Woody plant encroachment reduces annual runoff and shifts runoff mechanisms in the tallgrass prairie, USA

Woody plant encroachment into semiarid and subhumid rangelands is a global phenomenon with important hydrological implications. Observational and experimental results reported both increases and decreases in annual runoff for encroached watersheds and little is known regarding the underlying runoff generation mechanisms. To systematically study the effect of woody plant encroachment on runoff generation processes, seven experimental watersheds were instrumented in 2010, three on grassland sites and four on adjacent sites that were heavily encroached by eastern redcedar (Juniperus virginiana) in the southern Great Plains, USA. Results showed that the runoff coefficient was 1.460.6% in eastern redcedar encroached watersheds, significantly lower than 4.460.7% in grassland watersheds for the four water years from 2011 to 2014. Eastern redcedar encroachment resulted in reduction of both surface and subsurface flows and the magnitude of reduction depended on annual precipitation. While there were nearly equal contributions between overland flow and subsurface flow, 87% of the total runoff from grassland watersheds occurred under saturated or nearly saturated soil condition, while 86% of runoff under encroached watersheds was generated under unsaturated soil condition, suggesting a shift from saturation excess overland flow to infiltration excess overland flow. These results permitted reconciliation of observed difference of streamflow responses associated with Juniperus spp. encroachment in the region and provided insights to better predict change in water resources under vegetation changes in subhumid regions of the south-central USA. Plain Language Summary The rapidly increasing acreage encroached by juniper species in the southcentral Great Plains has recently attracted the attention of land owners, natural resource managers, water managers, and policy makers because of the potential effect on streamflow and water supply. However, there is no general consensus whether conversion from grassland to juniper woodland will increase or decrease streamflow. In 2010, we instrumented seven experimental watersheds, three on grassland sites, and four on adjacent juniper (eastern redcedar) woodland sites to directly measure streamflow from those watersheds in Payne County, Oklahoma. Results showed that the runoff depths in eastern redcedar encroached watersheds were much lower than from grassland watersheds for the 4 years from 2011 to 2014. To sustain water resources in the southern Great Plains, watershed managers should use best management practices to control or prevent rapid woody plant encroachment into the grassland.