Many tasks call upon the use of modeling for a more accurate and informed view of a system. The possibilities for model usage vary as widely as the functions of the water itself. Besides engineers, model operators include such groups as farmers, fish and wildlife managers, economists, and city planners. Although each occupation has specific problems to answer, the general concerns become relatively similar and facilitate grouping into general categories. The main issues discussed in this section include the use of modeling for economic evaluations, determination of engineered systems, and water resource management (NRC, 1982).
Economics encompasses the widest variety of reasons for model implementation. A main consideration for a model involves the necessity to predict a system's performance upon agricultural resources during times of drought or flood conditions. The output can help determine which water diversion permits are to be met. This information transforms into approximations of harvest amounts which directly relates to any economic impact of the area. Models also determine amounts of possible flooding, thereby providing for damage estimation.
Another large economic area entails recreational uses (Peterson, 1986). If model use helps regulate the release of stored water or predicts future water levels, park managers have the ability to alter their operational practices. This ability allows them to maximize the potential of aquatic recreational activities. Further recreational use appears in the employment of water models by wildlife officials. With the determination of possible stream levels, alternative f low regimes can be reviewed influencing the stream's administration. Testing of new instream f low requirements can also be accomplished to establish their actual result. These reasons along with other economic components provide several examples of the significance of modeling for financial investigations.
Engineers also employ models. One of, the main purposes of modeling in engineering involves the computation of maximum and minimum values in a body of water such as peak flood flow rates. These limits usually influence the adequacy of engineers' designs. Examples of their use include finding their effects on dams, lined channels, transmission structures, structures within floodplains, and environmental engineering concerns. With appropriate water level values known, the engineer's plan becomes more reliable and safe.
The most widely seen implementation of models, however, involves water resource managers. A system model allows them to continually manipulate the data to ascertain the changes that would occur on the actual system. This enables the testing of new permits before their issue. Also, managers hold the capability of testing new storage facilities or varying storage release amounts throughout the year to ensure that their operations do not affect any other part of the operating system. Another instance where administrators find models useful occurs due to lawsuits. During the proceedings, an accounting of the water through all types of seasons becomes a necessity to show significant need for the amounts of water being specified.
A major concern of water managers pertains to the estimation of resource adequacy. In this area of use, models can determine what permits will not be filled in a given year and the effects on reservoirs. With the proper model, water users receive advance notice of shortages or excessive flooding. Reservoirs can then be adjusted to assist in the shortfall or to take up the surplus water to assist downstream permits. Even though this purpose constitutes a majority of water management model use, all the tasks stated in this section show the essential need of this significant tool.
TYPES OF MODELS
Two primary types of models exist--causal and empirical (NRC, 1982). A causal model describes a system based on the dynamics of the processes; whereas, an empirical model is completely based on observations and relations. In a causal model, analytical methods characterize all the processes in the system with only basic measured values used as inputs (e.g. precipitation and soil properties). To describe a system, empirical models employ observed relationships and observed data like runoff values and evaporation rates. since all the models that are presented in this paper are empirical, the discussion will be constrained to only this type.
Empirical models have advantages and drawbacks. The main benefit of this type of model relates to the necessary knowledge of the actual mechanisms. Since this type of model is strictly based on observational relationships, the need to understand the actual phenomenon does not exist (NRC, 1982). This results in a short- circuiting of the actual complex causal chains. Without having to fully understand the actual workings, a model's time to completion becomes condensed compared to that of casual models. Unfortunately, since empirical models fit only the set of data upon which it was established, they become simple interpolation formulas and have no justification beyond that collection of measurements (NRC, 1982). Although this is a significant handicap, empirical models do give significant understanding of the systems operation and, in the range of the data, applicable results.
The primary goal of this thesis involves the construction of a water management model for the Green River drainage basin in western Wyoming. Since there have been no previous modeling attempts to describe this system in detail, a base model must first be constructed. To ensure that reality is retained, calibration of this model must reduce the differences between what the model predicts and actual measured values below a set guideline. In conjunction with the testing, a model must be fabricated which permits further enhancements as additional output requirements are needed. This basic analysis must also be able to crudely represent the system to permit the examination of large system expansions. In the final analysis of the model, its utility lies in the fact that it should give a basic understanding of how this river system behaves.
Throughout this study, an IBM compatible personal computer forms the basis of the hardware selection. Along with this system, several software packages assisted in the production of the final model. The two main modeling packages are discussed in the next chapter. To help with the formatting of the data, databases were constructed with DBASE III.
Although the origin of this thesis came from a need of the Wyoming Water Development Office and the Wyoming State Engineers' Office for a water accounting model on the Green River. There was no biased place on any software. Each piece of software underwent the same examination and testing to determine its ability to handle the system in question. The best software available that met the needs of the water accounting system for the Green River was used.
Meena, 1993 Table of Contents
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