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<br />, <br /> <br />( <br /> <br />5532 <br /> <br />The acceptability of the groundwater models that were developed and used in this study <br />depends on whether the information they provide allows selection of the appropriate water supply <br />alternative. It is with this purpose in mind that our opinion of the acceptability of the modeling <br />effort is based, <br /> <br />Regional Groundwater Mode' <br /> <br />Mer an initial review of the model, our peer review group made several suggestions for <br />correcting mistakes, and/or for improving the model calibration, A mistake in the input <br />parameters related to stress period duration was discovered, and subsequently fixed, A <br />mistake in the specific yield of the top model layer was also discovered, and subsequently <br />fixed, A careful screening of the calibration water-level data was recommended, since in a <br />few locations, fairly closely spaced data points had unreasonably large differences in water <br />levels, Review of the model also showed that some gradients in the simulated potentiometric <br />surface (e.g., in central Douglas County) did not match the relatively steep gradients implied <br />from the observed potentiometric surface, The model was recalibrated to improve the match <br />between the simulated potentiometric surface and the observed surface, <br /> <br />( <br /> <br />It is important to remember that the mode' was developed to answer a particular question, <br />and therefore mayor may not be appropriate for answering other questions, For example, it <br />would probably be unwise to presume that the transmissivity values in the model accurately <br />represent the actual aquifer properties, since the calibration was based on limited data, <br />Significant discrepancies between actual transmissiv~ies and model transmissivities in <br />various areas are likely, In some areas of the basin additional data may be available for <br />improving local estimates of aquifer properties, and in other areas of the basin there may be <br />no data available for improving the estimates. If improvements to the regional groundwater <br />model are ever implemented in the future, we feel that the following efforts may be helpful: <br /> <br />. Utilize additional historical water level data for calibration targets. For example, very <br />little water-level data from Arapahoe County were used as calibration targets, yet <br />additional data are available in this important area. The predictive ability of the model <br />in this region may be particularly questionable due to the lack of calibration targets <br />used here, <br /> <br />. Carefully screen the water-level data that are used for calibration targets. Some of <br />the data points appear to be possibly misleading. For example, in two different areas <br />of Douglas County, Denver aquifer wells that are relatively close together appear to <br />have potentiometric-head measurements that in some cases differ by hundreds of <br />feet. A careful screening of the data may indicate which data are more reliable, and <br />the model could be calibrated to the more reliable targets. <br /> <br />. Rather than estimating recharge using the assumption that mass imbalances can be <br />used as estimates of recharge, estimate recharge using some independent <br />methodology that has a more reasonable basis. <br /> <br />. Smooth out some of the abrupt changes between adjacent model cells. For <br />example, Figures 3.7a, b, and c. of the draft report show a very abrupt change in <br />hydraulic conductivity along a north-south line roughly through the middle of the <br /> <br />l, <br /> <br />. Page 2 <br />