CRDSS_Task1_15-3_EvaluationOfBESTSM

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made for the first priority, the flows at all of the nodes are adjusted downward to reflect the diversion, and the process is repeated for the next priority right. The amount of water available for the next priority can include any return flow that accrues to the river from the more senior diversions that occur in the same monthly time -step. Note that as a result of the minimum flow concept, no diversion would be allowed that would permit any node to become negative. Specific Aspects of Functionality Reservoir Accounting A key requirement of the CRDSS water rights planning model is the ability to model the operation of reservoirs and in particular the ability to operate multiple storage accounts within a reservoir, keeping track of the amounts stored and released for separate priorities and /or separate ownerships. From our inspection of the BESTSM source code, it appears that the model has this capability for handling any number of on- stream or off - stream structures. Each reservoir sub- account can be assigned a separate priority, and the model is reasonably flexible with respect to incorporating a set of operation criteria and rules. A mass balance is achieved using reservoir inflow, releases, evaporation, precipitation, and seepage. The losses can be prorated and allocated to the sub - accounts in a reservoir. Storage in a reservoir is limited by the decree and the maximum capacity and is further controlled by a single fill annually. Releases from a reservoir for diversion at another structure or node are managed by user - specified rules. It appears that many of the Blue River Reservoir operation features in the Green Mountain version of BESTSM are specifically written into the FORTRAN code, and use of the model code would likely require modification for more generic application. Exchanges Our review of the available version of BESTSM indicates that the model has the capability of accommodating water right exchanges on the river, i.e., a release of water to the stream system at one location (node) and diversion of a like amount at another location, generally upstream. The model apparently checks all of the intervening nodes to ensure that the upstream diversion, occurring by exchange, does not exceed the minimum flow available in the exchange reach. The model is limited in that it can only handle releases from a storage reservoir as the source of replacement in the exchange. It cannot handle an exchange from one direct diversion structure to another (or perhaps exchange of return flow from a direct flow use to another direct flow diversion). It appears that an exchange is assigned its own priority so that it can be operated in sequence during the allocation of available water. Return Flows Our inspection of the source code indicates that BESTSM has the ability to calculate return flows attributable to diversions at the nodes. It appears that the model allows for return flows to accrue to the river in the same time -step (month) that the diversion occurs. It is our observation that modeling return flows, in which a part of the returns are available for diversion within the same time -step, may require an iterative procedure in order for diversions pursuant to a water right to have the benefit of their own return flows. The amount of water available for diversion for a given water right is dependent upon the minimum flow in the network downstream of the point of diversion, and the observed minimum flow should reflect the return flows resulting from that current diversion. The returns can also be assigned to accrue at multiple node locations in the network. Lagged return flows can be specified by the user either as a percentage of the diversion or as a return flow table. Other Issues Additions and Modifications The input format for the BESTSM model is structured to allow additions of nodes (reservoirs, diversions, in- stream flows, etc.) relatively easily. However, for ,4275 05.10.94 3 1.15 -3 Fosha, Hyre