Observed and estimated (ERHYM-II model) water budgets for South Texas rangelands

Typescript (photocopy).A three-part study was conducted at the La Copita Research Area, 20 km southwest of Alice, in the eastern Rio Grande Plain of Texas. The research tested the hypothesis that honey mesquite-dominated shrub clusters have greater evapotranspiration rates than poor condition herb-dominated interspaces and they exceed evaporative losses from comparable but bare soil. Nine non-weighing lysimeters were used to determine the annual water budget for the different vegetation covers. Annual evapotranspiration rates of shrub clusters and interspaces were similar, and both were significantly greater than evaporative losses from bare soil. Surface runoff and deep drainage of water (> 2 m) from the bared soil were significantly greater than from herb-dominated sites and shrub clusters. Although there was no significant difference in any component for the annual water budget between the herb-dominated interspaces and the shrub clusters, there was a trend of increased water yield (31 mm) from the interspaces. There was no drainage of water below 2 m from beneath the shrub clusters. Beneath the interspaces a total of 22 mm of water percolated below 2 m during the study period. The potential for increased water yield from converting shrub-dominated rangelands to grass-dominated rangelands in South Texas is probably limited to years with above normal winter and spring precipitation. The second part of the study was conducted to determine if the ERHYM-II model was capable of simulating the water budget for the different vegetation covers. The estimated annual water budget was [plus or minus] 30 mm (4%) of the calculated annual water budget for the herb-dominated interspaces and the shrub clusters. The model under predicted evapotranspiration (18%) and over predicted soil water content (82%) for bare soil. A subroutine developed to calculate drainage as a function of unsaturated hydraulic conductivity should correct estimates of evaporation and soil water content from bare soil. Dimensional analysis was evaluated as a method of estimating shrub leaf biomass and leaf area. Four shrubs (honey mesquite, lime pricklyash, Texas colubrine, and Texas persimmon) were sampled. Relationship between leaf biomass and crown volume for mesquite were best expressed with the log-log or quadratic regression equation..