DETERMINATION OF PARAMETERS FOR BIMODAL HYDRAULIC FUNCTIONS BY INVERSE MODELING

To determine the hydraulic properties of soils, transient now experime nts on soil samples and their evaluation by parameter identification m ethods have become common. In the past, the feasibility of this method has been investigated for soils exclusively with unimodal pore-size d istributions. In this work we investigated, both from a theoretical an d from an experimental point of view, whether hydraulic properties of structured soils with bimodal pore-size characteristics can be identif ied by inverse simulation. Multistep outflow experiments were simulate d for three hypothetical soils with different degrees of bimodality. T he outflow data thus created were then used in the inverse procedure, both in the original form and with an added random error, An analysis of sensitivity coefficients fur the model parameters showed that if th e bimodality of the pore system is well developed, the parameters of t he bimodal hydraulic functions are uncorrelated and can be identified. This was confirmed by inverse optimization runs with simultaneous opt imization of up to six parameters, which consistently converged from d ifferent initial values to the true parameter values, indicating uniqu eness of the inverse problem. Attempts to fit the bimodal outflow data with simulations using unimodal hydraulic functions were not successf ul, as expressed by systematic disagreement between fitted and ''obser ved'' data and nonuniqueness of the inverse solution. We validated the se theoretical results with experimental data from an undisturbed sand y forest soil, which showed that it is possible to determine the reten tion parameters of bimodal hydraulic properties by inverse modeling of multistep outflow experiments.