FACTORS CONTROLLING INFILTRATION RATES IN A SEMI-ARID LANDSCAPE

Agricultural landscapes in semi-arid regions are often prone to gully erosion. Understanding rates and patterns of gully erosion requires documenting the driving hydrology, and in particular quantifying the capacity of soils to generate runoff during summer convective storms. Here we report measurements of soil infiltration capacity at a study site on the Colorado high plains. The site, known as the West Bijou Creek escarpment, lies 40 miles southeast of Denver. Rapid gully network propagation at this site appears to be driven mainly by head cut retreat during flash floods. Using a double-ring infiltrometer, measurements of infiltration rate were collected across a variety of soil and vegetation types to document typical rates and to determine the degree to which factors such as vegetation, slope steepness, soil porosity and permeability influence soil infiltration capacity.

We measured infiltration capacity in 15 different locations and observed infiltration rates ranging from 40 to 400 mm/hr. In general, the hillslopes were less permeable than valley floors. Hillslope infiltration capacity was in the range of 40 to 70 mm/hr. At the study site, it is typical to have rainfall intensities above 60 mm/hr in 10 minutes several times during the summer season. We have previously observed that overland flow is produced from these high rainfall intensities, and the infiltration tests strengthen our observations. Valley floors on the landscape show greater variability in the saturated infiltration capacity, with rates ranging from 70 to 320 mm/hr. Surprisingly, areas that are underlain by sandstone and shale show similar infiltration rates. The data suggest a conceptual model in which most runoff is generated on hillslopes, and the resulting flow routed through valley networks drives rapid erosion, sediment transport, and head cut retreat.