Infiltration during soil sealing: The effect of areal heterogeneity of soil hydraulic properties

The combined effects of soil surface sealing and areal heterogeneity of the soil hydraulic properties on the mean infiltration curve are studied here for the first time. Seal formation during rainfall is simulated according to the dynamic model of Assouline and Mualem [1997]. Areal heterogeneity is represented by lognormal distribution of the saturated hydraulic conductivity of the initially undisturbed soil and by related distributions of the other soil parameters. The mean infiltration curve of the heterogeneous field is evaluated presuming that horizontal fluxes are negligible. It is found that when soil surface sealing is taken into account in a heterogeneous field, the ponding time is only slightly affected. However, significantly more runoff is produced compared to nonsealing field (mulched), with the relative area contributing to surface runoff being increased from 65 to 95%. The relative effect of field heterogeneity on the infiltration curve increases for higher rainfall intensity. The formation of soil surface seal apparently reduces the effect of the field areal variability on the steady infiltration rate. The infiltration rate after 60 min of rainfall distributes lognormally under sealing conditions, while it distributes normally in the case of the unsealed soil. The final infiltration rate resulting from the assumption of a sealed uniform field is underestimated compared to sealed heterogeneous field, with the relative effect being larger for a loam soil (-51%) than for a sandy loam soil (-26%). Also, the approximation that the infiltration curve is a unique function of cumulative rainfall independent of rainfall intensity is not valid in a sealing heterogeneous field, and its application in such a case would be more erroneous than in uniform fields. On the other hand, one may disregard the variability of all soil parameters, except the hydraulic conductivity, when calculating infiltration.