Global drylands covering nearly 40% of the Earth’s land surface have been dramatically transformed by the replacement of desert grasslands with woody shrubs. Woody shrubs with deep roots have a competitive advantage over grasses with shallow roots since they can access deep subsurface water during dry seasons or drought periods. However, the role of deep subsurface water in supporting woody shrubs and the ecohydrological mechanisms by which this occurs are poorly understood globally and for areas with complex topography. 

Our overall hypothesis is that woody shrubs affect the redistribution of subsurface water depending on the terrain conditions. This is hypothesized to occur due to: (1) a redistribution of overland flow into deep subsurface areas that is enhanced by woody shrubs, (2) a redistribution of water from the wet season to the subsequent dry season, and (3) the access of this subsurface water exclusively by deep-rooted shrubs. This carry-over of deep subsurface water from the wet season to the following dry season and its access by woody shrubs is believed to have important consequences on carbon and water cycles in global drylands.

This proposal has a tight integration of ecosystem monitoring and simulation scenarios using a Modeling-Experimental (ModEx) approach. We formulate five specific hypotheses at four study sites in a 2x2 factorial design that quantifies the effects of terrain slope and of woody shrub density at the Jornada Experimental Range of New Mexico, USA. An ecohydrological model will be iteratively enhanced using the data collected at the four sites and then used to extrapolate findings across different terrain, plant cover, and rainfall conditions. This approach will yield new understanding of important, but poorly recognized, subsurface hydrological processes affecting the annual carbon and energy budgets.