Water in Soils - Rutgers Universitygimenez/SoilsandWater07/... · Soils and Water, Spring 2007...

Soils and Water, Spring 2007

Lecture 4, Soil Water 1

Water in Soils

Natural Resource Conservation Service, U.S.D.A.Natural Resource Conservation Service, U.S.D.A.

Too Much Water in the Soil

Use of Histosol for fuel (Scotland)

Subsidence in a drained Histosol(Everglades, Florida)

http://soils.ag.uidaho.edu/soilorders/histosols.htm

Not Enough Water in the Soil



The Dust Belt

Source: J. M. Prospero, J.M., P. Ginoux, O. Torres, S. E. Nicholson, and T. E. Gill. 2002. Environmental characterization of global sources of atmosphericsoil dust identified with the nimbus 7 total ozone mapping spectrometer (TOMS) absorbing aerosol product, Rev. Geophys., 40(1), 1002, doi:10.1029/2000RG000095.

Global distributions of dust and smoke: monthly frequency of occurrence of TOMS absorbing aerosol product over the period 1980 –1992. (a) January and (b) July. Scale is number of days per month when the absorbing aerosol index (AAI) equals or exceeds 0.7. In July the large dark area in southern Africa is due to biomass burning. In January, there is biomass burning in the region just north of the equator in Africa; part of the plume over the equatorial ocean is due to smoke. Essentially, all other distributions in Figure 1 are due to dust.

The Concept of Energy• Work, W, is the product of a force, F, acting over

a distance, s:

• Work done on fluids can either increase or decrease their Energy.

• There are different types of energy. According to the principle of conservation of energy when energy is transformed in exactly equivalent amounts.

]J[]Nm[FsW ==

Bernoulli’s Equation

• In practice, losses of energy occur as a result of non-negligible viscosity and a term accounting for the losses caused by friction or “drag” should be considered.

22

12 ]v

21zgP[]v

21zgP[ ρρρρ ++=++

gravitational kineticpressure

DATUM

A1

A2

z1 z2

v1

v1

P1

P2



Energy in Water

Figure 5.8 Whether concerning matric potential, osmotic potential, or gravitational potential (as shown here), water always moves to where its energy state will be lower. In this case the energy lost by the water is used to turn the historic Mabry Mills waterwheel and grind flour. (Photo courtesy of R. Weil)

Figure 5.7 Relationship between the potential energy of pure water at a standard reference state (pressure, temperature, and elevation) and that of soil water. If the soil water contains salts and other solutes, the mutual attraction between water molecules and these chemicals reduces the potential energy of the water, the degree of the reduction being termed osmotic potential. Similarly, the mutual attraction between soil solids (soil matrix) and soil water molecules also reduces the water’s potential energy. In this case the reduction is called matricpotential. Since both of these interactions reduce the water’s potential energy level compared to that of pure water, the changes in energy level (osmotic potential and matric potential) are both considered to be negative. In contrast, differences in energy due to gravity (gravitational potential) are always positive. This is because the reference elevation of the pure water is purposely designated at a site in the soil profile below that of the soil water. A plant root attempting to remove water from a moist soil would have to overcome all three forces simultaneously.



A Look at the Units• Each term in the following equation is expressed

in units of pressure, kPa:

• What is the relation between the definition of work, W, and the energy terms in the Bernoulli’s equation?– Pressure is W/volume

• Energy can also be expressed as:– W/mass– W/weight

2

21 vzgP ρρ ++

Energy in Soils

• Water moves slowly in soils. Thus, and:

• Where P is “pressure” or “matric” potential and ρgz is the “gravitational” potential.

][][ kPaPotentialTotalzgP =+ ρ

0~v21 2ρ

A1

A2

z1 z2DATUM

P1

P2

50 ml

Water Potentials



Darcy’s Experiments

LHHq 12 −−∝ L

HHKq S12 −−=

Lz1

h1

h2

z2

ΔH = H1-H2

H1

H2

Reference Level or Datum (z = 0)

Sand

Q/A = q

A

Justification of Darcy’s Law

• Energy loss is caused by frictional force, Fd, and is measured as: .

• Water movement in soil is slow (laminar). Thus,

Note: q=Q/A, where Q is flux (L3/T) and A is area (L2).

