ESTIMATING HYDRAULIC CONDUCTIVITY OF CRUSTED SOILS USING DISC INFILTROMETERS AND MINITENSIOMETERS

Although soil crusting has long been recognized as a crucial runoff fa ctor 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 fiel d method, based on the simultaneous use of disc infiltrometers and min itensiometers is proposed for determining the crust hydraulic conducti vity and sorptivity near saturation. On crusted soils, the classical a nalysis of the steady state water how was found to be inadequate. The proposed method is based on sorptivity measurements performed at diffe rent water supply potentials and uses recent developments of transient flow analysis. A minitensiometer, placed horizontally at the crust-su bsoil interface, facilitated the analysis of the infiltration regime f or the crust solely. Results are shown for representative soil units o f the East Central Super Site of the HAPEX-Sahel experiment: fallow gr asslands, millet fields and tiger bush. Non-crusted soils were also co nsidered and validated the transient method as demonstrated by compari son 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 th e 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, th e ratio of the saturated hydraulic conductivity of the crust to that o f 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 comparis on with the underlying soil. The results obtained here will be used in hydrological models to predict the partition of rainfall between infi ltration and runoff.