CRDSS_Task1_15-4_EvaluationOfUSBRModelHYDROSS

Home Browse Search

stream flow data, adjusted for historic diversions, return flows, and storage. The USBR has developed a virgin flow database which apparently is based on estimates of irrigated acreage and evapotranspiration (ET). The ET varies by month and year to simulate a time series history. Although the model involves a somewhat broad-scale analysis, it may be of use in the development of the CRDSS. It would however require additional effort by the CRDSS Project Team to review this existing virgin flow data set and make the modifications that may be necessary for the CRDSS applications. Once the pristine natural flow data are determined, HYDROSS begins the process of allocating water to the water rights in a strict sequential order of priority. The operations also proceed in a strict time sequence (the results from one month depend on the state of the river system at the end of the preceding month) and are cognizant of the spatial relationships in the system network (the results at one node are dependent upon what is occurring at other upstream and downstream nodes). HYDROSS attempts to meet demands for water for: (a) instream flow requirements; (b) power requirements; (c) diversions; and (d) storage. HYDROSS recognizes a difference between "natural flow" (that which would occur absent any man- related activity) and "project flow" (water that has been stored or otherwise appropriated as project water). The distinction is made between natural and project flows because originally the model was developed to assess water resources implications related to federal projects. Water rights decreed to divert natural flow are allowed to divert water subject to a number of constraints and limitations including consumptive demand, headgate capacity, etc. Diversions are also limited to the availability of supply after senior water rights in the network are satisfied. Project water rights generally operate by priority among themselves and typically do not divert natural flows until all natural water rights are satisfied. Project water cannot be withdrawn under a natural water right. In the CRDSS applications, most of the water rights would be considered as natural water rights, including those rights owned by the federal government (e.g., the Aspinall Unit water rights) since they do carry their own decreed priorities. HYDROSS does not specifically allow the assignment of a priority date to a reservoir for storage. Storage is apparently automatically assigned the lowest priority available, which in effect will allow storage of all remaining water not used in the basin. However, reservoirs with decreed priority dates can be modeled by using a diversion to deliver the water to the reservoir (the model treats a decreed reservoir as off stream and supplied via a feeder ditch). In the operation of HYDROSS, there are three basic components of operation to consider at each node. Inflow . This consists of: (a) the available flow from upstream nodes; (b) return flows from upstream diversions that are occurring within the current month (including lagged returns from prior months); (c) the incremental pristine gain (loss) to the river between nodes; and (d) a category for "other gains" which is a means for forcing water into the network, if desirable. Station Action . This represents the actions at a node (station) once the inflow is determined. These actions include: (a) diversions, taking into account the demands of more senior priorities at downstream nodes; (b) evaporation (if there is a reservoir at the node); (c) change in storage content (if there is a reservoir); (d) a component for "other losses," a specialized form of diversion; (e) an allowance for an in-stream flow bypass, if applicable; and (f) an inflow-outflow calculation to determine the "available flow" remaining after all other demands at the node are satisfied. 2 A275 05.10.94 1.15-4 Fosha, Hyre