Direct Measurement Instruments for Estimationof Vertical Water Movement

Pan LysimeterPassive Capillary Sampler

Tension Lysimeter

Joel HubbellAgE 558, Spring 2001

State Variables for Characterizing the Vadose Zone

Soil Water PotentialHydraulic gradient

Water ContentMass of water-changes in time and space

Flux Rate Concentration of Solutes

Pan LysimeterPassive Capillary Sampler (PCAPS)

Tension Lysimeter

Theory of operation Construction, installation and operation Advantages and limitations

Comparison of devices

Pan Lysimeter

Intersect flow and route to sampling locationImpermeable layer beneath siteNumerous configurations

Pan, plastic, glass block, funnel

Pan Lysimeter

Pan Lysimeter

Pan Lysimeter

Pan Lysimeter

Pan Lysimeters

Advantages: “Simple” designLower costs Available materials

Limitations: Divergent flow (under sampling)Collect 0 to 36-45% of flow (optimum)Difficult to design to predict fluxSamples macropores-”wet” conditions

Passive Capillary Sampler

Intercepts flowCollects with a constant pressure

Wick fluid with fiberglass rope

PCAPS

Passive Capillary Sampler

Passive Capillary SamplerAssets:Passive design

Macro/micro pore flowBest in sandsMore representative flux

103, 80, 64 % of WBWell published-12+ articles

Limitations:Soil water potential <-150 cm “wet”Clay and silts lower recoveriesDesign/materials more complexInstallation more complex-undisturbed

Tension Lysimeter

Tension Lysimeter

Tension Lysimeter

Advantages: Able to sample tensiometeric rangeGood for sands, silts and claysMatches SWP in device with existing conditions

Could be automatedShould produce best estimate of flux

Limitations:Complex designSophisticated instrumentHigh cost of operationFew papers

Comparison of InstrumentsFlux representiveness

Pan/Gravity Lysimeter PCAPS Tension LysimeterOverall

Low Medium-High High

For Low water potential(dry) Low Low High

High water potential (wet)Best Good Good

Chemical representiveness of sampleHighest Good Fair

Comparison of InstrumentsPan/Gravity Lysimeter PCAPS Tension LysimeterPrimary pore size sampled

Macro pore Macro pore Macro and micro

Water Pressure range of operation 0-unk (Wet range only) 0 to about –80 cm 0 to -700cm

(up to –150 cm)

Vary tension range for sampling? (flexibility)No Limited 0 to -80 cm Yes, 0 to –700 cm

Best to SampleSand Sand Clay-course sand

Comparison of Instruments

Cost to install and construct toolMedium-high Medium-high High

Complexity of design

Low High Very high Ease of operation

Easy Medium Difficult Mode of operation

Passive Passive Active

Cost of operationLow Low High

Pan/Gravity Lysimeter PCAPS Tension Lysimeter

Conclusions

Pan LysimeterLeast accuracy/complexity

PCAPSMore accurate/complex

Tension LysimeterBest/most complex

Problem is standard for comparisonAlso impedance match

Spatial variability in recharge Preferential flow-Finger, funnel flow, fast flow paths

Caution

Although the technique (flux meters) has worked well for those researchers that have perfected it subtleties,

it does not appear to be pragmatic on a large-area basis and should be used only for very localized

and specialized studies. (Wagenet, MOSA 1986)