Evaluation of some methods for measuring transient hydraulic properties of rain-induced surface seals on two clay soils

Abstract

Surface sealing of soils can markedly affect infiltration under rain. Robust infiltration models and methods for measuring hydraulic properties are needed. A review of methods of measuring seal hydraulic properties indicated that methods differ greatly in the hydraulic boundary conditions used and this may affect results and make a systematic analysis difficult. Two methods ('post-rain' suction permeameter and rainfall in situ) of measuring seal flux and conductivity under appropriate hydraulic boundary conditions were tested in a laboratory for two clay soils under simulated rain. The suction permeameter tests showed that conductivity of surface seal samples is altered by the suction applied during the measurement. This occurred in the absence of rain and appears to be more general than previously reported. Transient infiltration and conductivity measured by the two methods were in close agreement from ponding time onwards, provided suction equal to that measured under the seal was applied in the suction permeameter. The suction permeameter can provide conductivity data under a wider range of conditions than the rainfall in situ method, including prior to ponding, but requires measured subseal suction data. The two methods are seen to be complementary. The transient conductivity behaviour of the two soils was markedly different-the well aggregated, swelling clay exhibited an exponential decay, similar to commonly used models; the less aggregated, non-swelling clay did not. Desorption moisture characteristic curves (MCC) and measured subseal moisture content gave useful estimates of subseal suction after commencement of surface ponding, provided: (a) a correction from desorption to adsorption is not made, as desorption occurs in the subseal soil after time of ponding; and (b) MCC are measured using appropriate pre-treatment (rainfall wetting of sample in 10-20 mm depth in a soil bed).