Role of Biological Soil Crusts in Hydrologic Modeling of Western High-Desert Ecosystems

Biological soil crusts (biocrusts) are the dominant living cover of many dryland surfaces in the western United States and possess properties thought to influence local hydrology. Little agreement exists, however, about the potential effects of biocrust presence on infiltration and runoff processes relative to the controls of other soil and vegetation factors. To improve predictive capabilities of dryland hydrologic models, this study characterizes the soil moisture dynamics of areas with similar soil characteristics, but varying degrees of crust cover at a climate manipulation experiment near Moab, UT. Previous simulated rainfall treatments (now ceased) at this site have resulted in dramatic biocrust mortality over several plots monitored with multiple soil moisture sensors, thus allowing for comparison of soil water dynamics across a mosaic of different biocrust cover. We utilize on-site meteorological data as input to a plot-scale hydrologic model, modified to include local processes: (1) temperature-dependent precipitation partitioning, snow accumulation and melt; (2) seasonally-variable potential evapotranspiration, and (3) species-specific transpiration factors. Soil and vegetation parameters in the model were determined from field measurements or through calibration using the Shuffled Complex Evolution algorithm. Simulation results were compared to five years of soil moisture observations at each study plot. Residuals between the observations and model outputs were then regressed against measurements of biocrust percent cover, plant species, and surface roughness to identify potential physical controls. Study results are shown to help make inferences on the potential role of biological soil crusts in mediating infiltration and evaporation processes in dryland ecosystems that are important for water supply in the western United States.