The Modular Modeling System

The USGS Modular Modeling System (MMS) is being used for the watershed modeling component of this study. MMS is an integrated system of computer software developed to provide a framework for the development and application of models to simulate a variety of water, energy, and biogeochemical processes (Leavesley and others, 1996).

MMS has three major components: pre-process, model, and post-process (fig. 3). The pre-process component includes tools used to input and analyze spatial and time-series data for use in model applications. The model component includes tools to develop and apply models. The post-process component provides tools to display and analyze model results, and to pass results to management models or other types of software such as the HDB. All three components include graphical user interfaces (GUIs) and data-management interfaces (DMIs). Optimization (Opt) and sensitivity (Sens) tools are provided to analyze model parameters and evaluate the extent to which uncertainty in model parameters affects uncertainty in simulation results. A modified version of the National Weather Service's Extended Streamflow Prediction Program (ESP) provides forecasting capabilities using historic or synthesized meteorological data. A major feature of the model component is the module library that contains a variety of modules for simulating water, energy, chemical, and biological processes. The user, through an interactive model builder interface (xmbuild), selects and links modules to create a specific model.

A GIS interface, termed the GIS Weasel (Leavesley and others, 1997), is part of the MMS. The Weasel facilitates model development, and also can be used as a watershed analysis tool. A digital elevation model for the basin provided the input to Weasel for automatic delineation of the basin and for delineation of subbasins to be modeled or analyzed. Based on locations of streamflow gages, outlets of ungaged watersheds, and Reclamation water-management points, 59 subbasins were defined (fig. 4).

Models are being constructed for 38 subbasins, all of which are non-agricultural. The models are using the precipitation- runoff modeling system (PRMS; Leavesley and others, 1983) part of MMS that allows for the spatial distribution of hydrologic parameters by partitioning or characterizing a subbasin into hydrologic response units. The response units are assumed to be homogeneous in their hydrologic response. The GIS Weasel allows for the characterization (partitioning) of subbasins into modeling response units (MRUs), which in this case are equivalent to the hydrologic response units needed for PRMS.The GIS Weasel also provides a menu for examining and modifying the MRU boundaries for a subbasin; as part of the examination process, any attribute currently residing in the GIS for a MRU is provided. Data derived from the original digital elevation model (e.g., slope, aspect) and other gridded attribute data (e.g., vegetation, soil depth) can be used to estimate selected MRU and subbasin parameters. The MRUs were characterized using the Weasel based on information about landscape characteristics, precipitation, and soil characteristics. Initial characterization of subbasins resulted in about 1,000 MRUs.

Within the modeling process, a daily water balance is computed for each MRU on the basis of climate inputs (daily values of air temperature, precipitation, and solar radiation), and model parameters. The responses of all MRUs are summed on a unit-area basis to give a daily streamflow estimate at the outlet of the modeled area. If more than one subbasin is included in a particular watershed model, individual MRUs from each subbasin can also be summed to determine their contribution to the total streamflow hydrograph.

Output from the watershed models will be input into the HDB for use by the river and reservoir management model component of the DSS, called RiverWare. RiverWare is being applied by Reclamation's analysts.