• Since

12 HH −

qFvF dd ∝⇒∝

LHHKq

LHHq

orHHqLLFW

12s

12

12d

−−=⇒

−∝

−∝∝=

What is soil hydraulic conductivity?

The ease of water flow through soilThe ease of water flow through soil

μρ

=8

grK2

s

Saturated hydraulic conductivity is proportional to the square of pore radius

LPgrQ

μρπ

8

4 Δ=

rr

QQ4Q

2r

(Poiseuille’sLaw)

A A



Molecular Structure of Water

Source: http://www.physicalgeography.net/fundamentals/8a.html

Figure 5.3 Everyday evidences of water’s surface tension (left) as insects walk on water and do not sink, and of forces of cohesion and adhesion (right) as a drop of water is held between the fingers. (Photos courtesy of R. Weil and E. Tsang)

Surface Tension (Cohesion)

Water Arrangement Around a Charged Surface (Adhesion)

Source: http://www.physicalgeography.net/fundamentals/8a.html



Adhesion and Cohesion in Soil Pores

• Large pores “drain” faster and retain less water than small pores.

• Large pores are predominant in sandy soils, while small pores are typical of clay soils.

Large Pore Small Pore

Water Movement in Capillary Pores

Small Pore Large Pore

Figure 5.13 Saturated flow (percolation) in a column of soil with cross-sectional area A, cm2. All soil pores are filled with water. At lower right, water is shown running off into a container to indicate that water is actually moving down the column. The force driving the water through the soil is the water potential gradient, y1 2 y2/L, where both water potentials and length are expressed in cm (see Table 5.1). If we measure the quantity of water flowing out Q/t as cm3/s we can rearrange Darcy’s law (eq. 5.5) to calculate the saturated hydraulic conductivity of the soil Ksat in cm/s as:Remember that the same principles apply where the water potential gradient moves the water in a horizontal direction.



Clay Soil

Q1Q2

Sandy Soil

>>

Darcy’s Experiments

LHHq 12 −−∝ L

HHKq S12 −−=

Lz1

h1

h2

z2

ΔH = H1-H2

H1

H2

Reference Level or Datum (z = 0)

Sand

Q/A = q

A

Examples of Darcy’s Law

-L-hH2 – H1

00H2 (outlet)

LhH1 (inlet)

zhH

H1 – H2

H1 (outlet)

H2 (inlet)

H

-Lh

L0

0h

zh

Q

D = 2 R

SOIL L

h

WATER

1

2

SOIL

D = 2 R

WATER

Q

HL

h1

2Datum Datum



This diagram shows a general relationship between soil permeability and soil texture. In a profile, the texture of the least permeable horizon must be considered. Remember, however, that soil structure is also very important in determining permeability.

Texture and Soil Permeability

Unsaturated Soil

• The capillary model• The soil water retention curve

– Field capacity– Wilting point– Hygroscopic water

• Unsaturated hydraulic conductivity– Infiltration



Saturated vs. Unsaturated Water Flow

Saturated Flow Unsaturated Flowwaterair

Capillary Action

Dry glass beads glass beads + water

Large pore

Small pore

Water removal: stage 1 Water removal: stage 2Pipette

Adapted from: www.wtamu.edu/~crobinson/SoilWater/capact.html#1

Figure 5.4 The interaction of water with a hydrophillic surface (a) results in a characteristic contact angle (a). If the solid surface surrounds the water as in a tube, a curved water–air interface termed the meniscus forms because of adhesive and cohesive forces. When air and water meet in a curved meniscus, pressure on the convex side of the curve is lower than on the concave side. (b) Capillary rise occurs in a fine hydrophillic (e.g., glass) tube because pressure under the meniscus (P2) is less than pressure on the free water.



Hydrophobic Surfaces

Analysis of Capillary Rise

Final ExpressionFinal Expression Simplified [h and r are in cm]

Gravity (downward) Adhesion + Cohesion (upward)

Analysis of Forces

Surface Tension (γ), Contact Angle (α), Density (ρ)

Radius (r), Material (Glass)Properties of Interest

Fluid (Water)Capillary Tube

αγπ cosr2ghrgVgm 2πρρ ==

grh ραγ cos2=r15.0h =

The Capillary Model in Soils

• The pressure or matric potential is determined by the shape of the water-air interface:

Dry

Wet

Sat.

