol. 69, p.1063-1091.

Zollinger. W.D., C.S. Campbell and S.A. Taylor (1966) A com-parison of water-potential measurements made usingtwo types of thermocouple psychrometer. Soil Sci-ence, vol. 102, p. 231-239.

Zuber, A. and J.F. Cameron (1966) Neutron soil moisturegauges. International Atomic Energy Agency, AtomicEnergy Review, vol. 4, p. 143-147.

4

17

0*