Using Remote Sensing and GIS Techniques to Identify Watersheds for Brush Control to Maximize Water Yield BACKGROUND AND MOTIVATION Encroachment of woody species, most typically mesquite (Prosopis glandulosa) and juniper (Juniperus ashei and Juniperus pinchotii), has dramatically changed the landscape of semi- arid regions due to a host of human and environmental factors (Van Auken, 2000) and is believed to have contributed to the decrease of useful water yield (Wu et al., 2001). Small scale experiments indicate that the removal of juniper species, for instance, may reduce transpiration by as much as 40,000 to 100,000 gallons per acre per year (Owens, 1996). This shortage of water adds additional stress to semiarid systems which operate in a soil- water-deficient manner, meaning that the potential ET rate far exceeds the annual precipitation rate (Wilcox, 2002). Given the rate at which the population of these semi-arid regions is expanding, it is of the utmost importance that efforts are made to minimize wasteful losses of water (TWDB, 2002). It is by this means that brush control has entered the public focus as a possible way to mitigate the problem of reduced water supplies in relatively dry regions. Currently, there is no method for determining regions where the greatest increase in water yield will result from the implementation of brush management practices. Studies that have been conducted tend to focus on a watershed based approach to determine the overall effects of brush control. Research such as this has focused on hydrologic simulation of entire basins to add merit to the cause of brush management, rather than the finer points of implementing these practices (Bednarz at al., 2001; Red River Authority, 2000). This is especially important since Texas has begun the subsidization of these techniques as a way of compensating land owners who make the effort to increase the overall availability of water for a region (TWDB, 2002). If this process of state funded brush control is expected to be economically viable, there must be a way of ensuring that the public funds expended in the name of environmental, agricultural, and anthropocentric well-being are being spent to rehabilitate locations where the most benefit may be gained. Jason D. Afinowicz, E.I.T. 1 ; Clyde L. Munster, Ph.D., P.E. 1 ; Brad P. Wilcox, Ph.D. 2 ; Ronald E. Lacey, Ph.D., P.E. 1 1 Department of Biological and Agricultural Engineering, Texas A&M University 2 Department of Rangeland Ecology and Management, Texas A&M University SELECTION BT TOPOGRAPHY Wilcox (2002) suggests that recharge occurring through stream channels is enhanced by the ability of surface water to run off before being taken up by local vegetation. By decreasing interception by woody species on hill slopes (approximately 5% or greater) it is surmised that more water will be allowed to enter the stream channel and contribute to overall water yield. Selection of steep slope regions is performed with the use of the National Elevation Dataset for the Upper Guadalupe, available at a spatial resolution of 30 meters. SELECTION BY SOIL CHARACTERISTICS Wilcox (2002) suggests that thick soil layers may prevent water from infiltrating the profile and contributing to aquifer recharge. The karst geology of the Upper Guadalupe River watershed is optimum for taking advantage of the numerous shallow soil regions in the area. A threshold soil thickness of 1 m was selected to locate regions where infiltration past the root zone of overlying vegetation would be allowed. Analysis was conducted using NRCS SSURGO databases where possible, which represent the most accurate and detailed digital depiction of soil characteristics available. Where SSURGO coverage was not complete, STATSGO data was used. SELECTION BY LANDCOVER The amount of shrub cover is also indicated to be a major contributor to the success of brush control by Wilcox (2002). Though extensive landcover datasets exist, the information presented fails to give any indication of the amount of woody cover present. For this reason, a new landcover dataset for the Upper Guadalupe was created with the use of Landsat Enhanced Thematic Mapper Plus (ETM+) imagery, high resolution (1 meter) digital orthophoto imagery, and land cover information from the 1992 Multi-resolution Landcover Characteristics (MRLC) National Land Cover Data (NLCD) set. Since the future aims of the project look toward the creation of an easily computed index of prime brush control locations an emphasis was placed on the creation of a landcover set with the minimum need for ground-truth data for its creation. Minimizing this time and money consuming step benefits the system’s use by water planners and engineers. THE UPPER GUADALUPE RIVER Landsat ETM+ Image of the Upper Guadalupe River watershed, taken from Landsat Row 39, Paths 27 and 28. MRLC Land Cover Grid for the Upper Guadalupe River watershed. Provided in 30 meter resolution. Representative 1-m resolution DOQ showing detail of brush cover. Mosaics were formed for the entire watershed. The same DOQ classified into regions containing woody growth and no brush using a Maximum Likelihood classifier. Upon classification, the image was resampled to 30m and assigned a digital number based on amount of cover. The MRLC landcover grid was used as a basis to train the Maximum Likelihood classifier for the task creating a general landcover set. The new product represented a new dataset, contemporary to the 1999 satellite imagery that was used. This provided a coarse method for recognizing general brush covered regions and to provide additional land cover information to be used in later hydrologic analysis. Following classification of the satellite and orthophoto imagery, a final classification was performed using a visually determined threshold for the resampled DOQ classification. This allowed regions recognized as brush during the satellite image classification to be defined as either light (0-20%), moderate (20-50%), or heavy ( %) brush cover. PRIORITY BRUSH