Estimating hydraulic properties of rainfall-induced soil surface seals from infiltration experiments and X-ray bulk density measurements
Résumé
Soil and surface seal hydraulic properties were determined from simulated rainfall experiments by the inverse method applied to the Richards equation. Measurements used for the estimation include the soil water pressure head versus time at two distances from the soil surface, the transient infiltration rate at the soil surface and the drainage rates at the bottom of the soil profile. Seal properties were evaluated using a model that simulates changes in the seal bulk density with respect to time and space. Uncertainties, correlations and sensitivities of the soil and seal parameters were quantified to evaluate the accuracy of the model estimation and to compare the information content of each measurement type to parameter estimations. It appears that the uncertainties related to three seal parameter estimations, namely the parameter related to the dynamics of seal formation, the modelled seal thickness and the initial bulk density, were larger than 50% of the parameter values, because of the low sensitivity of the model to them and their multiple correlations. In addition to seal hydraulic parameter estimation, bulk density profiles of the soil surface were measured after the rainfall simulations using the X-ray method. The exponential-decay shape assumed in the soil surface seal model was found to correctly reproduce the measured distribution of bulk density with depth. However, the measurements showed a less developed seal than that suggested by the bulk density profile estimated from rainfall experiments. Finally, bulk density measurements were used as given input parameters of the model. Setting the initial bulk density and its maximal change over time at the measured values greatly decreased the seal parameter uncertainties. The method proposed could be used to improve the experimental design used to quantify the seal's hydraulic properties using inverse techniques.