Hydrological analysis in watersheds with variable-resolution global climate models (VR-CESM)

Traditionally, watershed-scale hydrology is simulated by distributed hydrologic models with offline coupling or forcing MET data, and/or the coupling of regional climate models and hydrological models that link atmospheric and land hydrology interactions. Global climate models are seldom used to study watershed-scale hydrology due to the relatively coarse grid resolution and simplified physical processes. Recently, however, watershed-scale hydrology analysis has become possible in Global Climate Models (GCMs) due to the development of variable-resolution GCMs that dynamically couple the hydrological processes between atmospheric and land systems at fine resolutions in selected regions and coarse resolution elsewhere. In this study, we use variable-resolution Community Earth System Model (VR-CESM) with refined-resolution (14 km) in the western U.S. and eastern China to evaluate watershed-scale hydrology. We compared the historical VR-CESM outputs with gauge measurements and other hydrological models (e.g., National Water Model in the U.S.), and calibrated the subsurface runoff capacities at each watershed. An RCP8.5 model projection from 2007 to 2050 is used to estimate the impact of changing precipitation and snow climatology on watershed hydrology. We also analyzed the extreme events and looked into the systematic hydrologic response to extreme weather conditions simulated by the VR-CESM. We conclude that refined-resolution VR-CESM can be used for watershed-scale hydrology analysis to understand water resources and water balance, in addition to the traditional watershed-scale hydrological models. It enables hydrological analysis at multiple watersheds in one simulation and helps understand the 2-way dynamics between land surface hydrology and atmospheric processes. However, it is challenging to apply VR-CESM for hydrologic analysis in regulated watersheds as human factors (e.g., pumping, irrigation, water diversion) that have not been fully resolved in VR-CESM.