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 h ydraulic conductivity functions. Using outflow data only in the object ive function, and applying the optimization algorithm of Marquardt's m aximum neighborhood, the multistep outflow method results in unique es timates 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 s oil hydraulic functions, four parameters (alpha, n, K(s) and lambda) w ere estimated for a medium-textured soil. Fixation of lambda negativel y affects the estimation of K(h). The reference curve as derived by si milar media scaling corresponded with curves derived by the direct ave raging of soil hydraulic functions, and with those obtained by optimiz ation of the mean outflow for a set of 20 soil samples.