Wet

Saturated

Dry



Dry soil: Water retained in small pores, high energy required to remove water from these pores. Water moves extremely slowly.

Wet soil: balanced content of air and water in pores. There is enough O2and water movement is rapid enough to sustain most aerobic processes.

Saturated soil: all pores filled with water. Highest flow rate of water, but typically O2 is absent. Thus, anaerobic processes dominate the system.

Capillary Effect in Field Soils

Mostly saturated soil

Saturated soil

hi

hihi

Figure 5.9 The matric potential and submergence potential are both pressure potentials that may contribute to total water potential. The matric potential is always negative and the submergence potential is positive. When water is in unsaturated soil above the water table (top of the saturated zone) it is subject to the influence of matric potentials. Water below the water table in saturated soil is subject to submergence potentials. In the example shown here, the matric potential decreases linearly with elevation above the water table, signifying that water rising by capillary attraction up from the water table is the only source of water in this profile. Rainfall or irrigation (see dotted line) would alter or curve the straight line, but would not change the fundamental relationship described.



Figure 5.17 The wetting front 24 hours after a 5 cm rainfall. Water removal by plant roots had dried the upper 70 to 80 cm of this humid-region (Alabama) profile during a previous 3-week dry spell. The clearly visible boundary results from the rather abrupt change in soil water content at the wetting front between the dry, lighter-colored soil and the soil darkened by the percolating water. The wavy nature of the wetting front in this natural field soil is evidence of the heterogeneity of pore sizes. Scale in 10 cm intervals. (Photo courtesy of R. Weil)

Figure 5.6 As this field irrigation scene in Arizona shows (left), water has moved up by capillarity from the irrigation furrow toward the top of the ridge. The photo on the right illustrates some horizontal movement to both sides and away from the irrigation water.

The Soil Water Retention Curveh

θv



0.11

500

0.06

1000

0.040.120.150.200.300.450.5θv

150020010033.39.85.92.9P (kPa)

Hanging Water Column Pressure Plate Extractor

Ceramic Plate

Saturated Soil Sample

Water RetentionWater Retention MeasurementMeasurement

h

Figure 5.24 Water content–matric potential curve of a loam soil as related to different terms used to describe water in soils. The wavy lines in the diagram to the right suggest that measurements such as field capacity are only approximations. The gradual change in potential with soil moisture change discourages the concept of different “forms” of water in soils. At the same time, such terms as gravitational and available assist in the qualitative description of moisture utilization in soils.

Animations: software



Figure 5.23 Volumes of water and air associated with a 100 g slice of soil solids in a well-granulated silt loam at different moisture levels. The top bar shows the situation when a representative soil is completely saturated with water. This situation will usually occur for short periods of time during a rain or when the soil is being irrigated. Water will soon drain out of the larger pores (macropores). The soil is then said to be at the field capacity. Plants will remove water from the soil quite rapidly until they begin to wilt. When permanent wilting of the plants occurs, the soil water content is said to be at the wilting coefficient. There is still considerable water in the soil, but it is held too tightly to permit its absorption by plant roots. A further reduction in water content to the hygroscopic coefficient is illustrated in the bottom bar. At this point the water is held very tightly, mostly by the soil colloids. (Top drawings modified from Irrigation on Western Farms, published by the U.S. Departments of Agriculture and Interior)

Figure 5.25 General relationship between soil water characteristics and soil texture. Note that the wilting coefficient increases as the texture becomes finer. The field capacity increases until we reach the silt loams, then levels off. Remember these are representative curves; individual soils would probably have values different from those shown.



Figure 5.15 Generalized relationship between matric potential and hydraulic conductivity for a sandy soil and a clay soil (note log scales). Saturation flow takes place at or near zero potential, while much of the unsaturated flow occurs at a potential of 20.1 bar (210 kPa) or below.

Infiltration: Lab ExperimentEarly Stage End of Experiment

Sandy loam Silt loam

Is there a model (mathematical expression) to quantify our observations?

