|Van liew, Michael|
Submitted to: Journal Hydrologic Engineering
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/19/2003
Publication Date: 9/1/2004
Citation: Garbrecht, J.D., Van Liew, M.W., Brown, G.O. 2004. Trends in precipitation, streamflow and ET in the Great Plains. Journal of Hydrologic Engineering. 9(5):360-367. Interpretive Summary: The central Great Plains experienced a decade-scale precipitation increase at the end of the 20th century. This study examined the annual and seasonal impact of this precipitation trend on streamflow and evapotranspiration (ET) for ten watersheds in Nebraska, Kansas and Oklahoma. Such an assessment is important to identify management actions to keep agricultural and urban water needs in line with water availability. In the Central Great Plains the increased precipitation led to a disproportionately large increase in streamflow and comparatively smaller increase in ET. The seasonal partitioning of the annual precipitation increase was, in most cases, biased toward the fall, winter and spring, with little or no change during the hot summer months. Evapotranspiration, a large portion of precipitation, increased primarily in the dry western watersheds, whereas in the wet eastern watersheds most of the additional precipitation contributed to streamflow. From this study one can conclude that the high water demand by agricultural and urban areas during the summer months did not directly benefit from the decade-scale trend of precipitation. Fall and winter crops, cool season grasses, and winter wheat grazing opportunities stood to benefit from the precipitation trend. Summer crops and warm season grasses only benefited from additional spring moisture stored in the soil. If the decade-scale precipitation trend was an indication of climate change, then this study suggested that the greatest impact of a wetter or drier climate in the Great Plains could be expected during fall, winter and spring, but not necessarily during summer, and the most notable impact would be on streamflow.
Technical Abstract: Planning and strategic management of water resources are contingent on a stable water availability. In this study the impact of decade-scale variations in annual and seasonal precipitation on streamflow and evapotranspiration (ET) were identified for 10 watersheds in Nebraska, Kansas, and Oklahoma. In the Great Plains an upward trend in precipitation over the last two decades of the 20th century had a strong impact on streamflow and a comparatively weaker impact on ET. Even though precipitation, streamflow and ET amount differed between watersheds, the trend resulting from the precipitation increase was similar for all watersheds. Increased precipitation led to a disproportionately large increase in streamflow and comparatively smaller increase in ET. On average, a 12% increase in annual precipitation led to a 64% increase in streamflow, but only a 5% increase in ET. The seasonal partitioning of the annual precipitation increase was, in most cases, biased toward the fall, winter and spring, with little change during the hot summer months. The strong streamflow response indicated that planning and management of surface water storage and supply can be critically impacted by decade long trends in precipitation. The lack of significant increase in precipitation and streamflow during summer suggests that any existing shortages will likely remain despite the observed annual precipitation increase. Finally, the ET response suggests that dryland farming and ecosystem vitality could benefit from the increased precipitation in fall, winter and spring, but the impacts are more modest compared to the streamflow response and do not occur during summer when potential ET is greatest.