The partitioning of evaporanspiration along the grassland-forest continuum: Ecohydrological implications of microclimatic trends and response to amount of woody plant cover

Evapotranspiration dominates the water budget in drylands, exerting important controls not only on the dynamics of water, but also on the amount and distribution of vegetation on a landscape. The spatial and temporal variability of vegetation cover imposes constraints on key ecohydrological processes that feedback to the dynamics of evapotranspiration and, most importantly, its partitioning between direct evaporation and transpiration from plants, one of the most significant ecohydrological challenges. Yet, lacking are systematic evaluations of how variations in woody plant cover---a fundamental vegetation attribute of landscapes that can vary spatially with amount of cover and temporally with leaf phenology---influence the dynamics of soil microclimate and ultimately the partitioning of evapotranspiration into its components. This study presents the results of field experiments that systematically evaluated the effects of amount of canopy cover and its seasonality in both surface microclimate and soil evaporation. These field observations are complemented by controlled experiments that directly evaluate the relationship between amount of canopy cover and the partitioning of evapotranspiration, with an assessment of its larger-scale implications using a regional land surface--atmosphere model. Finally, this study presents a classroom-adaptation of the evapotranspiration partitioning experiment that was used to effectively translate new scientific concepts and information into k-12 classrooms. Overall, the results from this study provide a comprehensive understanding about the interactive ways in which canopy cover, canopy structure attributes and plant phenology influence soil surface microclimate---characterized by near-ground solar radiation and soil temperature---and soil evaporation. More specifically, the results illustrate how the main control of deciduous-woody vegetation on soil evaporation is the addition of litter to the surface. However, in absence of litter, attributes of woody cover influence soil evaporation variably with season and phenology. Further, The results from this study illustrate how the partitioning of evapotranspiration exhibits a non-linear response to amount of woody canopy cover. Notably, when incorporated into a regional surface-atmosphere model, this non-linearity strongly affects water fluxes, highlighting the potential implications for ecological, hydrological, and atmospheric processes associated with the partitioning of evapotranspiration, providing important insights for natural resource management.