Effect of shrub management techniques on water quality and quantity on Coastal Bend rangeland

Abstract

Extensive studies on runoff from South Texas croplands and urban areas exist; however, little is known of the importance or impact of runoff generated from South Texas rangelands. Therefore, the objectives of this study are: (1) to assess the impacts of various rangeland management practices (grazing, herbicide treatment of the woody shrubs, and herbicide treatment of the shrubs followed by burning) on water yield and (2) to assess the impacts of various rangeland management practices on sediment loss and levels of nitrogen and phosphorus in runoff water. Three 1-ha watersheds, WS1-HB (Herbicide and Burn), WS2-HERB (Herbicide), and WS3-UNT (Control-Graze only), were surveyed and installed on Chaparral-Mixedgrass Communities located on a Monteola soil on the Welder Wildlife Refuge near Sinton, Texas. Ten months following treatment, woody shrub canopy cover was reduced by 41%, 60%, and 10% in WS 1, 2, and 3, respectively. Grass, forb, and litter standing crop increased in all 3 watersheds except for grass and forb standing crops in WS1-HB. Average infiltration rates for run 1 simulations were greater than 100 mm/hr except WS3-UNT brush plots (76 mm/hr). Run 2 average infiltration rates remained above 100 mm/hr except WS2-HERB woody shrub plots (96 mm/hr) and WS1-HB grass (72 mm/hr) and woody shrub plots. Run 3 average infiltration rates remained similar to run 2 grass and shrub in WS1-HB and WS2-HERB. WS3-UNT run 3 dropped to 67 mm/hr and 66 mm/hr in grass and shrub plots, respectively. Sediment in runoff waters was greatest on WS1-HB grass and shrub plots following treatment. WS2-HERB sediment increased in grass plots and was constant in shrub plots following treatment. WS3-UNT sediment production varied greatly between simulator runs and among shrub and grass plots. No significant increases in nitrates were found in WS1-HB and WS2-HERB grass or shrub plots following shrub treatment. Total Kjeldahl N (TKN) and P showed significant increases in 2 months following herbicide treatment and 3 weeks following the burn treatment. Ten months following treatment TKN and P levels had returned to pretreatment levels.