Effects of vegetation cover and grazing on rangeland runoff and erosion processes in the Asotin Creek Watershed, WA

Hussin Alshantiri

Nearly half of our planet's land surface is occupied by rangelands, which largely appear in arid to semiarid zones. Rangelands include pasture, scrub, non-crop, and non-forest lands. Since the early nineteenth century, erosion of rangeland soil has been recognized as an agricultural and environmental problem. In this MS thesis, to better assess and predict runoff and erosion rates, the Water Erosion Prediction Project (WEPP, v2010.1) was applied to two representative rangeland sites in the Asotin Creek watershed, Washington State. The goal of the study was to evaluate the effects of various grazing practices on rangeland runoff and erosion from the hillslopes of the Smoothing Iron and Pintler Creek grazing units of the Asotin Wildlife Area. The primary objective was to assess the effect of vegetative cover and grazing (both wildlife and livestock) on rangeland runoff and erosion processes in the study area using the WEPP model. The secondary objective was to evaluate and compare the performance of the WEPP model and a WEPP-based rangeland erosion model, the Rangeland Hydrology and Erosion Model (RHEM), in simulating runoff and erosion. Climate, soil, slope, and management data were compiled to represent site-specific conditions, and multiple grazing scenarios were simulated. Analysis of variance (ANOVA) was performed to compare WEPP-simulated yearly runoff and sediment yield as affected by the factors of grazing site, slope aspect, management scenario, and their interactions with the General Linear Model (GLM) procedure in SAS. Using WEPP simulation results for the current management conditions and the no-grazing scenarios, our statistical analysis found significant differences in simulated runoff between the sites (P < 0.0001), aspects (P = 0.0006), and current conditions vs. no grazing scenarios (P < 0.0001), respectively. The differences were caused by a substantially lower runoff under the no-grazing scenario at the north-facing slope of the Smoothing Iron site. Statistical analysis showed that the north-facing and south-facing slopes at the Smoothing Iron Unit were significantly different for both WEPP-simulated runoff and sediment yield with P < 0.0001, the northeast-facing and southwest-facing slopes at the Pintler Creek Unit were not significantly different for either simulated runoff or sediment yield with P-values of 0.47 and 0.42, respectively. Sediment yield values are ranging 1.9 to 8.2 t ha−1, a sediment yield of 1.9 t ha−1 for the Middle Branch South Fork subwatershed, which encompasses the Smoothing Iron grazing unit, and a sediment yield of 7.3 t ha−1 for the Pintler Creek subwatershed. The WEPP-simulated average sediment yield from a hillslope was 9.0 t ha−1 for the Smoothing Iron site, and 0.7 t ha−1 for the Pinlter Creek site, compatible with the literature values. The WEPP model predicted greater amount of biomass for the north slope than for the south slope of the Smoothing Iron site. The WEPP results showed minimum runoff and erosion when year-round elk grazing was removed. The WEPP-simulated runoff ranged from 12.1 to 21.6 mm depending on the site, aspect, and vegetation cover under current practices of grazing. In contrast, RHEM-simulated runoff values ranged from 0.06 to 1.5 mm. The discrepancy may be attributed to the fact that RHEM model does not simulate snow hydrology and runoff only occurs as a result of rainfall.

Effects of vegetation cover and grazing on rangeland runoff and erosion processes in the Asotin Creek Watershed, WA

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