Topographic Effects on the Distribution of Surface Soil Water and the Location of Ephemeral Gullies

ABSTRACT THE relationship between topographic attributes and the distribution of surface soil water content in a small bare-fallow cultivated catchment was examined. The most statistically significant regression equations describing this relationship contained the independent topographic variable aspect and the compound variable ln (A, ), where A, =Ab/S and A^ is the local upslope contributing area per unit width of contour line and S is the slope. The index ln (A, ), a measure of soil saturation, and the product of A^ and S (A^. S), a measure of the erosive power of concentrated surface runoff, were both capable of predicting the location of ephemeral gullies in certain parts of the catchment. However, combining them produced even better predictions throughout the catchment. Ephemeral gully locations were predicted in areas where the topographic indice ln (Ab/S) was greater than 6. 8 and A^. S was greater than 18. These results demonstrate that topographic nonuniformity within small catchments is a major factor controlling the spatial variability of soil water and the location and development of ephemeral gullies.