Long term Hindcasting of Surface Water Temperatures for the Highland Lakes on the Lower Colorado River in Texas

The Highland Lakes are six lakes on the Colorado River in Texas. The Lower Colorado River Authority (LCRA) operates the lakes, completed in 1951. The Highland Lakes provide water supply to municipal, industrial and agricultural users, and environmental flows for the river and Matagorda Bay. In Central Texas, 1.3 million people depend on the reservoirs as their primary water supply. LCRA and the Colorado River Water Network (CRWN) began monitoring surface water temperatures at the lakes from grab samples in 1981. LCRA developed regression relationships to replicate the monthly mean surface water temperature from PRISM gridded air temperature observations at each of the six lakes. For example, at Lake Travis, both current-month and prior-month mean air temperature show statistical significance of p<0.0001 with a standard error of less than 2 degrees Fahrenheit. During the monitored period, surface water temperature also exhibited strong seasonality. Air temperature improved water temperature estimates compared to seasonality alone during the cool season of October through May. In January and February, use of air temperature reduced mean error by +/- 1 degree Fahrenheit. In the warm season of June through September, the air temperature method generally was no better or worse than seasonality alone. LCRA used the air temperature relationships to reconstruct surface water temperatures back to 1890, prior to the lakes’ construction from the PRISM gridded air temperature. This work will support long-term analysis of computed evaporation and water quality evaluations.