I think there are several ways to define or achieve "sustainable" groundwater use here in NW Kansas. You have to pick your poison, though, because they all entail using less water than we are today. One intriguing approach involves the possibility of irrigating virtually every acre of the district to a lesser degree, rather than the 15% of the district we now irrigate fully.
The idea is based on limiting irrigations to match the long-term annual recharge rates - to assure an average dryland production rate every year, for whatever crop is grown. In theory, we should achieve long term sustainability if we can do so. For example, if it rains an average of 18 inches per year in Thomas County, and this precipitation regime produces a long term average dryland production of 60 bushels per acre of corn, how would we fair if every acre in the County was irrigated every year for the 60-bushel corn production level? When it rains 18 inches or more, no irrigation would be required or allowed. When it rains less, every acre could be irrigated only for the 60 bushel production target.
Another way to say this would be irrigation only as a supplement to average dryland production rates - be it wheat, corn, sorghum, beans or whatever. I wonder how a 60 bushel per acre corn production history on every crop acre, forever, would compare socially and economically to the 225 bushel per acre production levels we're now achieving on 12% of the acres - while the declining groundwater table continues to promise us an eventual end to this practice? I'm going to go out on a limb and predict that the long term economic outlook is positive, but I'm not sure how the current economy would respond. Cratering our current economic base to achieve any long term sustainable goal is always going to be problematic.
I'm sure there are a few things that would need to be factored in - like the very limited non-irrigation water use we have here (less than 2.5% of the total); the fact that crop production is not linear in it's regard to water use; the fact that annual rainfall is not known until after the crop year; and a few other things, but, these could be compensated for by either reducing the irrigated acres, or the crop production targets to some degree.
Such an approach would absolutely guarantee that the highest percentage possible of average annual precip would go toward crop production. Natural recharge would essentially cease, but with vitually no groundwater use coming out, the water table should stabilize over the long term (it'd still fluctuate a little bit in response to mid-term drought or wet cycles). With no surface water issues to be held accountable for, this situation could actually become an economic and hydrologic advantage.
I'd be interested in anyone's thoughts on these ideas. GMD 4 is NOT promoting this concept, but it'd be nice to know if it could ever be an option or not.