PS 24-68
Domestic grazing on saline soils: Run-off, erosion, and implications for Colorado River water quality
In arid and semi-arid ecosystems, overgrazing by domestic livestock can result in decreased vegetative cover, increased soil compaction, and breaking up of stabilizing soil crusts, leading to increased run-off and erosion. Such changes to hydrology and erosion are of particular concern for soils derived from saline parent material due to potential negative impacts on in-stream water quality. To evaluate how domestic grazing on saline soils (derived from Mancos Shale) is impacting run-off and erosion processes, a consortium of federal researchers and land managers established the Badger Wash study area in SW Colorado, USA in 1953. The measured reduction in run-off and erosion with long-term grazing exclusion from this earlier work is notable—over the first 13 years, exclusion of grazing resulted in a 35% reduction in sediment yield and a 21% reduction in run-off. This study area provides a unique opportunity to assess impacts of long-term domestic grazing exlusion on run-off and erosion processes. The goals of this study are to 1) determine if differences in sediment production between grazed and ungrazed pastures persists, 2) evaluate the how slope gradient interacts with grazing to effect sediment production, and 3) assess if the contrasting land-use histories effects in stream water quality.
Results/Conclusions
Results indicate that differences in erosion rates between grazed and ungrazed watersheds observed in the 1960s persist. The ratio of sediment lost per unit area of grazed : ungrazed measured in 2006 – 2012 is only slightly elevated compared to 1954 – 1966. This small relative change over the last 40 years suggests little further recovery of soil stability in this system occurred after the first 13 years. Erosion rates continue to be significantly greater in areas with gentle topography open to grazing than where grazing has been excluded. Analysis of the slope-grazing interactions suggests that grazing (current or historic) has altered soil and/or vegetation properties such that the relationship between site erodability and slope now differs. We attribute the lack of slope effect on erosion in the ungrazed areas to higher vegetation cover and increased soil stability counteracting the effect of slope. Lack of difference between grazing treatments in the salinity of eroded sediment or of in-stream flow (EC or elemental concentration) suggests that any potential impacts on in-stream water quality due domestic grazing on Mancos Shale derived soils is not due to greater salinity of overland-flow, but simply to increases in quantity of saline runoff originating from disturbed areas.