Among the key problems in atmospheric and hydrologic sciences are the modeling of the interaction between the atmosphere and land surface hydrology while also quantifying the surface/subsurface hydrologic flow processes both in vertical and lateral directions, and modeling the heterogeneity in surface and subsurface hydrologic processes. Meanwhile, in standard water resources engineering practice, the planning and management of the water resources is performed over the geographical region of a watershed. To address these issues, a model of coupled atmospheric-hydrologic processes at the watershed scale, the Watershed Environmental Hydrology Hydro-Climate Model (WEHY-HCM), has been developed. The atmospheric model PSU/NCAR MM5 (Fifth Generation Mesoscale Model) was coupled to the watershed hydrology model WEHY through the atmospheric boundary layer to form the WEHY-HCM. The WEHY-HCM is especially useful for producing nonexistent atmospheric data as input to the modeling of surface and subsurface hy...

The objective of this study is to evaluate the potential of the Watershed Environmental Hydrology Hydro-Climate Model (WEHY-HCM) for modeling runoff at ungauged or sparsely gauged watersheds. The WEHY-HCM employs an atmospheric module (fifth generation mesoscale model, MM5) that is coupled with its process-based watershed environmental hydrology (WEHY) module. In this study the atmospheric component of the WEHY-HCM was utilized for the dynamical downscaling of the coarse U.S. National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) historical global reanalysis atmospheric data over a foothills region in Northern California in order to reconstruct the historical hydro-climate data over four watersheds in the foothills region at 3 km grid scale at hourly intervals. The WEHY-HCM’s atmospheric module performance was evaluated by the comparison of model-reconstructed precipitation and air temperature against ground observations in time and space with satisfactor...

To establish a basinwide water management plan for the Tigris-Euphrates (TE) watershed, it is necessary to perform rigorous water balance studies of the whole watershed—at least for critical historical drought and flood conditions and under various water resources development scenarios. Water-balance studies over the watershed require climatic and hydrologic data sets, corresponding to historical critical flood and drought periods, at fine time and spatial grid resolutions provide the necessary hydroclimatic information. The Regional Hydroclimate Model of the Tigris-Euphrates (RegHCM-TE) and the associated geographic information system (GIS) were developed to downscale large-scale atmospheric data sets over the TE watershed and to reconstruct the aforementioned climatic and land hydrologic data sets at fine spatial and time increments. In RegHCM-TE, the earth system over the TE watershed is modeled as a fully coupled system of atmospheric processes aloft coupled with the atmospheric boundary layer, land-surface processes, surface and subsurface hydrologic processes, and the interactions among the various hydrologic and atmospheric processes. The parameters of the model are estimated by using existing global and local GIS data. By using the large-scale global historical atmospheric databases as its initial and boundary conditions, RegHCM-TE has been used to reconstruct historical hydroclimate data at a grid size of 15 km over the TE watershed, for which only very sparse hydrologic observations were available. RegHCM-TE and its model validation are described in this paper.

Field study is an essential component of hydrologic science because all hydrological studies must be conducted by such observation-based knowledge of real watersheds. Hillslope runoff processes have been intensively investigated, but the flow process at the boundary between the snowpack and ground surface has not been well documented. A field site at the northwest sector of the Ward Creek watershed, Lake Tahoe Basin, was built for observations of overland flow, subsurface stormflow, and channel flow, simultaneously with atmospheric measurements to examine the hydrology at a snow-covered hillslope. Also, the groundwater table under the snowpack was monitored by shallow wells at the hillslope. All field-measured atmospheric data were synthesized with an energy-balance snow model, and the snowmelt rate and energy balance were computed. The results of the analyses indicate that most of the snowmelt water infiltrated into the topsoil layers and that saturated subsurface flow was the largest contributor to the stream channel flow throughout the observation period. However, overland flow or longitudinal flow within the snowpack may still happen even over an unfrozen and unsaturated topsoil on a relatively mild hillslope (16%). The quantification of observations of overland/within snowpack flow showed that the snowpack enhanced the overland runoff, which caused higher peaks in flow discharge and sediment production. Analysis of the major rain-on-snow event of May 7, 2000, revealed that snowmelt water, caused by the energy flux from raindrops, might not be the major contributor to the runoff peak discharge since very little snowmelt was observed during the rain-on-snow event. Consequently, spring storm hydrographs in the Sierra Nevada seem to be affected by the high-soil water in the topsoil because of the daily water supply by snowmelt and by overland/within snowpack flow caused by the capillary suction forces in the snowpack.

Long-term water supply data are important for the current practice of water resources management at a target region. However, long-term water outflow data from reservoirs are typically limited at fine time resolution (hourly). In this study, the historical data on water supply from the Shasta Dam were reconstructed by a reservoir operation model with reconstructed reservoir inflow data. Before embarking on this exercise, first the inflow data were reconstructed by means of a watershed environmental hydrology hydroclimate model, with its input provided from historical atmospheric reanalysis data. The reanalysis data used in this study are from the National Center for Atmospheric Research and the National Center for Environmental Prediction (NCAR–NCEP) at spatial resolution of 2.5° (210 km at the modeled region). The NCAR–NCEP reanalysis data were dynamically downscaled to 3-km spatial grid resolution at hourly intervals by means of the regional atmospheric component of the hydroclimate model. The d...