Hydrologic impacts of sheep grazing on steep slopes semiarid rangelands

Abstract

Infiltration, sediment concentration of runoff, and sediment production from lightly grazed and ungrazed semiarid slopes were compared using a hand-portable rainfall simulator. The study slope was located in the Guadalupe Mountains of southeastern New Mexico. Average slope steepness was 50%. The objective of this study was to determine the impacts of light grazing by sheep (10 ha/ AU) on steep slope infiltrability and sediment production. Infiltrability on the grazed slopes was 12-17% lower than on the ungrazed slopes. These results are comparable to what has been reported from moderate slope gradients. Sediment concentration of runoff from the lightly grazed slopes was significantly higher than from the ungrazed slopes only at the end of the dry run (45 min). Sediment production was significantly greater from the grazed slopes for the dry run, but not the wet run. Percentage difference of sediment production between the grazed and ungrazed slopes was well within the range published for moderate slope conditions. These data give no indication that steep slopes (30-70%) in semiarid regions are any more hydrologically sensitive to light grazing than are moderate slopes (<10%).