Deriving hydrological parameters for modeling water flow under field conditions

The process of deriving the hydrological characteristics of a soil to parameterize a model at the Field scale is not straightforward, especially when various experimental procedures are used to estimate the parameter values. This paper shows the inconsistencies that were encountered when attempting to find the physical and hydrological parameters of two deep loam soils. The parameters were searched for with the aim, and the constraint, of compiling the input file of the LEACHM model, which implements a Campbell's modified K-theta-psi function approach. The soil water content at saturation was not consistent with the total porosity calculated from measured bulk and particle densities. The water retention curve (RC) measured in the laboratory and the conductivity curve measured in the field were described using modified Campbell functions that were separately parameterized, and results were then compared. The optimized values obtained for the theta-psi curve were remarkably different from those for the K-psi curve. Considering the physical nature of a model and the measurability of its parameters as the basic requirements for extending model predictions to sites where validation cannot be provided, possible solutions are to derive all soil properties simultaneously on the same sample and/or with a cofitting procedure, to adjust the measured values of the soil properties to the functional values, in order to combine the measured soil properties with the code requirements, and to modify the code to allow a separate parameterization of the two curves.