Seasonal variation of infiltration capacity and runoff on hillslopes in western Colorado

Abstract

Hillslope erosion was studied during 4 years along 25 slope profiles on Mancos shale hillslopes in western Colorado. Erosion was measured by the movement of markers and exposure of stakes. During winter the soil surface is loosened by frost action, and the stakes show minimum exposure in the spring. Rain-beat during the spring and summer compacts the soil, and the stakes show maximum exposure during the fall. Frost action and compaction cause creep to occur in the upper 2 inches of the lithosols. During spring and summer compaction of the soil by rain-beat decreases infiltration capacity, and runoff increases. Rills form on the slopes, but these are soon destroyed by winter frost action and creep, and the infiltration capacity of the soil is increased. Measurement of sediment yield and runoff from small drainage basins on the Mancos shale shows that mean annual runoff is relatively low, but sediment yields are normal for this type of terrain. An analysis of precipitation and runoff data reveals that average runoff and the ratio of runoff to precipitation are less in the spring than in the fall, reflecting the seasonal changes of soil characteristics. Therefore, seasonal changes in the soil, which cause a seasonal change in infiltration capacity, not only control the rate and process of hillslope erosion but also significantly affect the hydrologic characeristics of these small drainage basins.