Water in Soils. Too Much Water in the Soil. Not Enough Water in the Soil. Soils and Water, Spring Lecture 4, Soil Water 1

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1 Water in Soils 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) Not Enough Water in the Soil ecture 4, Soil Water 1

2 The Dust Belt Global distributions of dust and smoke: monthly frequency of occurrence of TOMS absorbing aerosol product over the period (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. Source: J. M. Prospero, J.M., P. Ginoux, O. Torres, S. E. Nicholson, and T. E. Gill. 00. Environmental characterization of global sources of atmospheric soil dust identified with the nimbus 7 total ozone mapping spectrometer (TOMS) absorbing aerosol product, Rev. Geophys., 40(1), 100, doi:10.109/000rg The Concept of Energy Work, W, is the product of a force, F, acting over a distance, s: W = Fs [ Nm ] = [ J ] 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. Bernoulli s Equation A v 1 A 1 P v 1 DATUM P 1 z 1 z 1 1 [ P + ρ g z + ρ v ] 1 = [ P + ρ g z + ρ v ] pressure gravitational kinetic In practice, losses of energy occur as a result of nonnegligible viscosity and a term accounting for the losses caused by friction or drag should be considered. ecture 4, Soil Water

3 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 matric potential. 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. ecture 4, Soil Water 3

4 A ook at the Units Each term in the following equation is expressed in units of pressure, kpa: 1 P + ρ g z + ρ v 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 Energy in Soils A A 1 P P 1 DATUM z 1 z 1 Water moves slowly in soils. Thus, ρ v ~ 0 and: [ P + ρ g z] = Total Potential [ kpa] Where P is pressure or matric potential and ρgz is the gravitational potential. Water Potentials 50 ml ecture 4, Soil Water 4

5 Darcy s Experiments h 1 ΔH = H 1-H A H 1 Sand h z 1 H Q/A = q z Reference evel or Datum (z = 0) H H q 1 H H1 q = K S Justification of Darcy s aw Energy loss is caused by frictional force, F d, and is measured as: H. H 1 Water movement in soil is slow (laminar). Thus, Fd v Fd q Note: q=q/a, where Q is flux ( 3 /T) and A is area ( ). Since W = F q H H or d 1 H H q 1 H H q K = 1 s What is soil hydraulic conductivity? The ease of water flow through soil A r 4 π r ρ g Δ P Q = 8 μ Q (Poiseuille s aw) Q 4 A r r ρg Ks = 8 μ Saturated hydraulic conductivity is proportional to the square of pore radius ecture 4, Soil Water 5

6 Molecular Structure of Water Source: Surface Tension (Cohesion) 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) Water Arrangement Around a Charged Surface (Adhesion) Source: ecture 4, Soil Water 6

7 Adhesion and Cohesion in Soil Pores arge Pore Small Pore arge pores drain faster and retain less water than small pores. arge pores are predominant in sandy soils, while small pores are typical of clay soils. Water Movement in Capillary Pores Small Pore arge Pore Figure 5.13 Saturated flow (percolation) in a column of soil with cross-sectional area A, cm. 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 y/, 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. ecture 4, Soil Water 7

8 Sandy Soil Clay Soil >> Q Q 1 Darcy s Experiments h 1 ΔH = H 1-H A H 1 Sand h z 1 H Q/A = q z Reference evel or Datum (z = 0) H H q 1 H H1 q = K S Examples of Darcy s aw D = R D = R 1 H WATER SOI h 1 Q SOI h Datum Datum WATER Q H h z H h z H 1 (inlet) h H (outlet) 0 0 H H 1 -h - H (inlet) H 1 (outlet) H 1 H h 0 h 0 - ecture 4, Soil Water 8

9 Texture and Soil Permeability 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. Unsaturated Soil The capillary model The soil water retention curve Field capacity Wilting point Hygroscopic water Unsaturated hydraulic conductivity Infiltration ecture 4, Soil Water 9

10 Saturated vs. Unsaturated Water Flow Saturated Flow water air Unsaturated Flow Capillary Action arge pore Small pore Dry glass beads glass beads + water Pipette Water removal: stage 1 Water removal: stage Adapted from: 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 (P) is less than pressure on the free water. ecture 4, Soil Water 10

