A system identification methodology for distributed-parameter model building is described and compared with other methodologies for the modeling of groundwater reservoirs. The methodology is then applied to the analysis of New Zealand's Hutt Valley-Port Nicholson groundwater reservoir data. It is established that there are too few measurement wells to allow for the identification of a model suitable for forecasting reservoir performance. Application of the methodology does, however, indicate where additional wells should be drilled, so that such a model could be identified. Certain field parameters are identified to within 100%, and this accuracy is found to be acceptable, provided all the important features of the reservoir are represented in the model.
INTRODUCTIONAn important problem in groundwater hydrology is the development of mathematical models that can be used to describe the behavior of the groundwater reservoir under different operating conditions. The groundwater reservoir, being a large natural system, is badly defined: its physics is well understood in qualitative terms, but not all of its quantitative characteristics can be easily established. The major impediment in modeling is the restriction on the number of available observations and experiments. The first is sometimes restricted in time for the lack of good historical records or in space for the lack of appropriate measurement wells. The second is restricted by the analyst's inability to control natural phenomena (e.g., rainfall), and for economic and environmental reasons, as he cannot easily control the amount of withdrawn water either.In this study, a system identification approach to groundwater reservoir model building is described and applied to the analysis of New Zealand's Hutt Valley-Port Nicholson groundwater reservoir data. The identification has been conducted with the purpose of building, data permiting, a model useful in forecasting reservoir performance under a set of different operating conditions. The Hutt Valley-Port Nicholson reservoir was chosen because it had been previously analyzed by Donaldson and Campbell [1977] with the help of a conventional model adjustment approach. Another aim of this work has therefore been to establish whether any new information could be extracted from the data using the new methodology by comparing the results with those obtained before.Any system identification methodology proceeds through three stages. The first stage, model structure identification, involves establishing the form of the (often parametric) equa-XNow at 15 tions appropriate for the system description. When these equations are parametric, the second stage, model identification, reduces to parameter estimation. During the third and final stage, model assessment, one ascertains whether the model is suitable for the purpose at hand. This approach is taken because, in philosophical terms, system identification is "a logical extension of the hypotheticodeductive procedures of the scientific method to handle the special case of bad...