Influence of Vegetation Management on Yield and Quality Surface Runoff

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Texas Water Resources Institute

Water requirements for the United States will triple by the year 2000 (Water Resources Council, 19689. In Texas and many western states about 75% of the total water used is from ground water and this source in many areas is rapidly being depleted. To meet future demands water will have to come from other sources (Runkles, 1972). A possible source is increased water yield from watersheds. The quantity and quality of this surface runoff is influenced by many factors which include precipitation pattern, vegetation-type, soil-type and land use. If surface runoff from watersheds is to be a potential water source, the impact of these factors on water quality and yield must be evaluated.

Forests, grasslands and shrublands cover vast watersheds in Texas and North America. Many watershed studies have been conducted in forested regions, but rangeland areas have received only limited attention, particularly in Texas. The significance of these latter types cannot be overlooked since 40% of the land surface in the United States and 60% of Texas support this type of vegetation.

The major use of rangeland is domestic livestock and wildlife production. The impact of this use on water yield and nutrient and sediment loss from watersheds requires investigation. The influence of various grazing systems and intensities must be determined in order to coordinate ranching practices with increased high quality runoff. The effect of brush control on runoff yield and quality has not been thoroughly investigated.

The purpose of this study was to determine the influence of vegetation characteristics, grazing systems and precipitation on surface runoff from rangeland on the Edwards Plateau region of Texas. Water yield, organic-N, N03-N, NH4-N, N02-N, total and ortho-P, Ca, Mg, K, pH, conductivity, total and calcium hardness, turbidity and suspended sediment load were quantitively evaluated.

Field sampling was conducted on small-gauged watersheds on the Texas A&M Agricultural Research Station at Sonora, Texas. These gauged watersheds, which have been established over the past 13 years by the Agricultural Research Service (ARS), represent a variety of grazing systems ranging from continuous heavy grazing with poor vegetation cover to fourpasture and seven-pasture deferred rotation systems with good cover. In addition several different techniques have been used for woody plant control on the watersheds. The Sonora Research Station, with over 25 years of grazing management research, provides a unique area for study of the effects of grazing management and brush control on surface runoff, nutrient load and sediment yield.