Vegetation and Water Yield Dynamics in an Edwards Plateau Watershed

Abstract

Woody cover, when expressed at the scale of the 207 km 2 Cusenbary Draw basin, remained unchanged (~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 production potential. Change in woody cover distribution at sub-range site scales, increased low and high woody covers and decreased intermediate woody cover, would be expected to lead to increased water yield at the basin scale because there was an apparent threshold woody cover (~20%) above which simulated evapotranspiration (ET) changed little with increasing woody cover. This potential increase, however, was more than offset by the decreased water yield due to increased ET loss associated with compositional changes of woody vegetation from oak to juniper. A set of woody cover-ET regression curves was developed for different range sites based on simulation studies using the SPUR-91 hydrologic model. Based on these woody cover-ET regression curves and GIS analysis, no brush management would result in a 35% decrease in water yield, while a hypothetical brush management cost-share program would increase water yield by 43% over the 1990 level. Benefits in water yield and forage production from brush management differ in different range sites. A brush management cost-share program that preferentially allocated brush management to sites with deep soil and the highest forage production 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 among range sites should be important concerns associated with developing and evaluating brush management policies.

Dryland ecohydrology and climate change: critical issues and technical advances

“…The degree to which woody and encroached communities increase water erosion (either through physical effects or by reducing infiltration) is poorly understood (Eldridge et al, 2003). Hydrological models have been used to study how the amount of rainfall reaching the ground, and hence the risk of erosion, varies according to changing cover of woody plant canopies (Wu et al, 2001). As woody vegetation tends to intercept more rainfall than understory vegetation (Thurow, 1991;Wu et al, 2001), it could be argued that woody vegetation reduces the risk of erosion to a greater degree.…”

Emerging Issues in Rangeland Ecohydrology: Vegetation Change and the Water Cycle

“…In the 1960s and 1970s, the potential for such a strategy was extensively investigated in the southwestern United States; anecdotal observations and estimates of ET savings associated with shifts in cover type from trees/shrubs to grasses led to the conclusion that there was significant potential for an increase in localized water yield (summarized by Hibbert 1983). In addition, modeling studies have indicated that the effects of shrub encroachment on water yield may be important on rangelands where drainage is rapid and deep (Wu et al 2001;Afinowicz et al 2005).…”

Removing Adult Overstory Trees Stimulates Growth and Transpiration of Conspecific Juvenile Trees

“…Concern has been raised that juniper may contribute to aquifer drawdown due to high year-round transpiration rates. Woody plant control may provide potential water savings to the region (Dugas and Mayeux 1991;Wu et al 2001;Wilcox 2002;Wilcox et al 2005). For woody plant management to successfully enhance groundwater recharge, the vegetation remaining after land clearing must use significantly less water than pretreatment vegetation.…”