Trends in Precipitation, Streamflow, and Evapotranspiration in the Great Plains of the United States

Planning and strategic management of water resources are contingent on trends in 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 amounts differed between watersheds, the trend due to 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 or no 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. The ET response suggests that dryland farming and ecosystem vitality could benefit from the increased precipitation in fall, winter, and spring, but the relative impacts are more modest compared to the streamflow response and do not occur during summer when potential ET is greatest. Finally, since the mid-1990s precipitation and streamflow in a number of Oklahoma watersheds have shown a gradual decline from peak values in the late 1980s toward more average conditions. This declining trend in streamflow may be important for planning and management of water resources systems that must meet an increasing demand for water by a growing society while at the same time considering environmental and recreational needs