STREAMFLOW RESPONSE OF AN AGRICULTURAL WATERSHED TO SEASONAL CHANGES IN PRECIPITATION.

Authors: Van Liew, Michael; Schneider, Jeanne; Garbrecht, Jurgen

Submitted to: Interagency Conference on Research in the Watersheds
Publication Type: Proceedings
Publication Acceptance Date: 10/1/2003
Publication Date: 10/27/2003
Citation: VAN LIEW, M.W., SCHNEIDER, J.M., GARBRECHT, J.D. STREAMFLOW RESPONSE OF AN AGRICULTURAL WATERSHED TO SEASONAL CHANGES IN PRECIPITATION. PROCEEDINGS OF THE FIRST INTERAGENCY CONFERENCE ON RESEARCH IN THE WATERSHEDS. 2003. p. 133-138.

Interpretive Summary: Management of water in reservoirs is complicated by the variation in streamflow that results from variation in seasonal precipitation. This is especially true during dry seasons like the summer months in the southern Great Plains, when demand for water is high, but supply from streamflow is low. The long-term precipitation forecasts that are issued monthly by the National Oceanographic and Atmospheric Administration's Climate Prediction Center may allow reservoir managers to better manage water releases and reservoir storage by anticipating when or whether streamflows will be sufficient to refill reservoirs. We used long-term precipitation measurements on a 13-square mile area of the Little Washita Experimental Watershed in southeastern Oklahoma and a computer model to predict the effects of less than (-20% and -40%) and more than (+20% and +40%) average precipitation during the fall months on streamflow. Results indicated that streamflow approaches base flow levels during dry periods, as expected, but increased proportionally more than precipitation during wet periods. For instance, streamflow increased by 39% when precipitation was 20% greater than average and by 96% when precipitation was 40% greater than average. However, a very wide range of variation in streamflow was predicted during periods of high precipitation, apparently due to differences in the size and intensity of storms during the fall months. Still, the study suggests that the forecasts can be used to predict streamflow levels and improve reservoir managers' ability to meet water demands and downstream flow requirements.

Technical Abstract: Seasonal variations in precipitation on a watershed lead to variations in streamflow that in turn result in uncertainties that impede the efficient management of available water resources. This is especially true for management of reservoir storage and water releases during and at the end of the dry season when water demand is highest and streamflow supply is lowest. Anticipating streamflow amounts based on seasonal precipitation forecasts holds promise to estimate the probability of replenishment of depleted reservoir storage and help identify best water supply management strategies related to anticipated streamflow and associated uncertainties. A study was conducted to evaluate the impact of hypothetical seasonal variations in precipitation on streamflow. The objective was to develop a prototype for streamflow response associated with a range of hypothetical precipitation forecasts. The prototype was developed for the 33 km2 subwatershed 442 located in the USDA-ARS Little Washita River Experimental Watershed in Southwestern Oklahoma. The Soil and Water Assessment Tool was used to determine streamflow responses to hypothetical precipitation forecasts that represent changes of +20% and +40% for the fall quarter. Measured precipitation data for a period of record from 1971 to 2000 on the subwatershed were used to develop the hypothetical precipitation forecasts. Test results of this study indicate that hypothetical precipitation forecasts that are drier than normal lead to streamflow responses that approach baseflow conditions on the watershed, while forecasts that are wetter than normal lead to higher streamflow values characterized by considerable variability due to variations in storm size, duration, and intensity during the fall months. Results of this study suggest that utilization of precipitation forecasts coupled with the corresponding anticipated streamflow changes could provide sufficient risk-based information that enable water authorities to more effectively manage reservoir storage and water releases to meet water demands and downstream flow requirements.