Development of hydrologic response unit delineation criterion and atmospheric hydrologic model link

Atmospheric models such as a Global Circulation Models (GCMs) or regional atmospheric models provide a basis for numerous applications ranging from weather forecasting to quantifying the effects of long-term global change. The results from the atmospheric models can be used effectively in distributed hydrologic modeling environs to provide better quantification of surface runoff and evapotranspiration. However, the resolution of atmospheric model estimates is of the order of tens or even hundreds of kilometers. Distributed hydrologic models require the spatial data at much finer resolution, and therefore one step of downscaling is essential.; The spatial structure to a distributed hydrologic model can be provided by Hydrologic Response Units (HRUs). HRUs are spatial units with homogeneous hydrologic response. Several studies have used the concept of HRUs; however, there exists no analytical method in literature for HRU delineation. This study provides a statistical criterion for delineation of HRUs using first-order subwatersheds and variables of choice, including remotely sensed elevation, a Normalized Difference Vegetation Index (NDVI), vegetation type, and soil texture data. Adjacent first-order subwatersheds are merged together to form one HRU after comparing the characteristics of variables of neighboring subwatersheds. The effectiveness of this merge criterion is tested by comparing the resulting hydrologic response in the form of streamflow from the subwatersheds. This HRU delineation criterion is tested using a Climate Vegetation Hydrologic Model (CVHM); however, this methodology is not specific to any hydrologic model.; HRUs can serve as an effective link between atmospheric and hydrologic models. This study employs a simple orographic model, called ZOOM, to provide the regional atmospheric model (RegCM2) estimates at the center of each HRU, which in turn are used by a distributed hydrologic model, known as Climate Vegetation Hydrologic Model (CVHM). Comparison between the downscaled precipitation and temperature data with observed data shows reasonable agreement. A detailed analysis of these downscaled estimates in CVHM to quantify hydrologic responses (such as evapotranspiration and streamflow) from a mountainous watershed in northern Utah is presented.