1. Presented by Jason Afinowicz Biological and Agricultural Engineering Department, Texas A&M University CVEN 689 Applications of GIS to Civil Engineering Instructor: Francisco Olivera, Department of Civil Engineering April 29, 2002 Background on Shrub Encroachment and Brush Removal Vegetation plays a key role in determining the availability of surface runoff and groundwater in the hydrologic cycle. Any water removed from the soil profile and transpired into the atmosphere is unavailable for aquifer or stream recharge. Therefore aquifer and stream recharge depends on the type and amount of vegetative cover. Some varieties of invasive woody plants have been identified as excessive water users when compared to the native grass species that they have replaced. Unfortunately this alteration of the land cover has occurred in the arid and semi-arid regions where water resources are already scarce. In particular, in the southwestern U.S., a very noticeable change has occurred in the ground cover due to both human factors (due to changes in management practices, grazing, and decreased fire frequency) and climate change (including atmospheric carbon dioxide enrichment). In places where water is already a scarce resource, unnecessary transpiration losses continue to detract from the sustainability of the range ecosystem. Availability of surface water is also a key issue regarding the recharge of aquifers, which provide the majority of water for local populations. Clearly, these range management practices serve both a natural and an anthropocentric importance. The clearing of non-native, woody vegetation has been incorporated into best management practices (BMPs), which have been implemented in a number of previous and current studies. Hydrologic simulation of these process have also produced favorable data for the benefits of brush management by demonstrating increased stream flows, decreased evaporation and transpiration, and increased shallow aquifer recharges. The implementation of these BMPs is a difficult process that involves the removal of relatively hearty growth and replacement by grass species. However, the method of applying these practices is a generally unguided process of removing unwanted vegetation without consideration of optimum sites for water yield change. Ultimately, a method of targeting optimum locations for these BMPs is required to maximize efficiency in increasing water supply. Location of Honey Creek (yellow) the Upper Guadalupe (green) and the Edwards Aquifer (pink) The Honey Creek Watershed The area of study for this project is the Honey Creek watershed located in the Upper Guadalupe Basin (HUC 12100201) in western Comal County. The stream drains approximately 6000 acres of land and is fed year-round by a number of active springs. The watershed lies in the Edwards Aquifer contributing area and flows that exit into the Guadalupe are carried downstream to the recharge zone along with the rest of the Upper Guadalupe. Research is currently being conducted in the watershed to determine the effects of brush control on water yield. The data from this portion of the project will be later incorporated into much more advanced brush removal simulations with GIS Data Sources Used RF3 Reach File – Reach data for the Upper Guadalupe was used to determine stream layouts and to aid in delineation. Digital Elevation Models (DEMs) – 30- meter resolution of the Honey Creek area were used to determine elevation contours for the watershed. SSURGO Soil Data – High resolution soil survey information to aid in determining surface characteristics. Landsat ETM+ Imagery – Remote sensing data, in the form of Landsat spectral imagery, was used to determine land cover classifications in Honey Creek. Climate Data - Information, obtained form conventional databases, was used to generate weather for the simulation. Results from the Simulation Water Yield As expected, water yield increased with the additional removal of brush covered land, overall. Aquifer Recharge Aquifer recharge, contrary to expectation, decreased considerably with the removal of brush. Evapo-Transpiration Unexpectedly, ET losses increased slightly overall with the additional removal of brush. Experimental Procedure To determine the effects of brush control techniques in the watershed, a hydrologic simulation was conducted using the Soil and Water Assessment Tool (SWAT) incorporated into the Better Assessment Science Integrating point and Non-point Sources (BASINS). The Honey Creek watershed was simulated in its current state, and then using the same variables, was simulated with two brush-cleared conditions. The original land cover used for the control simulation was determined to be 25% range with grasses, 32% range with brush, and 43% evergreen forest. In the first experiment, the area covered with range with brush was electronically converted to range with grasses. In the second experiment, in addition to the changes in experiment one, the evergreen forest land cover was reduced to range with brush. Future Goals and Considerations Ultimately, the final goal of this research is to produce a tool to aid in determining the ideal locations for brush management. By simulating the hydrology of a river basin and electronically altering the land cover to represent brush control practices, certain subbasins can be recognized as those that will benefit the most from a given amount of removal. In reaching this goal, there are certain problems which must be considered and solved to obtain a reliable, usable model: Goal 1: Calibrate the model with real life hydrologic data and climate records from Honey Creek Goal 2: Better integration of available data sources such as SSURGO Goal 3: Create a reliable method for recognizing land cover from cheap, readily available Landsat ETM+ imagery