Examples of Motion• When a=1, the motion is easy to

solve: linear function.• When a≠1, the equation is

linearized:

• Which case represents water movement in soils?

atx ∝

Dis

tanc

e, x

Time, t

a=1

a<1

a>1

)tlog(a)xlog( ∝



Water Movement in Soils

x = 0.30 t 0.42x = 0.26 t 0.61

v=dx/dt=0.13 t -0.58v=dx/dt=0.16 t -0.39

Silty LoamSandy Loam

log (x) = 0.61 log (t) - 0.59R2 = 0.9971

log (x) = 0.42 log (t) - 0.52R2 = 0.9845

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 1 2 3 4

log (t/s)

log

(x/c

m)

Sandy LoamSilt Loam

Two ways to Represent the Data

0

5

10

15

20

25

30

0 1000 2000 3000 4000

time, s

dist

ance

, cm Sandy Loam: 1

Silt Loam: 2Predicted 1Predicted 2

0.000

0.005

0.010

0.015

0.020

0.025

0.030

0.035

0.040

0.045

0 1000 2000 3000 4000

time, s

velo

city

of w

ettin

g fr

ont,

cm/s

Sandy LoamSilt LoamExtrap. Sandy LoamExtrap. Silt Loam

dxdy

xy⇒

ΔΔ

risky: no data

x = 0.26 t 0.61

v=dx/dt = 0.16 t -0.39

d[AxB]/dy= AB x B-1

From Wetting Front to Infiltration Rates

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

0.016

0.018

0 1000 2000 3000 4000

time, s

q, c

m/s


q = velocity x porosity

0.000

5.000

10.000

15.000

20.000

25.000

0 0.2 0.4 0.6 0.8 1 1.2

time, h

q, in

/h


If porosity = 40%



0.000

5.000

10.000

15.000

20.000

25.000

0 0.2 0.4 0.6 0.8 1 1.2

time, h

q, in

/h

Sandy LoamSilt LoamExtrap. Sandy LoamExtrap. Silt LoamRainfallRainfall x 4

Runoff in Sandy Loam

Infiltration and Runoff

1.25 in/2h Design Storm for Stormwater BMP’s

8 times the Design Storm

XX

Ponding Time

Runoff in Silt Loam

Unsaturated Flow is Equal Upward, Downward, or Sideways

Permeability over a finer texture or a more Dense Layer of the same texture



Thin band of Lowered Permeability begins to cause Lateral Flow

Finally some water is held at a tension low enough to equal larger pores below.

Figure 5.20 One result of soil layers with contrasting texture. This North Carolina soil has about 50 cm of loamy sand coastal plain material atop deeper layers of silty clay-loam-textured material derived from the piedmont. Rainwater rapidly infiltrates the sandy surface horizons, but its downward movement is arrested at the finer-textured layer, resulting in saturated conditions near the surface and a quick-sandlike behavior. (Photo courtesy of R. Weil)



Clay Layer with Slow Permeability over Sand Layer with Higher Permeability (Note: Tension control the

rate)

Finally some water is held at a tension low enough to equal larger pores below. (Note: rate is controlled by permeability of

the surface layer)

Water Makes An End Run



Only Pores Connected to Surface Increase Flow Rate

Pores Not Connected May Actually Slow Downward Movement



Flow is lateral and saturated over Layer with Slow Permeability. (Note: Permeability controls the

downward rate)

Examples: Mass/Vol. Relationships• A crop root zone is 0.4 m deep and has a bulk density value of 1.2

Mg.m-3. The mineralogical composition of the soil is mainly quartz and clay. On mid August the average water content in the root zone is 0.2 kg kg-1. Expressing your results on a 1 m2 area, calculate the:

– Volume of water (m3) contained in the root zone, – Volume of air (m3) contained in the root zone, and – Total surface area (m2) in the root zone if the value of the soil specific

surface area is 50 m2 kg-1.

• Calculate the porosity and the bulk density of a soil sample whose dry weight is 100 g and that at saturation holds 40 cm3 of water. The mineralogical composition of the soil sample is dominated byquartz and clay.

Example Darcy’s Law• The diagram below shows a ditch intercepting seepage under a

roadbed. The saturated hydraulic conductivity of the pervious material is 0.4 m day-1. Assuming that the length of the ditch is 400 m, calculate the volume of water seeping into the ditch during one day.

Diagram 1. Ditch intercepting water flowing underneath a road.

Water in Soils - Rutgers Universitygimenez/SoilsandWater07/... · Soils and Water, Spring 2007...

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Transcript of Water in Soils - Rutgers Universitygimenez/SoilsandWater07/... · Soils and Water, Spring 2007...