Estimating hydraulic conductivity of crusted soils using disc infiltrometers and minitensiometers

Although soil crusting has long been recognized as a crucial runoff factor in the Sahel, very few field methods have been developed for the measurement of the crust hydraulic conductivity, which is difficult to achieve because of the small thickness of most surface crusts. A field method, based on the simultaneous use of disc infiltrometers and minitensiometers is proposed for determining the crust hydraulic conductivity and sorptivity near saturation. On crusted soils, the classical analysis of the steady state water flow was found to be inadequate. The proposed method is based on sorptivity measurements performed at different water supply potentials and uses recent developments of transient flow analysis. A minitensiometer, placed horizontally at the crust-subsoil interface, facilitated the analysis of the infiltration regime for the crust solely. Results are shown for representative soil units of the East Central Super Site of the HAPEX-Sahel experiment: fallow grasslands, millet fields and tiger bush. Non-crusted soils were also considered and validated the transient method as demonstrated by comparison with Wooding's steady state solution. This validation was obtained in the case of fallow grasslands soil but not for the millet fields. In this latter case, the persistent effects of localized working of the soil to remove weeds caused large variations in infiltration fluxes between the sampling points, which tended to dominate over effects of differences in applied potential. For the tiger bush crusted soils, the ratio of the saturated hydraulic conductivity of the crust to that of the underlying soil ranges from 1 3 to 1 6 , depending on whether the crust is of a structural (ST) or sedimentation (SED) type. The method also allows the estimation of a functional mean pore size, consistent with laboratory measurements, and 40% less for the crusts in comparison with the underlying soil. The results obtained here will be used in hydrological models to predict the partition of rainfall between infiltration and runoff.