Agenda - Council - 07/28/2015 - Laserfiche WebLink

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observation wells are sparse CWI drives head during calibration. While broad spatially, CWI data are uncertain due to the following: • Inaccurate water -level measurements • Inaccurate well location • Inaccurate elevation • Unstable water level at the time of measurement • Misidentification or incorrect assignment of hydrostratigraphic units in databases • Seasonal pumping affects of water levels • Long-term changes in water levels due to climate or growing water demand The single biggest contributor to predictive uncertainty is uncertainty in future water demand. We do not know for sure how many people will live in the metro, where they will live, how much water they will use, or if sources of water will remain the same. This is where input from City Administrators and Engineers comes in. We recognize that no one knows the city and its water supply better than the city or utility staff. Therefore, we have been asking for input on population, population served, per capita water use, water sources, and well locations. It is hard to predict water use given all the variables, but historically water use has been in about a +1- 20% range, which is why we are presenting results with this range. Calibrated MM3 The steady-state Metro Model 3 model estimates average water levels between 2003 and 2011, within a range (plus or minus) about 17 feet. Because it is a steady-state model, it does not represent water levels for a specific day and time. Instead, it is intended to illustrate where aquifer water levels will come to equilibrium under a given water budget (recharge, pumping, baseflow). In other words, it illustrates where things will ultimately end up. In general the model uncertainty is spread fairly evenly throughout the model. Areas where model uncertainty appears to be concentrated are: • Northwest Hennepin County o Areas of faulting o Geologic atlas updated in 1989 o Few observation wells • Eastern Scott County o Areas of faulting • Rice County (directly to south) geologic atlas updated 1995 o Few observation wells • Le Sueur County (note: not in 7 county metro, directly south of Scott County)