Assuming shallow ponding depth and high hydraulic conductivity of a drainage media laid under-neath the amended soil layer, the seepage rate (or percolation rate, PC) becomes the same as the vertical hydraulic conductivity of the amended soil:
(10.5)
where SP is the seepage rate (or percolation rate, PC) of the soil water, 
is the coefficient introduced to consider anisotropy of the soil and other uncounted factors controlling hydraulic conductivity, and K is the hydraulic conductivity of the amended soil layer (mm/hour). The value of may be determined by a model calibration. When the soil is not saturated, a hydraulic conductivity is estimated using the van Genuchten equation (van Genuchten, 1980) based on soil water con-tent, field capacity, and wilting point, see equation (10.3) and (10.4).
