Understanding groundwater recharge rates has direct relevance for management of the Edwards Aquifer, which serves as the main source of fresh water for the city of San Antonio and surrounding communities. As population around San Antonio continues to grow, so does the demand for water and the stress placed on the aquifer. A method that is commonly believed to augment water yields is brush management. Over the last 150 years an increase in juniper density on the Edwards Plateau has coincided with decreasing streamflow. This has led many to believe that removing juniper would increase available water. Due to its karstic nature, the recharge zone of the Edwards Aquifer is assumed to be a prime location for augmenting water yields through vegetation manipulation. This study assesses the dynamics of recharge and the effects of manipulating surface vegetation. To accomplish this, a shallow cave located in the Edwards Aquifer recharge zone in San Antonio, Texas was instrumented to monitor drip recharge in response to simulated rainfall events. In 2004, simulations were conducted over the cave to measure recharge rates with a dense Ashe juniper canopy. The data and observations from the initial simulations were used to establish a baseline with the juniper in place. In February 2008 the juniper stand was cleared and the rainfall simulations were reproduced in June and July 2008, and again in early June 2009. Results from the study indicated that even though the amount of rainfall reaching the surface increased, a decrease in the amount of recharge occurred. The decrease can be possibly be explained by the elimination of stemflow, which is believed to direct higher concentrations of water to preferential flow paths near the base of the tree, and an increase in surface runoff. However, because such a large portion of the water reaching the surface is not accounted for by the budget, it is difficult to conclude that a change in recharge did not occur at a larger scale.

With the expanding population of Texas and the resulting increase in demand for water, the scarcity of water is becoming an increasingly urgent issue and research is being conducted to find ways to improve water yield, the amount of water that is used for aquifer recharge and riparian areas (lakes, streams, etc). Rangelands provide a major catchment for both surface reservoirs and aquifers. The Edwards Aquifer Recharge Zone and the Twin Buttes Drainage Area were selected to determine the tradeoffs between brush management for increased water yield and wildlife habitat improvement. In April 2002, a mail survey of 300 randomly selected landowners was conducted in each of the two watersheds to gain information regarding their demographics, land cover, and willingness to enter into a cost-share brush removal program. A total of 131 usable questionnaires (43.7%) were returned by Edwards Aquifer area participants, and 141 usable questionnaires (47.0%) were returned by Twin Buttes area participants. Respondents were asked questions including attitudes towards brush amounts and management strategies, and certain constraints that may be included in a cost-share brush removal program. Most respondents indicated interest in enrolling at least part of their land in such a program. Reasoning behind interest varied in each study area. However, interest in various contractual agreements was neutral at best. Policy implications entail considering wildlife habitat mitigation, landowners' preferences, and potential for increasing water yield and determining if all can be managed, or if trade-offs must be considered.

With the increase in human population, water resources have become more and more precious. A comprehensive study of water yield characteristics is imperative, especially in water-limited semiarid regions. The objective of this study is to examine spring flow and vegetation cover in a first-order watershed and investigate the herbaceous community structure of upland riparian zones. This study consists of two major components: (1) the effects of environmental factors and vegetation cover on spring flow at Pedernales River upland catchments, and (2) the ecological responses of vegetation to altered flow regimes that result from brush management at the upland riparian zones. The study finds that an average of 3.67% of the monthly water budget of first-order catchments in central Texas is made up of spring flow. The influence of woody plant cover on streamflow was evaluated by comparing spring sites with different percentages of woody cover three times during 2003 and 2004. Our findings indicate that changes in woody plant cover had no influence on the amounts of streamflow from these catchments, and the surface catchment area had only a minor influence. This suggests that the real spring catchment area might be different from the surface watershed boundaries that have been delineated by topography. Plant species richness and diversity gradually decreased with increasing lateral distances from the stream bank. Herbaceous richness and diversity declined with increasing Ashe juniper cover in the riparian zone. Ashe juniper canopy cover had a larger effect on the understory composition than the cover of other woody species. Herbaceous diversity and production was greater in areas with sparse tree density than in areas with no trees, but was lowest at high tree densities. The complete removal of Ashe juniper in the riparian zones is not recommended because of the potential loss of grass cover. The recommended management would be to leave a sparse cover of canopy trees to maintain understory plants.

