RECHARGE AND WATER BALANCE STUDIES AT CAVE WITHOUT A NAME, TEXAS: WHAT WE LEARN FROM INTERDISCIPLINARY METHODS

Results from an eight-year study in the Edwards Plateau in Kendall County, TX, USA show that diffuse and direct recharge can be differentiated at a variety of spatial and temporal scales, and highlight the importance of quantifying antecedent conditions for understanding dynamics of both types of recharge. We used ecohydrologic and hydrogeologic studies at a variety of scales (from individual trees, to drip sites, to whole-watershed) at Cave Without A Name (CWAN) to better understand the mechanisms controlling, and dynamics of, infiltration and recharge in semi-arid karst systems, how environmental and biological processes affect the amount and timing of recharge, and how diffuse and direct recharge variability affect both baseflow hydrology and storm responses.

Sapflow and liquid water stable isotope data suggest that most trees in the region do not tap into deeper water resources, and obtain much of their water from the upper few meters of soil/epikarst in these systems. Despite the regionally thin soils, significant water capacity exists in this and deeper horizons, and these zones strongly control responses to rainfall in the form of runoff, diffuse and direct recharge, and in plant-water use. Deeper vadose water in porous and fractured bedrock supports baseflow in these systems, which is different from Paleozoic systems in which baseflow is supported by drainage from deep/thick soils above a relatively impermeable bedrock. Additionally, liquid/gas phases in the unsaturated zone respond to variations in barometric pressure, which raises unanswered questions about the importance of pressure in affecting flow and chemistry of downward percolating waters; especially during drought conditions.

Direct measurements of diffuse recharge and use of a conservative tracer (Cl) suggest that the groundwatershed for the CWAN system is between 20 and 40 km², but provided little information regarding its geographic location. Recent dye tracing work highlights how variable recharge conditions are across the watershed, and has begun to delineate the boundaries of the watershed.