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Water Resources Planning Model <br />The water resources planning model is a key component of the Colorado River Decision Support <br />System (CROSS). The purpose of this chapter is to provide a brief description of the model and <br />the data management interface utilities used to create the model's input files. <br />1.0 Introduction <br />2.0 StateMod Input File Development <br />3.0 Model Operation <br />4.0 Su~portin~ Materials <br />1.0 Introduction <br />The State of Colorado's Stream Simulation Model (StateMod) is the water resources planning <br />model used to assess past, present, and future water management policies of the river basins of <br />CROSS. StateMod is a monthly water allocation and accounting model that starts by computing <br />the base streamflows (i.e., those flows available in a stream system absent a portion of man's <br />diversions and use of the water resource). The model then allocates the available baseflows to the <br />water rights (diversions, storage, and instream flow) in strict conformance with the prior <br />appropriation doctrine. <br />StateMod represents the river basin using a network of nodes that are used to reflect the stream <br />system's physical and legal operational parameters. The nodes are typically located at major <br />stream features and include stream gaging stations, reservoirs, diversion structures, instream flow <br />reaches, and locations for imports/exports from the basin. Water rights (direct flow, storage, <br />instream rights, exchanges, and operational rules) are connected to a node in the network and are <br />assigned a priority number. Each structure (diversion, reservoir, or instream flow node) may be <br />further defined to reflect decreed water right amounts, structure capacity limitations, and ranking <br />by priority. <br />StateMod may compute baseflows or may use baseflows developed outside of the model. <br />Baseflows are generated by StateMod using an inverse modeling approach in which the monthly <br />streamflow data at the known gages are adjusted by adding back into the gage values the man- <br />made depletions attributable to modeled historic diversions and storage at the nodes. Once this <br />baseflow file is created, the model allocates water in the assigned priority sequence. For each <br />priority, the amount of water available at a structure is calculated by observing the flow at that <br />node and at all other downstream nodes to the lower end of the basin. The flow available is <br />generally the minimum of physical availability, legal availability, demand, and capacity. After <br />the diversion is made for the first priority, the flows at all of the nodes are adjusted downstream <br />