SHRUB ENCROACHMENT AND CLIMATE CHANGE CONTROLS ON MOUNTAIN FRONT RECHARGE IN DRYLANDS

Mountain system recharge is often the primary groundwater recharge mechanism in dryland systems, out of which focused recharge through ephemeral streambeds is the most common pathway. The relative importance of this pathway and the factors that control it, however, are poorly constrained. In this study, we utilize multiple lines of observational evidence to demonstrate that focused recharge via an ephemeral streambed is an important component of the water balance (~20% of annual precipitation) in a mountain front ecosystem located in the Jornada Experimental Range, New Mexico, USA. This system has undergone extensive changes via woody plant encroachment (WPE) and climate change over the last 100 years. Both changes are global phenomenon that alter dryland system structure and function. The hydrologic impacts of these changes, however, have been difficult to quantify due to nonlinearities in ecohydrological processes and a changing climate. Using a process-based hydrologic model constrained by watershed observations, we first test the climate factors that facilitate focused recharge by driving the model with stochastically-downscaled meteorological forcings from a variety of climate models, emissions scenarios, and future periods. We demonstrate that the daily storm size is the primary climate factor that controls streambed infiltration and that extreme events larger than the 97^th percentile generate almost 50% of the focused recharge. We then utilize the model to examine the impact of WPE on the water budget and find that the effects of WPE depend primarily on the desertification pathway. We show that the conversion of grasslands to shrublands with high bare soil coverage increases channel transmission losses by 29%. When shrubs replace grasses without creating extra bare soil, however, transmission losses are reduced by 18%. Finally, model simulations are used to assess the relative roles of WPE and climate change in controlling channel transmission losses for a late 21^st century condition. The combined simulations indicate that changes in transmission losses are primarily determined by the WPE pathway and not the climate change signal. As a result, shrub encroachment should be given consideration when assessing the vulnerability of mountain groundwater aquifers to climate change.