Recently groundwater has been getting a lot of attention. Particularly, scientists have focused on characterizing our groundwater use over the last few decades, and producing estimates of future water availability. Many aquifers are being depleted at alarming rates, some projected to become depleted as early as the end of the century (Jay Famiglietti has done a lot of good work on this topic, see his articles in Nat Geo here). If we continue along the path we are currently on, the question is not if we will deplete our groundwater resources, but when.
Given this reality, it seems appropriate that we begin to model management scenarios in which groundwater plays either a substantially reduced role, or no role at all in providing water for irrigation. That is no small task when you consider that in the U.S. groundwater provides 61% of total water used for irrigation (Siebert et al., 2010). These scenarios may involve switching to more efficient irrigation technology (drip irrigation as opposed to our current water-intensive practices) or reducing the irrigated area altogether.
Irrigation has a substantial impact on regional climate, and the implications of altering our current irrigation practices need to be studied further. It has been well documented that irrigation provides a regional cooling effect, a property that alleviates heat-stress in crops during droughts and heat waves (see van der Velde, 2010). Less well understood is whether switching to drip irrigation will increase water efficiency at the expense of this regional cooling effect. It is worth exploring whether during heat waves plants are in greater need of the cooling effect or the water provided by irrigation.
An alternative management scenario – allowing portions of fields to lay fallow due to insufficient water for irrigation – may have a time-dependent component, particularly when considering the time required for transitional vegetation to take root. If multi-year droughts lead farmers to leave large percentages of their fields fallow all-at-once, the vegetative landscape is likely to look very different than if the transition is gradual. Vegetation cover has an effect on atmospheric dust loading (think dust bowl) and on the latency with which the landscape reacts to drought (see work by Ning Zeng on this subject).
So, back to the point: how do we dynamically incorporate potential management decisions into climate projections? We need to conduct analyses that provide an envelope of uncertainty around management decisions, and a few scenario analyses in which different management options are explored in detail. Part of understanding the future of the hydroclimate is understanding how we are playing a dynamic role in that system and to what degree our actions will impact our hydrologic future.