Candidate Indicators for Long Term Persistence of Stream Flow Losses for the Lower Colorado River in Texas

The Highland Lakes are operated by the Lower Colorado River Authority (LCRA) in Texas to provide water supply to municipal, industrial, agricultural users and environmental flows for the river and Matagorda Bay. The Highland Lakes also provide for hydroelectric generation and recreation. Subject to extended droughts interrupted by intense rainfall, the region has the nickname of Flash Flood Alley. The Lower Colorado River has recently experienced a historic drought. While precipitation has been 80% of average, runoff into the Highland lakes have been at historic lows.

Texas entered 2011 in La Niņa conditions which generally contributes to lower than average precipitation in Central Texas. Then in early spring, an intense high pressure ridge became established over the south central US, keeping rain and storms away from practically all of Texas. This resulted in the development of a Flash Drought, creating unprecedented dry and hot weather conditions. Furthermore the Central Texas area experienced a brutal summer with about 90 days with temperatures at or above 100 degrees Fahrenheit. Estimates are that water lost in the State from all sources was 16,000,000 acre-feet to 60,000,000 acre-feet (Caldwell, 2013) of which 70 to 80% appears to have come from soil moisture (Long, 2013).

Multiple regression relationships were developed to predict runoff from precipitation and other factors which explain about 2/3 of the variation of observed inflows. This explanation is good considering the inherent error in stream flow measurement and inflow estimation. Review of the residuals from the relationships reveals periods of unfavorable non-stationarity in inflows after accounting for statistically significant climate and seasonality variables. In particular the periods from 1977 to 1987 as well as 2002 to present showed uncharacteristically low runoff. Using this analysis, potential indicators of periods of run-off non-stationarity were tested and candidate indicators are presented.