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Redistribution

Redistribution. Redistribution. the continued movement of soil water after infiltration ends rate decreases over time influences plant available water influences solute transport. Deeply wetted profiles. Extend Darcy’s Law Buckingham-Darcy Law

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Redistribution

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  1. Redistribution

  2. Redistribution • the continued movement of soil water after infiltration ends • rate decreases over time • influences plant available water • influences solute transport

  3. Deeply wetted profiles • Extend Darcy’s Law • Buckingham-Darcy Law • Assume that the matric potential gradient is negligible, • Gravitational gradient drives flow • if z is positive down, then • Flux equals the hydraulic conductivity

  4. Estimating drainage rates for deeply wetted profiles • Use Campbell’s hydraulic conductivity model • Calculate the drainage rate for a loam soil with • Ks = 310 mm d-1; b = 4.5; s = 0.46; and  = 0.40 • What is the value of  when the drainage rate drops to 1 mm d-1?

  5. Reading assignment • More on redistribution, p. 303 – 313 • Optional • Search Youtube for “Water Movement in Soil” and watch a classic soil physics video or go directly to: http://www.youtube.com/watch?v=jWwtDKT6NAw

  6. Redistribution in partially wetted profiles

  7. Field capacity • the water content at which internal drainage allegedly ceases • often incorrectly considered to be a soil property • arbitrary because redistribution has no “break-points” • sometimes estimated as  when m = -10 kPa or m = -33 kPa (-337 cm)

  8. Field capacity and irrigation management • Available water capacity (AWC) = (fc-wp) x z • Allowable depletion (AD) selected based on management goals: 0.30 x AWC  AD  0.50 AWC • Irrigation applied when the soil water deficit reaches the allowable depletion • Irrigation amount is set equal to or slightly less than the soil water deficit

  9. What is the “field capacity” for this soil? • table 16.1

  10. “Field capacity” estimation • Calculate the equilibrium water content at the surface of a uniform loamy sand assuming no evaporation and a water table at 102 cm • s = 0.39 ; r = 0.05 ;  = 0.035 cm-1; n = 1.74 • What if the water table were lowered to 337 cm?

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