**4.3 Models developed by federal agencies in the United States**

Most of the large federal reservoirs in the U.S. were constructed and are operated by the U.S. Army Corps of Engineers (USACE) or U.S. Bureau of Reclamation (USBR). The USACE has over 500 reservoirs in operation across the nation as well as many navigation locks, hydropower plants, and flood control structures. The USACE operates essentially all of the reservoir projects that it has constructed. The USBR has transferred operation of many of its projects to non-federal sponsors upon completion of construction but continues to operate about 130 reservoirs and appurtenant structures in the 17 western states. The USACE plays a dominant role in the U.S. in operating large reservoir systems for navigation and flood control. The USBR water resources development program was originally founded upon constructing irrigation projects to support development of the western U.S. The responsibilities of the two agencies evolved over time to emphasize comprehensive multiple-purpose water resources management.

The USACE and USBR developed many models for specific reservoir systems during the 1950's-1970's (Wurbs, 1996, 2005a). Many of these system-specific models have since been replaced with generalized models. The USBR currently uses RiverWare and MODSIM, which are described later in this chapter, and several remaining system-specific models. The USACE Hydrologic Engineering Center (HEC) maintains a suite of generalized simulation models that are widely applied by water agencies, consulting firms, and universities throughout the U.S. and the world. This chapter later focuses on HEC-ResSim but several other HEC products are also noted below.

Generalized Models of River System Development and Management 13

The four modeling systems were developed by water agencies and university research entities and have been extensively applied in both the U.S. and other countries. The software was developed for application by model-users other than the original developers and is accessible by water management professionals throughout the world. The ResSim, MODSIM, and WRAP software and documentation can be downloaded free-of-charge at the websites listed in Table 1. RiverWare is a proprietary software product which is available for a licensing fee as described at the website shown in Table 1. The four software packages all run on personal computers operating under Microsoft Windows and all have also been executed with other computer systems as well. RiverWare was developed primarily for Unix workstations though it also is used on personal computers with Microsoft Windows. The four alternative modeling systems and their predecessors have evolved through many versions over more than twenty years of research and development, with new versions being released periodically. The modeling capabilities provided by each of the models have changed significantly over time in the past and continue to be improved and expanded.

**5.1 Hydrologic Engineering Center (HEC) Reservoir Simulation (ResSim) model** 

The USACE HEC initiated development of ResSim in 1996. ResSim was first released to the public in 2003 with the intention of eventually replacing HEC-5, which has been extensively applied for over 30 years. Documentation currently consists of a Users Manual (Hydrologic Engineering Center 2007) and other information found at the website in Table 1. ResSim is designed for application either independently of the previously discussed Corps Water Management System or as a component thereof. Applications have included the Sacramento and San Joaquin River Basins in California and Tigris and Euphrates River Basins in Iraq.

ResSim is comprised of a graphical user interface, computational program to simulate reservoir operation, data management capabilities, and graphics and reporting features. Multiple-purpose, multiple-reservoir systems are simulated using algorithms developed specifically for the model rather than formal mathematical programming methods. Meeting the needs of USACE reservoir control personnel for real-time decision support has been a governing objective in developing ResSim. The model is also applicable in planning studies. The full spectrum of multiple-purpose reservoir system operations is modeled. Particularly

The user-selected computational time-step may vary from 15 minutes to one day. Stream flow routing options include Muskingum, Muskingum-Cunge, modified Puls, and other methods. Stream flow hydrographs provided as input to ResSim can come from any source, including being generated with the HEC-HMS Hydrologic Modeling System. Multiplereservoir systems, with each reservoir having multiple outlet structures, are modeled. Release decisions are based on specified storage zones that divide the pool by elevation and a set of rules that specify the goals and constraints governing releases when the storage level

MODSIM is a general-purpose reservoir/river system simulation model based on network flow programming developed at Colorado State University (Labadie 2006; Labadie & Larson 2007). The model has evolved through many versions, with initial development dating back

detailed capabilities are provided for modeling flood control operations.

**5.2 MODSIM river basin management decision support system** 

falls within each zone.

The Corps Water Management System (CWMS) is the automated information system used by the USACE nationwide to support real-time operations of flood control, navigation, and multiple-purpose reservoir systems (Fritz et al., 2002). The CWMS is an integrated system of hardware and software that compiles and processes hydrometeorology, watershed, and project status data in real-time. A map-based user-friendly interface facilitates modeling and evaluation of river/reservoir system operations. CorpsView, a spatial visualization tool developed by the HEC based on commercially available geographic information system (GIS) software, provides a direct interface to GIS products and associated attribute information. The CWMS combines data acquisition/management tools with simulation models which include HEC-HMS (Hydrologic Modeling System), HEC-ResSim (Reservoir Simulation), HEC-RAS (River Analysis System), and HEC-FIA (Flood Impact Analysis).

The HEC-5 Simulation of Flood Control and Conservation Systems model (Hydrologic Engineering Center 1998) has been used since the 1970's in many USACE and non-USACE studies, including investigations of storage reallocations and other operational modifications at existing reservoirs, feasibility studies for proposed new projects, and support of real-time operations. The HEC plans to eventually replace HEC-5 with HEC-ResSim. However, HEC-5 is still available at the HEC website and continues to be applied by various model-users.

HEC-5 simulates multiple-purpose reservoir system operations for inputted unregulated stream flows and reservoir evaporation rates using a variable time interval. A monthly or weekly time step may be used during periods of normal or low flows in combination with a daily or hourly time step during flood events. HEC-5 makes release decisions to empty flood control pools and to meet user-specified diversion and instream flow targets based on reservoir storage levels and stream flows at downstream locations. Flood routing options include modified Puls, Muskingum, working R&D, and average lag. Optional analysis capabilities include computation of expected annual flood damages and water supply firm yields.

The HEC Prescriptive Reservoir Model (HEC-PRM) was developed in conjunction with studies of reservoir systems in the Missouri and Columbia River Basins. Later applications include studies of systems in California, Florida, and Panama (Draper et al., 2003; Watkins et al., 2004). HEC-PRM is a network flow programming model designed for prescriptive applications involving minimization of a cost based objective function. Reservoir release decisions are made based on minimizing costs associated with convex piecewise linear penalty functions associated with various purposes including hydroelectric power, recreation, water supply, navigation, and flood control. Schemes have also been devised to also include non-economic components in the objective function. HEC-PRM applications to date have used a monthly time interval.
