Geospatial Hydrology Group The Effects of Land Cover, Geology, and Groundwater Levels on Spring Discharge in Pedernales River Basin Y. Suharnoto1, C. Munster1, B. Wilcox2, L. Shen2 1 Department of Biological and Agricultural Engineering, Texas A&M University MS 2117 College Station, TX 77843 2 Department of Rangeland Ecology and Management, Texas A&M University 2126 TAMU College Station, TX 77843 Geospatial Hydrology Group http://ghg.tamu.edu/ Abstract The discharge at 21 upland springs in the Pedernales River basin was measured in the summer 2003. Groundwater levels in the upper layer of Edwards-Trinity Aquifer were simulated using the Analytical Element Method (AEM). The interface between the Edwards – Glen Rose and the Glen Rose – Hensel geologic formation were mapped. Surface catchments for each spring were delineated using a 30 m resolution digital elevation model (DEM) and subsurface catchments were derived from the geologic interface map. Land cover was determined from field observations and Landsat imagery. 15 of the sampled springs were located at interfaces (mostly Edwards – Glen Rose interface). Subsurface catchment areas were generally smaller than surface catchment areas and did not coincide. Simulated groundwater levels closely matched springs and well data. Oak dominated landscapes had the lowest spring discharge rates. Bottom: A group of maps shows Sampling points, watershed delineation, land cover, geologic interfaces, and groundwater contour Right: A group of graphs shows spring discharge vs catchments area, simulated groundwater vs surface elevation, spring discharge vs land cover, spring discharge vs juniper and oak land cover, spring flow depth vs flow velocity. Methods Groundwater levels were simulated using the Analytic Element Method (AEM) with an average saturated hydraulic conductivity of 6.5 ft/day (TWDB) and a recharge value of 219 mm (28 % of average precipitation). Visual Bluebird 1.5 with the SPLIT engine was used as the AEM modeling interface. The Geology interface maps were generated by spline interpolation with tension spline type from digitized 1:24,000 scale surface geology maps and well log data from the TWDB and the HCUWCD. Land cover was determined using LANDSAT imagery from mid-1990s by Erdas Imagine 8.7 software using supervised classification with the maximum likelihood method. The signatures were created from 32 field measured sampling points (4 ha/sampling) for each vegetation type. Band combinations 3, 4 and 5 were used for classification based on the best minimum and average separability signature calculation using transformed divergence formula. Surface and subsurface watershed delineation was performed using EPA BASINS 3.0 software. Conclusions Most of the springs were located at the Edwards – Glen Rose formation interface and the spring discharges were generally very low. The oak dominated land cover exhibiting the lowest spring flows but there was no obvious relationship between spring flow and catchments area, surface elevation or juniper cover. Subsurface catchments were generally smaller than surface catchments. The AEM successfully simulated groundwater levels in the upper layer Edwards – Trinity Aquifer. The interface maps for the geologic formations will be useful in future groundwater studies. HCUWCD Presented in Edwards Water Resources in Central Texas: Retrospective and Prospective, South Texas Geological Society and Austin Geological Society, May 21, 2004, San Antonio, TX.