TRANSITIONARY BIOMES AND THE VERTICAL FLUX OF WATER IN SEMIARID RANGELANDS

For more than a century, western semiarid rangelands fluctuated between native and exotic ecosystems. While grassland savannas in more mesic regions were encroached by mesquite, ashe and juniper woodlands, native sagebrush steppe were yielding to exotic grasses such as cheatgrass, red brome and lovegrass. Both transitional biomes are the likely result of changing herbivory, landuse, and fire frequency. For example, in Texas, woody species are removed in grasslands to maintain streamflow, while in Nevada exotic grasses are controlled in shrublands to reduce fire potential and maintain biodiversity. Regardless of management approaches, any transgression between biome will disrupt hydrologic functions of the ecosystem and impact water balance. Understanding these disruptions are important for a host of practical reasons. In this study, we hypothesize that grassland (native or exotic) transpiration is lower, resulting in higher flux below the root zone, and we investigate unsaturated zone processes spatially and temporally in two regions of the US characterized by climatic gradients and transitional grassland biomes, one encroaching, the other encroached. We use a one-dimensional soil moisture model (HYDRUS) distributed at 14km scale across the CONUS-Soil, a multi-layer soil characteristics data set for the conterminous United States, and 100 years of synthetic climate data generated from ~30 years of North American Regional Reanalysis gridded data. The model is implemented across a precipitation/temperature gradient from (1) central to west Texas and (2) southern Idaho to central Nevada. Response variables include estimates of water flux below the root zone and travel time to bedrock. We simulate biome transition using a (1) linear diffusion approach over 10, 50, and 100 years, and (2) instantaneous annual change such as by fire. Results indicate that soil water retention and depth are as critical to deep soil recharge as plant species transition, and that groundwater response times are several decades in areas of deeper profiles or finer textured soils. In shallower soils, little effect is noted because either all moisture is lost to ET or runoff (which could ultimately result in groundwater recharge elsewhere).