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• Paired watershed is effective in studying runoff response to woody encroachment. • Rooting zone soil moisture increased after converting juniper woodland to grassland. • Juniper removal in central Great Plains resulted in fivefold increase in runoff. Mechanical removal of encroached juniper (Juniperus spp.) is a common practice to restore native grasslands. However, the hydrological responses to grassland restoration remain mostly unquantified for the climate transition zone in the southern Great Plains of the USA where ecosystem evapotranspiration is highly sensitive to the change of vegetation functional type. We used a paired watershed approach to directly quantify the impact of mechanical removal of eastern redcedar (Juniperus virginiana L., redcedar) and restoration to native prairie or establishment of switchgrass (Panicum virgatum L.) on root zone soil moisture and event-based runoff for eight years including three main phases - calibration, transition, and restored grassland in north-central Oklahoma, USA. Results showed that the root zone soil water storage on average increased 1.6 and 1.9 times for restored prairie and switchgrass, respectively, after juniper removal. The regression model estimation based on the relationships established in the calibration phases between event-based runoff from control watershed and the treatment watersheds found that the restored prairie and switchgrass production system increased annual runoff by 4.46 and 4.54 times, respectively. These results indicated that both soil moisture and runoff are highly responsive to land use change in the southern Great Plains. Reestablishment of herbaceous dominance by mechanically removing encroached woody species is closely followed by restoration of soil moisture dynamics and watershed runoff regime. [ABSTRACT FROM AUTHOR]

In the Great Plains of the central United States, water resources for human and aquatic life rely primarily on surface runoff and local recharge from rangelands that are under rapid transformation to woodland by the encroachment of Eastern redcedar (redcedar; Juniperus virginiana) trees. In this synthesis, the current understanding and impact of redcedar encroachment on the water budget and water resources available for non-ecosystem use are reviewed. Existing studies concluded that the conversion from herbaceous-dominated rangeland to redcedar woodland increases precipitation loss to canopy interception and vegetation transpiration. The decrease of soil moisture, particularly for the subsurface soil layer, is widely documented. The depletion of soil moisture is directly related to the observed decrease in surface runoff, and the potential of deep recharge for redcedar encroached watersheds. Model simulations suggest that complete conversion of the rangelands to redcedar woodland at the watershed and basin scale in the South-central Great Plains would lead to reduced streamflow throughout the year, with the reductions of streamflow between 20 to 40% depending on the aridity of the climate of the watershed. Recommended topics for future studies include: (i) The spatial dynamics of redcedar proliferation and its impact on water budget across a regional hydrologic network; (ii) the temporal dynamics of precipitation interception by the herbaceous canopy; (iii) the impact of redcedar infilling into deciduous forests such as the Cross Timbers and its impact on water budget and water availability for non-ecosystem use; (iv) land surface and climate interaction and cross-scale hydrological modeling and forecasting; (v) impact of redcedar encroachment on sediment production and water quality; and (vi) assessment and efficacy of different redcedar control measures in restoring hydrological functions of watershed. [ABSTRACT FROM AUTHOR]