Our GMD just started using a Modflow groundwater model developed by Steve Larson of S.S. Papadopolos, Inc. Actually, it is a converted version of the Republican River Compact model co-developed by the states of Kansas, Colorado and Nebraska under order of the US Supreme Court as part of the compact settlement stipulation. We have run about 6 different scenarios - 1 representing continued pumping as in the past (status quo) and 5 scenarios of reduced irrigation pumpage in our 6 designated high priority areas (HPAs). One of these runs was no irrigation pumpage at all in the 6 HPAs.
The status quo was, well, status quo. The water table declines continued downward over the next 60 years on an very consistent trend as we've seen in the past 40 years. The various percentage reductions (90%, 70%, 50%, 20%) showed virtually proportional trends for each run. It was the zero pumpage run that showed some interesting results.
For this run (100% reduction of irrigation water pumpage) the water levels in all 6 HPA's drifted upward (negative decline) for about 20 years, then started a much slower decline trend for the last 40 years - to finish 60 years out at slightly below the model starting levels. The model is likely telling us that halted pumpage immediately benefits the specific region that does it (for about 20 years) as the long term pumping cone fills back in. Eventually, the pumpage outside the HPA (which continued the entire time in excess of safe yield) begins to affect the HPA causing it to begin to decline again, but at a much slower rate.
The model confirms the common notion that the very slow rate of groundwater movement will find all benefits of an area's reduced pumpage accruing to that specific area for about 2 decades. Our next step is to couple an economic model to tell us the economic impacts of reducing any pumpage rate by any specific method. We feel that if there are 4 ways to reduce pumpage by 30%, for example, the economic impacts are likely to vary between the methods. It'll be fun and games for a while in GMD 4.