Woody Plant Encroachment has a Larger Impact than Climate Change on Dryland Water Budgets

Woody Plant Encroachment (WPE) into dryland grasslands is a global phenomenon that alters dryland structure and function. It has long been known that functional changes in vegetation alter hydrologic processes, but quantifying these changes at the watershed scale has been difficult due to the nonlinearities inherent to ecohydrologic processes. In this contribution, we utilize a process-based ecohydrologic model, the TIN-based Real-time Integrated Basin Simulator (tRIBS), constrained by extensive watershed observations, to quantify the ecohydrologic impacts of WPE on focused recharge through streambeds. We demonstrate that when grasslands convert to a shrubland with high bare soil coverage ('xerification') streambed infiltration is increased by 29% and the ratio of channel transmission losses to precipitation ( T _L / P ) raises from 0.17 to 0.22. When shrubs replace grasses without creating extra bare soil ('thicketization'), however, T _L is reduced by 18% and T _L / P decreases from 0.17 to 0.14. These changes to transmission losses are compensated by changes in plant available water in hillslope soils, where reductions (increases) in plant available water reduce (increase) ET for the xerification (thicketization) pathway. This finding highlights that feedback between shrub encroachment and plant available water may lead to further shrub encroachment. We utilize model simulations under independent and combined effects of climate change and WPE to assess the relative importance of each phenomenon in a late 21 ^st century condition. Results indicate that changes to focused channel recharge are determined primarily by the WPE pathway and not by the climate change signal. As a result, WPE should be given consideration when assessing the vulnerability of dryland groundwater aquifers to climate change.