CONTROL SITES Chosen using the following criteria: Slope greater than 5% Average annual rainfall greater than 450 mm Soil Depth of less than 1 m Brush cover greater than 50% The selected regions covered an area of 36,330 ha, or approximately 9.7% of the entire watershed. SECONDARY BRUSH CONTROL SITES Chosen using the following criteria: Average annual rainfall greater than 450 mm Soil Depth of less than 1 m Brush cover greater than 20% The selected regions covered an area of 142,282 ha, or approximately 38.0% of the entire watershed. The Upper Guadalupe River watershed encompasses over 374,000 ha of the Texas Hill Country, north of San Antonio. The watershed boundaries are contained within Bandera, Blanco, Comal, Gillespie, Kendall, Kerr, and Real counties. The watershed ABSTRACT Shrub encroachment has drastically transformed the landscape of arid and semi-arid rangelands of the southwestern U.S. over the past century. It has been hypothesized that this replacement of natural herbaceous growth with woody species has reduced the volume of streamflow and aquifer recharge in these landscapes. Several small scale field experiments and computer-based hydrologic simulations have been conducted to support this theory. In recent years, the State of Texas has begun to subsidize brush control as a means of augmenting streamflow. However, if brush control is to be regarded as a viable option for increasing water availability in arid or semi-arid populated areas, a technique must be determined for targeting locations where brush management funding will provide the greatest addition to water yield. The working hypothesis is that water yield from brush control will be maximized in watersheds where; 1. Precipitation is greater that 450 mm, 2. Shrub coverage is greater than 20%, 3. Soil depth measures less than 1 m, and 4. Limestone aquifers underlie the soil profile. Locations that meet the listed criteria for candidate brush control sites can be easily identified by querying a number of data sources using Geographic Information Systems (GIS). A classification scheme was developed for remotely sensed multi-spectral imagery of arid and semi-arid areas that allows for the recognition of regions meeting the required criteria for brush cover. Selection of optimum sites based on soil and climate characteristics was conducted using high resolution databases. The methods utilized throughout the entire process were designed to be easily adapted to a variety of locations. SELECTION BY CLIMATE CHARACTERISTICS Hibbert (1983) established a guideline for the necessary precipitation required for a region to demonstrate an increase in stream flow. Though Hibbert’s threshold value of 450 mm/year was selected from studies in Arizona and California, this values has been applied to Texas rangelands. This amount of average annual rainfall is considered a requirement for vegetation change to influence water yield. By using data from Oregon State’s Parameter-elevation Regressions on Independent Slopes Model (PRISM) datasets for annual rainfall, it was verified that the Upper Guadalupe met this criteria. lies above the Edwards Aquifer recharge zone and ultimately contributes to the groundwater supply for the entire region, including San Antonio. Springs fed by the aquifer host a variety of unique species and contribute to the ecology of this unique ecosystem. Future Research Beyond the selection of potential brush control target sites, several research activities have been planned to continue the advancement of GIS techniques in brush control for water resources planning: Validation of Landcover Set The landcover set derived in this research will be verified by GPS survey to validate its use in further research and to evaluate the proposed methodology for use by professional planners. Calibration and Validation of the SWAT Model Data from an in progress brush study at Honey Creek in the Guadalupe watershed will be used to calibrate and validate the Soil and Water Assessment Tool (SWAT) for brush control analysis. Simulation of Target Sites The SWAT model will be used with the described calibration to verify the validity of the proposed selection methodology by electronically simulating brush in place and brush removed conditions for the Upper Guadalupe. SELECTION CRITERIA For purposes of site selection for brush control, we will propose that the optimum sites as those meet the following criteria: Slopes greater than 5% Average annual rainfall greater than 450 mm Soil depth of less than 1 m Brush cover greater than 20% Underlying limestone aquifers REFERENCES Bednarz, S.T., T. Dybala, R.S. Muttiah, W. Rosenthal, W.A. Dugas Brush management/water yield feasibility studies for eight watersheds in Texas. Texas Water Resrources Institute, CollegeStation, TX. Hibbert, A.R Water yield improvement potential by vegetation management on western rangelands. Water Resources Bulletin. 19: Owens, M.K The roll of leaf and canopy-level gas exchange in the replacement of Quercus virginiana (Fagaceae) by Juniperus ashei (Cupressaceae) in semiarid savannas. American Journal of Botany. 83: Red River Authority Assessment of Brush Management/Water Yield Feasibility for the Wichita River Watershed Above Lake Kemp, Hydrologic Evaluation and Feasibility Study. Red River Authority of Texas. Wichita Falls, TX. TWDB Water for Texas – Texas Water Development Board, Austin, TX. Van Auken, O.W Shrub invasions of North American semiarid grasslands. Annual Review of Ecology and Systematics. 31: Wilcox, B.P Shrub control and streamflow on rangelands: A process based viewpoint. Journal of Range Management. 55: Elevation dataset showing the regions meeting the 5% slope criteria (Shown in Red). Regions meeting the 450 mm rainfall requirement (Entire Upper Guadalupe). Map derived from electronic soil datasets showing regions with soil depth less than 1 m. Dataset representing the regions meeting the brush cover greater than 20% criteria. Dataset representing the regions meeting the brush cover greater than 50% criteria.