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Soil Water Balance

Soil Water Balance

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Soil Water Balance

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  1. Soil Water Balance • Reading: Applied Hydrology Sections 4.3 and 4.4 • Topics • Soil water properties • Soil water flux • Soil water balance

  2. Subsurface water • Infiltration • Soil moisture • Subsurface flow • Groundwater flow

  3. Porous Medium Flow • Groundwater • All waters found beneath the ground surface • Occupies pores (void space not occupied by solid matter) • Porous media • Numerous pores of small size • Pores contain fluids (e.g., water and air) • Pores act as conduits for flow of fluids • The storage and flow through porous media is affected by • Type of rocks in a formation • Number, size, and arrangement of pores • Pores are generally irregular in shape because of • differences in the minerals making up the rocks • geologic processes experienced by them.

  4. Zones of Saturation • Unsaturated zone • Zone between the land surface and water table • Pore contains water and air • Also called as vadose zone or the zone of aeration • Saturated zone • pores are completely filled with water • Contains water at greater than atmospheric pressure • Also called phreatic zone • Water table • Surface where the pore water pressure is atmospheric • Divide between saturated and unsaturated zone • Capillary fringe • Zone immediately above the water table that gets saturated by capillary forces

  5. Soil Water Three categories • Hygroscopic water • Microscopic film of water surrounding soil particles • Strong molecular attraction; water cannot be removed by natural forces • Adhesive forces (>31 bars and upto 10,000 bars!) • Capillary water • Water held by cohesive forces between films of hygroscopic water • Can be removed by air drying or plant absorption • Plants extract capillary water until the soil capillary force is equal to the extractive force • Wilting point: soil capillary force > plant extractive force • Gravity water • Water that moves through the soil by the force of gravity • Field capacity • Amount of water held in the soil after excess water has drained is called the field capacity of the soil.

  6. Soil Sieves http://www.rtg.wa.edu.au/loanpool/belmont/sieves.jpg

  7. Table 1. Size limits (diameter in millimeters) of soil separates in the USDA soil textural classification system. Soil Particle Sizes(USDA Soil Classification System 1 mm 0.1 mm 0.01 mm . 0.001 mm

  8. http://www.uga.edu/srel/kidsdoscience/soils-planets/soil-particle-size.pdfhttp://www.uga.edu/srel/kidsdoscience/soils-planets/soil-particle-size.pdf

  9. Soil Texture Triangle Source: USDA Soil Survey Manual Chapter 3

  10. Soil Water Content Soil Water Content

  11. Soil Water Flux, q q = Q/A

  12. Energy fluxes at Freeman Ranch

  13. Evaporation (mm/day) November Average = 1.1 mm/day August Average = 3.1 mm/day

  14. Daily Rainfall Aug 6 – Aug 13 30 min Rainfall August 6, 2004

  15. Soil Water Content Rainfall mm/30min

  16. Soil Water Balance at Freeman Ranch E P 0 cm Net Exchange 10 cm 20 cm

  17. Soil Water Tension, y • Measures the suction head of the soil water • Like p/g in fluid mechanics but its always a suction (negative head) • Three key variables in soil water movement • Flux, q • Water content, q • Tension, y Total energy head = h z=0 z1 q12 z2

  18. Darcy’s Law • K = hydraulic conductivity • q = specific discharge • V = q/n = average velocity through the area

  19. Definitions Element of soil, V (Saturated) Pore with water solid Pore with air Element of soil, V (Unsaturated)

  20. Continuity Equation z dy dx dz y x

  21. Continuity (Cont.) Continuity Equation

  22. Surface Tension • Below surface, forces act equally in all directions • At surface, some forces are missing, pulls molecules down and exert tension on the surface • If interface is curved, higher pressure will exist on concave side • Pressure increase is balanced by surface tension, s • s = 0.073 N/m (@ 20oC) air Interface Net forceinward water No net force

  23. Air R Water Solid Solid Capillary Action • Capillary Pressure • Related to saturation • Suction Head • Total Head

  24. Pressure Distribution in Subsurface Clay Sand Top of capillary fringe Water table z = 0, P=0

  25. Richard’s Equation • Recall • Darcy’s Law • Total head • So Darcy becomes • Continuity becomes Soil water diffusivity