Effects of Rainfall, Vegetation, and Microtopography on Infiltration and Runoff

Abstract

Apparent, or effective, infiltration rates on grassland hillslopes vary with rainfall intensity and flow depth because of the interaction between rainfall, runoff, and vegetated microtopography. The higher parts of the microtopography are occupied by greater densities of macropores and therefore have much greater hydraulic conductivities than the intervening microdepressions. On short hillslopes and plots the apparent infiltration rate is simply the spatial average of the saturated and unsaturated conductivities of this surface. The proportion of the surface which is saturated and the value to which the unsaturated conductivity is raised depends on the rainfall intensity. On longer hillslopes the downslope increase in flow depth in microtopographic depressions progressively inundates more permeable, vegetated mounds so that the hydraulic conductivity of a greater proportion of the surface is raised to its saturated value. For this reason the apparent infiltration rate increases downslope, even in the absence of spatial trends in any of the surface characteristics that affect infiltration. Apparent, or effective, infiltration rate depends on hillslope length. Consequently, steady state discharge does not increase linearly with distance downslope. These two fundamental relationships between infiltration, rainfall intensity, and runoff are analyzed on the basis of sprinkling-infiltrometer measurements and a mathematical model. the higher parts again increases the spatially averaged hydraulic conductivity of the surface. In this case the effect of rainfall intensity on infiltration rate is a result of interactions of flow depth with the hydraulic conductivity of the mineral :wil, microtopographic form, vegetation density, and hillslope gradient and length. The first four of these factors often show systematic trends along hillslope profiles. However, •ve avoid consideration of such trends here to illustrate how i•afiltration varies systematically with rainfall intensity and Copyright 199! by the American Geophysical Union. !•aper number 91WR01585. t!}43.1397/91/91WR-01585505.00distance downslope in the presence of a constant microtopography, cover density, and gradient. In this paper we explore these effects on the basis of field measurements and a mathematical model. REVIEW OF PREVIOUS WORK Effect of Rainfall IntensityCook [1946] noted a dependence of infiltration rate on rainfall intensity and attributed it to more complete ponding of the surface. Moldenhauer et al. [ 1960], examining rainfall and runoff records from plots during natural rainstorms, found the tb index ((total storm rainfall -total storm runoff)/duration of excess precipitation) to be strongly dependent upon rainfall intensity. They reasoned that, in addition to the more complete ponding proposed by Cook, small-scale spatial variations of conductivity would allow increased infiltration as the more permeable zones were inundated by the more abundant runoff generated at higher rainfall intensities. Hawkins [ 1982] reviewed other published interpretat...

Seasonal and spatial variations in infiltration rates in badland surfaces under Mediterranean climatic conditions

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Seasonal variability of infiltration rates under contrasting slope conditions in southeast Spain

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The relative efficiency of four representative cropland conversions in reducing water erosion: evidence from long‐term plots in the Loess hilly area, China

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