11 Hydrophobic Surfaces Analysis of Capillary Rise Capillary Tube Radius (r), Material (Glass) Properties of Interest Fluid (Water) Surface Tension (γ), Contact Angle (α), Density (ρ) Analysis of Forces Adhesion + Cohesion (upward) π r γ cosα Gravity (downward) m g = ρ V g = ρ π r h g Final Expression h = γ cosα ρ gr Final Expression Simplified h = 0.15 r [h and r are in cm] The Capillary Model in Soils Dry Wet The pressure or matric potential is determined by the shape of the water-air interface: Dry Wet Sat. Saturated ecture 4, Soil Water 11

12 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 O and water movement is rapid enough to sustain most aerobic processes. Saturated soil: all pores filled with water. Highest flow rate of water, but typically O is absent. Thus, anaerobic processes dominate the system. Capillary Effect in Field Soils h i Mostly saturated soil Saturated soil h i h i 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. ecture 4, Soil Water 1

13 Figure 5.17 The wetting front 4 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 Curve h θ v ecture 4, Soil Water 13

14 Water Retention Measurement Ceramic Plate h Saturated Soil Sample Hanging Water Column Pressure Plate Extractor P (kpa) θ v Figure 5.4 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 ecture 4, Soil Water 14

15 Figure 5.3 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 5.5 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. ecture 4, Soil Water 15

16 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 0.1 bar (10 kpa) or below. Infiltration: ab Experiment Early Stage End of Experiment Sandy loam Silt loam Is there a model (mathematical expression) to quantify our observations? Examples of Motion Distance, x a x t a>1 a=1 a<1 When a=1, the motion is easy to solve: linear function. When a 1, the equation is linearized: log( x ) alog( t ) Which case represents water movement in soils? Time, t ecture 4, Soil Water 16

17 Water Movement in Soils log (x) = 0.61 log (t) R = log (x/cm) Sandy oam Silt oam log (x) = 0.4 log (t) R = log (t/s) Sandy oam x = 0.6 t 0.61 v=dx/dt=0.16 t Silty oam x = 0.30 t 0.4 v=dx/dt=0.13 t Two ways to Represent the Data distance, cm Δy dy 10 Δx dx time, s x = 0.6 t 0.61 Sandy oam: 1 Silt oam: Predicted 1 Predicted d[ax B ]/dy= AB x B-1 velocity of wetting front, cm/s risky: no data time, s v=dx/dt = 0.16 t Sandy oam Silt oam Extrap. Sandy oam Extrap. Silt oam From Wetting Front to Infiltration Rates q = velocity x porosity If porosity = 40% q, cm/s Sandy oam Silt oam Extrap. Sandy oam Extrap. Silt oam time, s q, in/h Sandy oam Silt oam Extrap. Sandy oam Extrap. Silt oam time, h ecture 4, Soil Water 17

18 Infiltration and Runoff q, in/h times the Design Storm X X time, h Ponding Time Runoff in Silt oam Runoff in Sandy oam Sandy oam Silt oam Extrap. Sandy oam Extrap. Silt oam Rainfall Rainfall x in/h Design Storm for Stormwater BMP s Unsaturated Flow is Equal Upward, Downward, or Sideways Permeability over a finer texture or a more Dense ayer of the same texture ecture 4, Soil Water 18

19 Thin band of owered Permeability begins to cause ateral Flow Finally some water is held at a tension low enough to equal larger pores below. Figure 5.0 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) ecture 4, Soil Water 19

20 Clay ayer with Slow Permeability over Sand ayer 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 ecture 4, Soil Water 0

21 Only Pores Connected to Surface Increase Flow Rate Pores Not Connected May Actually Slow Downward Movement ecture 4, Soil Water 1

22 Flow is lateral and saturated over ayer 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. 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. kg kg - 1. Expressing your results on a 1 m area, calculate the: Volume of water (m 3 ) contained in the root zone, Volume of air (m 3 ) contained in the root zone, and Total surface area (m ) in the root zone if the value of the soil specific surface area is 50 m 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 cm 3 of water. The mineralogical composition of the soil sample is dominated by quartz and clay. Example Darcy s aw 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. ecture 4, Soil Water