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 Assume that the matric potential gradient is negligible, Gravitational gradient drives flow – if z is positive down, then Flux equals the hydraulic conductivity

Estimating drainage rates for deeply wetted profiles Use Campbell’s hydraulic conductivity model Calculate the drainage rate for a loam soil with – K s = 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 ?

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:

Redistribution in partially wetted profiles

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)

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

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

“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 ;  = cm -1 ; n = 1.74 What if the water table were lowered to 337 cm?