Vegetation and water yield dynamics in an Edwards Plateau watershed

Woody cover, when expressed at the scale of the 207 km(2) Cusenbary Draw ba sin, remained unchanged (similar to 23%) from 1955 to 1990. When expressed at the scale of range sites, woody cover declined on sites with relatively high production potential and increased on sites with relatively low produc tion potential. Change in woody cover distribution at sub-range site scales , increased low and high woody covers and decreased intermediate woody cove r, would be expected to lead to increased water yield at the basin scale be cause there was an apparent threshold woody cover (similar to 20%) above wh ich simulated evapotranspiration (ET) changed little with increasing woody cover. This potential increase, however, was more than offset by the decrea sed water yield due to increased ET loss associated with compositional chan ges of woody vegetation from oak to juniper. A set of woody cover-ET regres sion curves was developed for different range sites based on simulation stu dies using the SPUR-91 hydrologic model. Based on these woody cover-ET regr ession curves and GIS analysis, no brush management would result in a 35% d ecrease in water yield, while a hypothetical brush management cost-share pr ogram would increase water yield by 43% over the 1990 level. Benefits in wa ter yield and forage production from brush management differ in different r ange sites. A brush management cost-share program that preferentially alloc ated brush management to sites with deep soil and the highest forage produc tion potential increased water yield by 50%, compared to a 100% increase if brush management were preferentially allocated on sites with shallow soil and highest water yield potential. These model results illustrate that the spatial scale of assessment and spatial distribution of brush management am ong range sites should be important concerns associated with developing and evaluating brush management policies.