CHARACTERIZING SMALL PERENNIAL SPRINGS DURING DROUGHT CONDITIONS, WESTERN TRAVIS COUNTY, TEXAS

Small watersheds in preserves and parklands of western Travis County contain perennial karstic springflows that sustain unique and fragile ecosystems. The springs issue from Middle Trinity Aquifer and are composed of the karstic Cow Creek formation (springflow horizon) that is overlain by the siliciclastic Hensel Formation, which is overlain by karstic limestones (Lower Glen Rose). These critical springflow systems have never been documented or quantified. Increasing groundwater development could threaten to reduce springflow of these springs. This study is a first attempt to characterize three springs: Elder Springs (1.3 km2), Grotto Spring (1.9 km2), and Hamilton Pool Spring (21.2 km2) within three adjacent watersheds. The study occurred over a 6-month period from May through November 2022. During this period, the study area experienced severe to extreme drought conditions receiving less than 30.5 cm (12 in) of rainfall which represents around 1/3 of the average annual rainfall for the area. Springflow measurements include periodic measurements (n= 8 to 10 per site) using an Acoustic Doppler Velocimeter (ADV) and bag and graduated cylinder. Continuous measurements with a 0.75 ft H-flume and pressure transducer (Hobo Mx2001) occurred with manual measurements at one location. Spring flows and water levels in each watershed demonstrate an overall correlation and recession, with periodic response in water levels and springflow to rainfall events in the watershed. Results include median springflows of 0.375 lps at Elder Springs, 0.400 lps at Grotto Spring, and 1.42 lps at Hamilton Pool Spring. There is a strong positive correlation between spring discharge and the total watershed and recharge areas for each corresponding spring, further supported by potentiometric mapping of the study area. A first order comparison of springflow and rainfall volumes over the 6-month period suggest rainfall within the recharge zone is up to ~3% of springflow. Quantification of springflow and springshed areas will help determine water budgets for springflow modeling and ultimately the conservation and protection strategies.