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1 <br /> Vadose Zone Model <br /> 1 <br />' this objective, the unit gradient approach has been shown to work reasonably well <br /> (Unla et al , 1992) <br /> A 5,1 Unit Gradient Approach <br /> ' For more information on this subject the reader is referred to the two papers <br /> mentioned above as well as Carsel and Parrish (1988) Briefly, Darcy's equation for <br /> ' the unit gradient case may be written as <br /> q = k,K. (A-25) <br /> ' where <br /> q = infiltration rate or net recharge rate [cm/d) <br /> kr = relative permeability [unitless] <br /> KPu = hydraulic conductivity of the unsaturated zone if it were <br /> fully saturated [crn/d] <br /> • When using RISC, the user is asked to input the infiltration rate, q, and the <br /> unsaturated zone's hydraulic conductivity (Ks„) This term, K„,, is the hydraulic <br /> ' conductivity of the unsaturated zone if it were assumed to be fully saturated, i e no <br /> air-filled porosity in many applications of RISC, K,., is assumed equal to the <br /> hydraulic conductivity of the soil below the water table if the value entered for the <br /> ' infiltration rate exceeds KSY,then q is set to K,s„ assuming that the excess flow will be <br /> diverted as runoff <br /> ' <br /> Having values for q and K,,,, Equation A-25 is used to solve for the relative <br /> ' permeability,kl <br /> k, — Kn, (A-26) <br /> q <br /> where the terms are as defined for Equation A-24 <br /> A-15 <br />