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Chapter II
Model Comparison

Before any basin data can be developed for input, a model must first be chosen in order to decide what data needs to be obtained. The two models which were selected for investigation are WIRSOS and HYDROSS. WIRSOS (Wyoming Integrated River System Operation Study) was developed by Leonard Rice Consulting Engineers for the State Engineer and Attorney General's Office of Wyoming. HYDROSS (Hydrologic River Operation Study System) was authored by the United States Bureau of Reclamation, Upper Missouri Region. Version 4.0 constituted the generation of HYDROSS that was tested. Both models consist of fortran code and can be run on IBM-PC compatible computers as well as workstations or mainframes. These two models were chosen because they are the two primary models that are suited to handle Wyoming water allocation systems and acceptable to the State Engineer's Office, Bureau of Reclamation and Wyoming Water Development Office.

Each model, however, is slanted toward the main concerns of the organization that operates it. HYDROSS is specifically designed to best evaluate the effects of reservoirs in the model; whereas, WIRSOS is better adapted to handle water rights issues in Wyoming and contains more river management capabilities. These two differences in use manifest themselves throughout every input file required and in the output that they generate.

One of the most important inputs into any water management model is the diversions. These determine how the water is dispersed, what quantities of water are available for storage, and what permits will not be filled. Being central to a management model, the discrepancies in the diversion input files for the two models are noticeable and significant. The differences range from how the priority date is treated to the number of reservoirs on which a single diversion can call.

The priority date encompasses the heart of the priority permit system on which Wyoming water law is based. This date allows every user on the system to be prioritized and allotted water in correct priority. The day that the permit is legally filed results in its priority date in Wyoming; therefore, the date consists of a day a month, and year. WIRSOS recognizes this format and allows for an eight digit number (mmddyyyy) to be used. HYDROSS, on the other hand, only tolerates a four digit number for the priority date. The four digits can either represent the year of the permit or an independent numbering system assigned to each permit. This distinction forces the modeler to decide between the accuracy of the full date or the efficiency of less input. Both models do allow for multiple diversions with different priority dates at each station. HYDROSS additionally permits a priority date for stored water to be included.

The modeler must also be concerned about the overall efficiency of the diversion. Efficiency in this case means the percentage of the water diverted that is actually consumed. WIRSOS lumps all losses into one value and applies this to the water which will become the return flow. Since WIRSOS only allows one efficiency value, this one percentage is then taken as the efficiency for every month. Although this method is not completely accurate, even this one value is not readily available through the literature or basic flow data on most systems; therefore, it must be evaluated from data near the investigation site, a more intense field study or educated estimate. Efficiencies in HYDROSS require more intricate data for both canal and site losses. HYDROSS also permits the variation of efficiency values throughout the year. This compels the programmer to either devote a large amount of time to the development of the specific conditions at the points of diversion or to estimate significantly more efficiency values than are generally known. This results in a model that is farther from actual conditions in many cases. With both models dictating the use of data which is generally unavailable, the less approximations or estimates that have to be made will assist in the overall simplicity of the model; therefore, in real basin modeling, a lumped value for a diversion efficiency can be considered to be a more acceptable estimation than a more detailed accounting of losses when there is no data to support the larger input requirements.

With the priority and efficiency problems addressed, the focus can be turned to the manner by which the actual amounts of water per diversion are encoded. WIRSOS takes the managerial position and only allows a monthly table. In addition to this method, HYDROSS allows for other types of input. The most useful is a per unit table. The units can either be an irrigation requirement or a per capita value. In conjunction with the number of acres or population, a value can be determined and a demand placed on the system. HYDROSS also employs a maximum annual amount per diversion, a bypass diversion value, and off-channel storage. With both models able to apply basic monthly table inputs, the added ability of HYDROSS is quite attractive; however, there are few times that a model will deviate from the permitted amounts of water.

The last major difference in the diversion file is reservoirs. WIRSOS is limited to call upon only one reservoir for additional water for a diversion. HYDROSS offers the capability to link multiple reservoirs to a single diversion. This feature requires additional input space to be used thus increasing the size of the input files.

As with all river studies, one of the key components of the data input is the actual amount of water that actually occurs in the system. These values can be found from USGS gauging stations, previous studies, hydrologic analysis or local agencies. These data, however, can be some of the most difficult to obtain and apply. Also, HYDROSS requires pristine channel flow at every station in the model. This involves taking any data that are available and converting them back in time before there were any demands on the system. Not only is this impractical in the amount of time that must be spent altering all the data, the size of the file that results limits HYDROSS to only the running of smaller models due to computer memory allocated.

WIRSOS averts this problem by only requiring data from the basin headwater sources and developing the flow at stations downstream through its algorithms. Along with this feature, WIRSOS allows mid-basin runoff stations to be specified to alter the f low that the model would predict. Runoff data are difficult to collect; yet, having to alter them and determine values at every point in an analysis, as is the case with HYDROSS, is overly complex and an extremely time consuming process.

