Soil Water. Soil Water. Soils and Water, Spring Lecture 3, Soil Water 1

Transcription

1 Soil Water Importance. Global distribution. Properties of water. Amount and energy of water in soil. Hydraulic properties (hydraulic conductivity and water retention). Soil Water Objectives Gain an appreciation of the importance of soils as mediators of matter (gas and liquid) and energy (temperature) exchanges at the surface of the earth. Distinguish between water quantity and water energy. Learn the principles of water movement. SCAN plotter at Lecture 3, Soil Water 1

2 Soil Climate Limitations to Agriculture Climate driven Soil driven Soil driven Source: Linking Land Quality, Agricultural Productivity, and Food Security K. Wiebe. Resource Economics Division, Economic Research Service, U.S. Department of Agriculture. Agricultural Economic Report No. 83. Irrigation Irrigated land account for only 0% of cropland but it produces 40% of the global harvest. However, between 0% to 30% of the irrigated land is affected by salinization caused by improper management. The other problem is water availability. Population and food consumption continues to grow, while water availability for agriculture is expected to decrease from 70% to 6% by 00. Water in Soils Natural Resource Conservation Service, U.S.D.A. Lecture 3, Soil Water

3 Distribution of Wetlands 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 Lecture 3, Soil Water 3

4 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 Expressing the Amount of Water in Soil Volume percentages of soil components Air Water Mineral Organic Gravimetric water content: mass of water/mass of dry soil [g of water/g of soil] Volumetric water content: volume of water/volume of soil [m 3 of water/m 3 of soil] Molecular Structure of Water Source: Lecture 3, Soil Water 4

5 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: Summary of Concepts Water is retained in soil in small crevices: cohesion Dry Wet adhesion Sat. The smaller the pores the greater the forces retaining the water in place. Lecture 3, Soil Water 5

6 0.1 mm A and B are pores filled with water, C is an empty pore. Courtesy of Prof. Haim Gvirtzman, Inst. of Earth Sciences, The Hebrew University of Jerusalem 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 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. Lecture 3, Soil Water 6

7 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. Lecture 3, Soil Water 7

8 A Look 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? N N m = 3 m m J W = 3 m volume Expressing Potential Energy of Water in Soils Type of Expression Units W vol W Weight J N = = 3 m m W J = mass kg = J = kg ms m kg ms kg ms Pressure (kpa) No special unit Length ( head ) Three dimensional view of pores in sand W D Lecture 3, Soil Water 8

9 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 Darcy s Experiments h 1 ΔH = H 1-H A H 1 Sand h z 1 L H Q/A = q z Reference Level or Datum (z = 0) H H L q 1 H H1 q = K S L Lecture 3, Soil Water 9

10 Justification of Darcy s Law 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 (L 3 /T) and A is area (L ). Since W = F L ql H H or d 1 H H q 1 L H H q K = 1 s L Examples of Darcy s Law D = R D = R 1 H WATER SOIL L h 1 Q SOIL L h Ref. Level WATER Q H h z H h z H 1 (inlet) h L H (outlet) 0 0 H H 1 -h -L H (inlet) H 1 (outlet) H 1 H h 0 -h 0 L L 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. Lecture 3, Soil Water 10

11 Conceptualization of Soil Pore Systems CT representation of pores in a sandy material Soil pores represented as capillary tubes What is soil hydraulic conductivity? The ease of water flow through soil A r 4 π r ρ g Δ P Q = 8 μ L Q (Poiseuille s Law) Q 4 A r r ρg Ks = 8 μ Saturated hydraulic conductivity is proportional to the square of pore radius Adhesion and Cohesion in Soil Pores Large Pore Small Pore 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. Lecture 3, Soil Water 11

12 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, cm. Sandy Soil Clay Soil >> Q Q 1 Lecture 3, Soil Water 1

13 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 Lecture 3, Soil Water 13

14 Saturated vs. Unsaturated Water Flow Saturated Flow water air Unsaturated Flow Capillary Action Large 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. Lecture 3, Soil Water 14

15 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 Lecture 3, Soil Water 15

16 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. Lecture 3, Soil Water 16

17 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 Lecture 3, Soil Water 17

18 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 Lecture 3, Soil Water 18

19 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. Lecture 3, Soil Water 19

20 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: Lab 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 Lecture 3, Soil Water 0

21 Water Movement in Soils log (x) = 0.61 log (t) R = log (x/cm) Sandy Loam Silt Loam log (x) = 0.4 log (t) R = log (t/s) Sandy Loam x = 0.6 t 0.61 v=dx/dt=0.16 t Silty Loam 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 Loam: 1 Silt Loam: 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 Loam Silt Loam Extrap. Sandy Loam Extrap. Silt Loam From Wetting Front to Infiltration Rates q = velocity x porosity If porosity = 40% q, cm/s Sandy Loam Silt Loam Extrap. Sandy Loam Extrap. Silt Loam time, s q, in/h Sandy Loam Silt Loam Extrap. Sandy Loam Extrap. Silt Loam time, h Lecture 3, Soil Water 1

22 Infiltration and Runoff q, in/h times the Design Storm X X time, h Ponding Time Runoff in Silt Loam Runoff in Sandy Loam Sandy Loam Silt Loam Extrap. Sandy Loam Extrap. Silt Loam Rainfall Rainfall x in/h Design Storm for Stormwater BMP s Lecture 3, Soil Water