INVERSE METHOD TO DETERMINE SOIL HYDRAULIC FUNCTIONS FROM MULTISTEP OUTFLOW EXPERIMENTS

Outflow experiments, in combination with inverse modeling techniques, offer a time-efficient way to simultaneously determine retention and hydraulic conductivity functions. Using outflow data only in the objective function, and applying the optimization algorithm of Marquardt's maximum neighborhood, the multistep outflow method results in unique estimates of the retention and hydraulic conductivity functions. For the same experimental conditions the one-step outflow method often yields nonunique solutions. For the Mualem-van Genuchten expression of the soil hydraulic functions, four parameters (alpha, n, K(s) and lambda) were estimated for a medium-textured soil. Fixation of lambda negatively affects the estimation of K(h). The reference curve as derived by similar media scaling corresponded with curves derived by the direct averaging of soil hydraulic functions, and with those obtained by optimization of the mean outflow for a set of 20 soil samples.