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The MODSIM model has been widely used for water resource planning purposes by private, <br />municipal, quasi-municipal, and federal water users including: the cities of Fort Collins and Greeley, <br />the Northern Colorado Water Conservancy District, Denver Water, Colorado Water Resource and <br />Power Development Authority, the U. S. Bureau of Reclamation, and others. <br />Basic Description of Model Operation and Function <br />MODSIM is written in FORTRAN 77 and is currently operational in the PC and workstation <br />environment. It can be operated on a monthly, weekly, or daily time-step. MODSIM is a <br />capacitated network flow model in which the components of the system are represented by an <br />interconnection of nodes (diversions points, reservoirs, points of inflow/outflow, demand locations, <br />stream gages, etc.) and links that have a specified direction of flow and maximum capacities (canals, <br />pipelines, and natural river reaches). This structure generally reflects the "real" network that requires <br />knowledge by the user and data. In order to consider the demands, inflows, and desired reservoir <br />operating rules, MODSIM internally (and on its own) creates a number of "accounting" nodes and <br />linkages that are intended to ensure mass balance throughout the network. <br />The network can be visualized as a resource allocation system through which the available water <br />resource can be moved from point to point to meet various demands. The underlying principle of a <br />network solver is that this movement of the water is to be accomplished in a manner that will <br />maximize its value (or, conversely, minimize its "cost"). In a water rights sense, the costs can be <br />based on water right priorities or some other weighting factor serving to prioritize water allocation. <br />MODSIM employs an optimization algorithm to interactively solve the entire flow network to <br />achieve this minimum cost while satisfying mass balance and maintaining flows through the links <br />within required limits. This form of network solution ensures that available flows in the system are <br />allocated according to user specified operational rules and demand priorities. Originally MODSIM <br />incorporated a procedure known as the "out-of-kilter" algorithm. Current versions, including the <br />workstation version, employ a network flow algorithm based on Lagrangian Relaxation theory. <br />In developing the real network, all of the nodes and linkages must be defined and constrained by user <br />supplied upper and lower capacity limits (e.g., diversions and storage), and flow though the linkages <br />must be unidirectional with respect to positive flow. <br />MODSIM operates two types of demand nodes, flow through demands and terminal demands. The <br />former represent demands that flow past a demand node and remain in the network for subsequent <br />diversion (e.g., in-stream flow rights). Terminal demands can represent diversions that will generate <br />return flows or consumptive demands in which all of the water contributing to a demand is <br />consumptively used. The model can also accommodate reservoir operations and accounting, <br />hydropower, channel routing, and imports and exports from the network. Although not contemplated <br />as an immediate need for the CRDSS, MODSIM also can simulate the interaction between the <br />surface stream and the ground water aquifer. <br />Specific Aspects of Functionality <br />Reservoir Accounting . From inspection of the source code and interviews with Dr. Labadie and <br />users familiar with the application of MODSIM, it was concluded that the model has the capability to <br />simulate reservoir operations, including multiple accounts and ownerships within a reservoir. The <br />latter is accomplished by creating a "parent-child" relationship in which the parent represents the <br />2 <br />A275 05.10.94 1.15-5 Fosha, Hyre <br />