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' RISC User's Manual Version 4 Q <br /> I <br /> 1 A.5.2 Estimating the Unsaturated Zone Hydraulic Conductivity <br /> ' In the unsaturated zone, the hydraulic conductivity vanes as a function of the moisture <br /> content It is assumed that this relationship can be described by the Brooks and Corey <br /> (1964)model <br /> T ly <br /> I <br /> ' where <br /> 81 = total porosity in unsaturated zone [-] <br /> 0. = volumetric water content or water-filled porosity [cm3 <br /> c' water/cm3 soil] <br /> 0, = irreducible water content cm3 water/cm3 soil] <br /> ' y = pore size dtstribution parameter [-] <br /> ' The pore size distribution parameter is estimated from the van Genuchten "n" <br /> parameter using the following relationship(Lenhard et al , 1989) <br /> ' <br /> Y_ 2 (A-28) <br /> �+ n <br /> ' (n-1)(1-0 5�"-'�} <br /> ' The model requires the user to enter n Carsel and Parrish (1988) have a large <br /> database of van Genuchten's "n"for various soil types This database is also available <br /> ' in the RISC software <br /> ' A 5 3 Estimating Moisture Content and Seepage Velocity <br /> The water-filled porosity, BW„ is calculated using Equation A-27 since values of all the <br /> other terms are either specified or previously denved The calculated water-filled <br /> porosity is then used to estimate the seepage velocity, v <br /> 1 <br /> A-16 <br />