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...

On long hillsides one can examine systematic downslope changes in the ground surface as Horton overland flow gathers into depressions and eventually incises the surface to form channels. Microtopography plays an important role in this process. We have sampled the microtopography of two long hillslopes in a savanna region of southern Kenya, and defined the spectral characteristics of its roughness at various distances from the drainage divide. The microtopography is fractal with a dimension that decreases systematically downslope, and the overall roughness varies between and along hillslopes in response to: (i) the weathering characteristics of the underlying bedrock; (ii) the type and density of patchy vegetation, and (iii) the tendency for wash to incise the surface with increasing intensity as runoff discharge increases downslope. The downslope decrease in fractal dimension reflects the progressive development of low-frequency roughness, here referred to as "swaley" microtopography, which the wash develops even far upslope of the channel head. The statistical analysis of surface roughness motivates a discussion of the role of microtopography in the interaction between wash and diffusive sediment transport processes that ultimately determines the critical distance from the divide at which channels begin. t Le e CD 0.02 5

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