AgroEcoSystem-Watershed (AgES-W) Model Delineation and Scaling

Water movement and storage within an agricultural watershed can be simulated at different spatial resolutions of land areas or hydrological response units (HRUs). Interactions between HRUs in space and time vary with the HRU sizes, such that natural scaling relationships are confounded with the simulation scales. Scaling concepts can be tested using the AgroEcoSystemWatershed (AgES-W) model with different resolutions of HRUs. Here, we delineated a 56-ha agricultural watershed in northern Colorado, USA farmed primarily under a wheat-fallow rotation. Factors used to distinguish HRUs included topography (surface flow paths), land use (crop strips and native grass), and mapped soil units (three types). The delineation algorithm was able to produce HRUs that follow the land use and soil boundaries, but adjustment of some of the inputs remains a trial-and-error process to avoid excessive aggregation and undesired spatial features. AgES-W model parameters were first calibrated using single-HRU simulations to fit measured daily soil moisture at different landscape positions and depths where soil moisture is being measured. Simulations (not shown here) are being conducted using these fixed parameter sets applied across the watershed. Questions to be addressed include: How do spatial soil moisture and runoff at the outlet vary with different HRU delineations? Is there an optimal or threshold spatial resolution of HRUs with respect to model fit to data? The results will help guide HRU delineation and hydrological simulations in this and other semi-arid agricultural watersheds.