With the expanding population of Texas and the resulting increase in demand for water, the scarcity of water is becoming an increasingly urgent issue and research is being conducted to find ways to improve water yield, the amount of water that is used for aquifer recharge and riparian areas (lakes, streams, etc). Rangelands provide a major catchment for both surface reservoirs and aquifers. The Edwards Aquifer Recharge Zone and the Twin Buttes Drainage Area were selected to determine the tradeoffs between brush management for increased water yield and wildlife habitat improvement. In April 2002, a mail survey of 300 randomly selected landowners was conducted in each of the two watersheds to gain information regarding their demographics, land cover, and willingness to enter into a cost-share brush removal program. A total of 131 usable questionnaires (43.7%) were returned by Edwards Aquifer area participants, and 141 usable questionnaires (47.0%) were returned by Twin Buttes area participants. Respondents were asked questions including attitudes towards brush amounts and management strategies, and certain constraints that may be included in a cost-share brush removal program. Most respondents indicated interest in enrolling at least part of their land in such a program. Reasoning behind interest varied in each study area. However, interest in various contractual agreements was neutral at best. Policy implications entail considering wildlife habitat mitigation, landowners' preferences, and potential for increasing water yield and determining if all can be managed, or if trade-offs must be considered.

abstract: Urbanization and woody plant encroachment, with subsequent brush management, are two significant land cover changes that are represented in the southwestern United States. Urban areas continue to grow, and rangelands are undergoing vegetation conversions, either purposely through various rangeland management techniques, or by accident, through inadvertent effects of climate and management. This thesis investigates how areas undergoing land cover conversions in a semiarid region, through urbanization or rangeland management, influences energy, water and carbon fluxes. Specifically, the following scientific questions are addressed: (1) what is the impact of different urban land cover types in Phoenix, AZ on energy and water fluxes?, (2) how does the land cover heterogeneity influence energy, water, and carbon fluxes in a semiarid rangeland undergoing woody plant encroachment?, and (3) what is the impact of brush management on energy, water, and carbon fluxes? The eddy covariance technique is well established to measure energy, water, and carbon fluxes and is used to quantify and compare flux measurements over different land surfaces. Results reveal that in an urban setting, paved surfaces exhibit the largest sensible and lowest latent heat fluxes in an urban environment, while a mesic landscape exhibits the largest latent heat fluxes, due to heavy irrigation. Irrigation impacts flux sensitivity to precipitation input, where latent heat fluxes increase with precipitation in xeric and parking lot landscapes, but do not impact the mesic system. In a semiarid managed rangeland, past management strategies and disturbance histories impact vegetation distribution, particularly the distribution of mesquite trees. At the site with less mesquite coverage, evapotranspiration (ET) is greater, due to greater grass cover. Both sites are generally net sinks of CO2, which is largely dependent on moisture availability, while the site with greater mesquite coverage has more respiration and generally greater gross ecosystem production (GEP). Initial impacts of brush management reveal ET and GEP decrease, due to the absence of mesquite trees. However the impact appears to be minimal by the end of the productive season. Overall, this dissertation advances the understanding of land cover change impacts on surface energy, water, and carbon fluxes in semiarid ecosystems. / Dissertation/Thesis / Doctoral Dissertation Civil, Environmental and Sustainable Engineering 2017

Over the past 100–150 years Texas rangelands have dramatically changed from native open savannahs to dense woodlands. On the Edwards plateau, a major management concern is the increasing encroachment of Ashe juniper (Juniperus ashei). Preceding an anticipated brush management program, I investigated the presence, co-occurrence, and habitat relationships of 7 breeding bird species in the Leon River Watershed in central Texas, USA: black-capped vireo (Vireo atricapillus), golden-cheeked warbler (Dendroica chrysoparia), northern bobwhite (Colinus virginianus), white-eyed vireo (Vireo griseus), Bell’s vireo (Vireo bellii), painted bunting (Passerina ciris), and brown-headed cowbird (Molothrus ater). Vegetation characteristics were compared between sites occupied by each species and unoccupied sites using univariate analysis. Models for predicting species site occupancy were developed (using logistic regression) based on habitat characteristics correlated with the presence of each species. Two species of special concern, the endangered black-capped vireo and golden-cheeked warbler occupied 5.6% of sites and 13.8% of sites respectively, while the brood parasite brown-headed cowbird was the most widespread, occupying 86.8% of sites. Species co-occurrence patterns revealed significant associations between the golden-cheeked warbler and each of 5 other species. For most species, variables included in habitat models could be explained by knowledge of species known habitat associations. For example, the black-capped vireo was positively associated with increasing low-growing (<1.5 m) hardwood cover and with Low Stony Hill ecological sites. The golden-cheeked warbler was positively associated with increasing density of larger juniper trees, increasing variability in vertical vegetation structure, and decreasing midstory canopy of deciduous nonoaks (e.g., cedar elm [Ulmus crasifolia]). It also preferred Low Stony Hill and Steep Adobe ecological sites. Site occupancy seemed to be driven by variables that describe overall vegetation structure. In particular, cover of low-growing non-juniper vegetation and juniper tree density appeared to be important in determining site occupancy for several species. Although the models constructed were not very robust, resource managers can still benefit from such models because they provide a preliminary examination of important controlling variables. Managing rangelands to maintain or restore a mosaic of juniper patches and open shrublands are likely to help meet the habitat requirements of these bird communities.