Effect of Trends and Long-term Fluctuations of Rainfall on Watershed Runoff

This study investigates the impact of long-term trends and fluctuations in rainfall characteristics on runoff from the Little Washita River watershed. For this 610 km2 watershed, mean daily discharge, peak flow, flow frequency, and volume increased significantly over a 22-year period, despite the installation of floodwater retarding structures which controlled 45% of the drainage area. Changes in runoff are shown to be associated with long-term increases in rainfall amount, intensity, and frequency. Time series analyses of rainfall and watershed runoff demonstrated that changes in rainfall patterns and amounts can mask the beneficial impacts of floodwater retarding structures. Concurrent meteorologic conditions must be measured and evaluated for proper interpretation of downstream impacts of upstream conservation measures.