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INTRODUCTION
Meena (1993) compared the WIRSOS planning model with the USBR's HYDROSS model. The comparison revealed that WIRSOS was not as good at modeling reservoir operations as HYDROSS. This was an expected result, since HYDROSS was written to determine the availability of water for reservoirs.The algorithms of HYDROSS were analyzed in detail for this study to determine how HYDROSS performs reservoir operations. Once sufficient data on HYDROSS and its operation were gathered, its best features (from Wyoming's perspective) were incorporated into WIRSOS. In addition to improving reservoir operation by WIRSOS, the post-model operation of restructuring output files was incorporated into the model and critical data files are checked for proper arrangement for model operation.
Reservoir operations performed by WIRSOS corresponded to Wyoming State Water Law. However, most large reservoirs in Wyoming are not owned or operated by the state, and even fewer are operated exactly according to state law. One of the reasons for modifying WIRSOS was to more precisely simulate the amount of water available in a river system.
Subroutines were written or modified to:
How these tasks were accomplished is discussed in detail in the following sections.
OPTIONAL INPUT
When modifying a program, its former users should be able to utilize the new version with only minor modifications to existing data bases; therefore, all modifications to WIRSOS are optional, not mandatory, for model operation. However, the inclusion of off-channel storage reservoirs in the WIRSOS model has necessitated the addition of one line to each reservoir data record. At the start of the program (when the user is prompted to name the input files, name the output files, name the run, declare the number of stations where runoff data is input, and declare whether reservoirs are to be modeled), the user will be able to choose which options are used during the current simulation. Except for the reservoir data files, model users will not have to update existing data bases to run the modified program. They also do not need two versions of the program to run old and new models. The model user will have to modify existing data bases if they want existing models to benefit from the modifications made to WIRSOS through this study.OUTPUT
To be most acceptable to the model users, two WIRSOS output files are modified into an easier to understand format. This was accomplished by executing a program that reads these two files, modifies the data format, and prints the information to a new file. This restructuring of output files is now done internally by WIRSOS reducing the number of post model operations performed by the user. The result is easier model operation.DATA CHECKING
WIRSOS is sensitive to the order in which data is entered into the model. The diversion rights, instream flow rights, junior project rights, and reservoir rights data files must be sorted by priority data and station number. For these data bases,, the oldest right is entered first then the second oldest right. This is repeated until the most junior right has been entered into the data file. When two rights have the same priority date, the right furthest upstream is entered first. The station data file is also sorted by station number, the station furthest upstream is entered first then the next furthest. This is repeated until the most downstream station has been entered. Because the way these files are arranged is critical to model operation, the model checks the arrangement of these files prior to processing the information contained in the files.RESERVOIR OPERATION
The principal difference between WIRSOS and HYDROSS is the method in which priority dates assigned to reservoirs and the amount of water a reservoir can divert. In WIRSOS, each reservoir has a priority date and the maximum amount of water that can be stored by a reservoir in a year is assigned similar to other water rights. HYDROSS also assigns a priority to reservoirs. The priority assigned to each reservoir is after all other water rights have been satisfied. In addition, the amount that is diverted to a reservoir in HYDROSS is any water not used by diversions or Instream Flow Rights (IFR) during the current month. The first method simulates how Wyoming State Water Law declares a reservoir should be operated; and the second represents how reservoirs are often operated. WIRSOS had to allow both the diversion of water to a reservoir to satisfy the legal water right and the storage of extreme water in reservoirs after Wyoming State Water Law had been satisfied.WIRSOS is already equipped to handle legal diversions to reservoirs and a new subroutine was added to divert an extreme supply of water (water not needed by any other right on the river system) to reservoir storage. The subroutine is called after all water rights, including IFR's, are satisfied. The program then checks the flow at each reservoir and diverts any available water to storage. The subroutine does not comply with Wyoming State Water Law. It allows a reservoir to fill more than once during a water year and to retain more than its maximum permitted right during that year. This modification was necessary to allow modeling of reservoir operations on Wyoming rivers where this type of procedure can occur.
A drawback to allowing excess water to be stored in reservoirs is that a river can theoretically be drained, negatively impacting tourism, sport fishing, and public opinion. Therefore, the USBR and other reservoir operators often release a minimum amount of water through the outlet works to augment the flow below a reservoir. This water in general is used to preserve the esthetics of the river and support fish populations.
To model a reservoir, one of the new input variables to WIRSOS is a minimum release from a reservoir. This release can be any combination of project water, natural water, and IFR's. If the minimum release is not met and the release is less than the flow into the reservoir, the reservoir releases water to maintain the minimum release. The water is counted as a non- project release. The model user can set this value for each reservoir a lower minimum release can be set to reduce the amount of water released from storage.
OFF-CHANNEL STORAGE RESERVOIRS
A reservoir that is not on the main stream is considered an off-channel storage reservoir. For an offchannel storage reservoir, water is diverted to the reservoir at a station upstream from the reservoir station. This water is returned at another station downstream from the diversion station or to a station another stream. The size of the diversion becomes the limiting factor in deciding how much water is diverted to an off-channel storage reservoir. WIRSOS did not allow off-channel storage reservoirs; it returned water diverted to a reservoir at the point of diversion. To model an off-channel storage reservoir, the user had to use false diversions and elaborate delay tables (a very difficult and often inaccurate procedure). As an improvement to the operation of WIRSOS, the amount of water diverted to an off-channel storage reservoir is limited to the size of the diversion canal.RESERVOIR RIGHTS
One area that caused WIRSOS users considerable frustration was the method used to allocate water on a multiple right reservoir. Water stored in a reservoir in a year and held in storage until the next year was not assigned to a reservoir right. This allowed an early priority reservoir right to fill its full allocation every year. In actuality, the water stored by a given reservoir right, minus the water used from that right during the year, is the amount of water that is allocated to the reservoir right for the next year. This method was programmed into WIRSOS to assign water stored in a reservoir to a reservoir right. The output from WIRSOS was also modified to give an account of the status of the rights in each reservoir at the end of every year and to give the model user an idea of what was the limiting factor for reservoir storage at the reservoir right's priority date.Allocating stored water to each reservoir right created an additional problem with WIRSOS. Project water rights in WIRSOS were assigned to a reservoir not a reservoir right. Therefore, if there was water in the reservoir, the project water right is receives this water regardless of ownership. WIRSOS has been programmed to allow the user to assign a project water right to a reservoir right. If project water rights are not tied to a reservoir right, WIRSOS will subtract water release from a reservoir from the reservoir rights. The water is first removed from the oldest reservoir right and then from the junior reservoir rights.
A related problem was that a project water right was only allowed to call on one reservoir to fulfill its requirement. However, there are instances where a project water right is allowed to call on more than one reservoir to satisfy its demand. To meet this requirement, WIRSOS has been modified to allow a project water right to call on two reservoirs.
WATER EXCHANGES
A water exchange occurs when a project right is not physically downstream from its associated reservoir. A water exchange is when a project right removes water from the stream or river and that water is replaced by water from the associated reservoir. WIRSOS has been modified to allow water exchanges for senior project rights.CONCLUSION
The WIRSOS model was developed to assess the water rights disputes on the Wind River which passes through the Wind River Indian Reservation. Since this dispute was a legal battle, the model was structured to approximate Wyoming water law. The program provided a model for water planning. With the modifications made in this study to WIRSOS, it can became an excellent tool for accurate water planning on any stream system in Wyoming.
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