RESERVOIRS Reservoirs represent a major contributor to the amount of data in any model. Their complexity and data requirements are enormous compared to that of a diversion, but since they are major structures in a system, the data are usually available. Since most major reservoirs in Wyoming are generally owned and operated by the Bureau of Reclamation, HYDROSS's main concern is reservoirs. Although WIRSOS does account for reservoirs, it treats them in a simple manner, not as the complex systems that they are. The differences in the two models include use of the stored water, the input parameters, the water rights, and pooling.

From a managerial point, the quantity of water that a reservoir has been permitted is as important as that of a diversion. For this reason, priority dates are also given to reservoir rights. A dam operator has limited control over the timing of reservoir filling. If he has a late priority, a downstream user with earlier rights has precedence over the water and therefore a reservoir must pass water in dry years instead of filling. Realizing this fact, WIRSOS assigns each reservoir right its proper priority date and maintains its priority throughout the running of the model. HYDROSS does not. A priority of 9999 is assigned to each reservoir right in the system. This represents the lowest priority of all and can result in a reservoir never filling in dry years regardless of its actual priority date. To circumvent this method, the Bureau of Reclamation suggests treating each reservoir as an "offstream reservoir" supplied by a diversion with the reservoir,' s actual priority date. Although this becomes an effective tool, it further removes the model from the reality of how the system works. seeing the necessity of proper modelling of priority dates, HY.DROSS contains a definite flaw which is not present in WIRSOS.

The characteristics that are input into the two models for each reservoir are essentially identical. Both include minimum and maximum content, maximum spillway capacity, and area-capacity relationships. The variances in the two models' handling of reservoir parameters exist in HYDROSS's input of absolute maximum content, target content, and the use of tables for relationships. Absolute maximum combines with downstream channel capacities to prevent flooding by filling a reservoir past its maximum content. With table input, the HYDROSS reservoir parameters are those actually measured. WIRSOS, however, uses equations to relate area and capacity. There are five choices of equations in WIRSOS and, depending on the available area and capacity relationship for each reservoir, they can be expressed as:

1.	AREA = CF1 + CF2*(VOL**CF3)
2.      AREA = CF1 + CF2*(ALOG10(VOL))
3.	AREA = CF1 * (CF2**(CF3*VOL))
4.	AREA = 10**((CF2*ALOG10(VOL)+CF1)
5.	AREA = CF1 + (CF2*ALOG(VOL))

     CF1,CF2,CF3 = Input Constants 
     AREA = Reservoir Surface Area (Acres)
      VOL = Reservoir Storage Volume (Acre-Feet)

one reservoir can actually utilize all five equations by dividing it into as many as five parts. Even though the equations give a more continuous set of points, a regression must be achieved f or each relationship resulting in some degree of error. This error, however, can become negligible with a good regression fitting of the equation(s) , but it still adds error to a model who's attempt is to accurately portray the actual river system.

Now that the differences in the reservoir characteristics have been discussed, how the water is actually used can be examined. Although both handle diversion and bypass releases in approximately the same manner, a couple of areas do exist that separate the two programs--power operations and pooling. HYDROSS allows for more detailed power input. Compared to WIRSOS's only inputs of a release goal month and volume, HYDROSS does much more, almost to the extent that it appears to be overdone. It requires a monthly power release table, a priority date for power use, a power plant efficiency, and an optional tailwater elevation table. The efficiency and tailwater information are combined with the upstream head to determine the amount of power being produced each month allowing for more accurate and complete reservoir reports. To further enhance these reports, HYDROSS can also perform pooling of the reservoirs. Pooling involves attempting to keep all reservoirs at their specified target volume by releases from upstream reservoirs which are in excess of their target volumes. This feature can be turned on or off for each reservoir depending on a difference in ownership or to simply prohibit a reservoir from being altered by the routine. With this trait along with the power manipulation and the input of physical properties, HYDROSS exhibits a distinct advantage over WIRSOS in the domain of reservoirs even with its severe priority problem stated earlier.

As in the case of the input categories, the output deviates between the two models. The main differences in the output also relate to the main purpose of each program. HYDROSS gives detailed information with respect to what is happening to the reservoirs. Unfortunately, it ignores individual diversions and only reports what is happening at each modeling station. Conversely, WIRSOS has specific information on which permits are called out with specific amounts and percents as well as rudimentary reservoir data. This allows for exact effects to be seen for all users of the system instead of just reservoir owners.

With the added input characteristics and simplifications that the Bureau of Reclamation installed in HYDROSS, its intent is obvious. The manipulation of reservoir data constitutes its primary responsibility. . This ability is fitting when reservoirs are the only area of interest in the model. For an entire river basin, though, every influence must be analyzed and their results revealed. WIRSOS does demonstrate the aptitude to accomplish this goal, but it comes up lacking in its operation of reservoirs. An ideal model would combine the water rights aspects of WIRSOS and the reservoir attributes of HYDROSS. Since neither model illustrated an overall ability to deal with all inputs in an exemplary manner, some sacrifices have to be made. WIRSOS's definite superiority in diversion input and output in combination with the ability to run reservoirs in a more coarse form offered the best alternative. With this in mind, all the data for the Green River basin was constructed in a WIRSOS format and WIRSOS was used as the model for the basin.

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