CLIMATE CHANGE IMPACTS ON HYDROLOGIC RESPONSES IN A SPRING FED WATERSHED NEAR THE 100TH MERIDIAN – LLANO RIVER WATERSHED

Stream-subsurface interactions of post-storm events (streamflow recessions) play a vital role in maintaining riparian ecosystem services but are affected by droughts and other climatic shifts. Understanding how recessional streamflow characteristics relate to climatic conditions in the region is therefore important for sustainable water resources management, particularly in climate hot-spots such as the 100^th meridian . The Llano River watershed is a spring-fed watershed near the 100^th meridian with significant ecological and economic significance to the Central Texas region. Recession time, defined as the time taken for streamflow to decrease from median flow to low-flow thresholds were compared to the Standardized Precipitation Index (SPI) to understand how stream-aquifer interactions vary under different climatic regimes.

The recession times showed a strong seasonal behavior across the watershed. Recession times being lower during the summer months compared to the winter months. The recession times during winter droughts were significantly longer than those of wet periods. However, no statistical differences were noted in recession times between drought and wet periods during summer months. The study also indicated that the South Llano River contributed significantly to downstream flows. The data showed that contributions from the South Llano River were at least 50% more than the North Llano River. Therefore, to understand surface water (SW) /groundwater (GW) interactions, a gain-loss study was conducted along a stream reach of 16.6 miles over the South Llano River upstream of the city of Junction, TX. The yearlong study (September 2015 – August 2016) indicated that the South Llano River is a losing stream over a length of 12.25 miles upstream while the stream gains from groundwater over a downstream length of 4.35 miles. The results of the study are useful to guide future watershed management and groundwater development activities.