Extending the life of the High Plains Aquifer
Corn producers and cattle operations in western Kansas depend largely on groundwater from the High Plains (Ogallala) Aquifer to maintain and increase production levels. Groundwater levels are being depleted, however, and this has implications for current and future economies of the region.
A team of engineers and scientists at K-State has conducted a study of groundwater use as it relates to corn and cattle production through 2110. The goal of the study was to provide a basis for scientific discussions and planning purposes beyond the current generation. Given this aim, and the intricacy of the issues involved, the study required a highly interdisciplinary approach. David Steward, K-State Professor of Civil Engineering led the study with participants including Stephen Welch, Professor of Agronomy, Scott Staggenborg, Adjunct Professor of Agronomy, and others. Steward said, "the intended audience for the study was family farmers wishing to pass the farm onto subsequent generations along with the information providers and others positioned to aid in that process."
The results were published recently in the Proceedings of the National Academy of Sciences, available online at: http://www.pnas.org/content/early/2013/08/14/1220351110.full.pdf+html
So far, 30 percent of the groundwater from this aquifer has been pumped and another 39 percent will be depleted over the next 50 years, given existing trends. Recharge supplies 15 percent of current pumping and would take an average of 500 to 1,300 years to completely refill a depleted aquifer. Significant declines in the region's pumping rates will occur over the next 15 to 20 years given current trends, yet irrigated agricultural production might increase through 2040 because of projected increases in water use efficiencies in corn production.
Model development and assumptions used
The study noted that water use efficiencies in irrigated agriculture have been increasing with time. These savings can provide a buffer for those working to improve corn genetics, irrigation methods, and cultural practices. There will be better corn hybrids as time goes on, which will be able to produce higher yields per unit of water used. Irrigation technology efficiency will continue to improve, resulting in more productive use of irrigation water. And other methodologies such as improved residue management will also contribute to enhanced water use efficiency.
Any water savings made currently will mean more water will be available in the future. Since we are assuming future water use will be more efficient and result in higher corn yields than if the water is used now, we have found that making reductions in water use now will significantly extend the useful life of the aquifer - and the future of corn and cattle production in the region.
The tradeoff, of course, is that water use reductions today decrease current agricultural production.
Our model also assumes that cattle production will closely follow the level of corn produced in these regions of Kansas. The model was tested by applying it to previous years for which corn production and cattle production values are known. It was found to be accurate, thus lending credibility to its projections through time.
Figures 3A and 3C in the published paper (reproduced here) show the results of our modeling of the different water use scenarios and their implications for future trends in corn and cattle production. Five scenarios were examined beginning with "0% reduction," a continuation of current practice with the main influence on pumping being whatever limitations on capacity might be caused by drawdown. Further analyses studied the impacts of 20, 40, 60, and 80% reductions. These levels were chosen to ascertain the sensitivities of the system to different levels of change. The 80% reduction scenario was included because it approximates the level required to match natural recharge rates.
Figure A. The panels in Figure A (above) illustrate the potential future of High Plains Aquifer groundwater use, corn production, and cattle production in the three regions of western Kansas under the 0% reduction, i.e. if no changes in water use from current useages are made.
In this status quo scenario, groundwater use in southwest Kansas, for example, is projected by our models to peak about 2020-2030, then decline rapidly through 2110. Production of corn from irrigation would peak about 2040-2050, then decline rapidly. Cattle production follows the same trend.
In any projection of possible futures, it is important to include an indication of potential uncertainties. To do so, we ran our models 10,000 times for each scenario, changing the combinations of input values on each run in accordance with the levels of variation in current data. This produced the band of possible outcomes shown for each curve in the graph. The width of each band at each point in time is the 95% confidence limits on the particular projection. The line at the approximate center of each band is the median projection- that is, half of the 10,000 runs exceeded this value and half fell below it. Tables in the published paper supply numerical details of these median projections.
Figure C. The panels in Figure C illustrate the potential future of High Plains Aquifer groundwater use, corn production, and cattle production for southwest Kansas only- but shows the projected results under all the water use reduction scenarios studied. As in Figure A, a 95% confidence band was calculated for each scenario, with the different reduction levels indicated by the color coding in the legend. The different bands are graphed on top of each other but the solid confidence limit and median lines from the overlaid scenarios show through so the trends for each projection can be visually followed.
To make things simple when looking at these graphs, it might be useful just to compare the top two curves for corn and cattle production. The top curve in each graph is the corn or cattle production under the 0% reduction scenario. This is the same as what is shown in Figure A. The next curve below is the projected corn and cattle production if water use in southwest Kansas would be reduced by 20%, starting now.
Comparing these two curves, the 0% reduction results in a higher level of corn and cattle production until 2040-2050, compared to the 20% reduction scenario. After that time period, however, corn and cattle production under the status quo starts a sharp downward trend and falls below the 20% reduction scenario by 2070. Beyond that time, production under the 20% reduction scenario is greater than under the 0% reduction scenario.
This is the key finding of this study. Using our assumptions, a 20% reduction in irrigation from ground water, starting now, would maintain corn and cattle production at a higher level in the long-term, beyond 2110. As noted earlier, this happens due to increased water use efficiencies that do not have time to accrue under the 0% reduction scenario. The study also details the implications this has for remaining production capacity beyond the 2110 horizon, which is increased by near-term reductions. The 40%, 60%, and 80% reduction scenarios show the same trends, but with even more pronounced results.
The model developed by our team of researchers shows how corn and cattle production in the areas of Kansas that use groundwater from the High Plains Aquifer for irrigation would change under different water use reduction scenarios.
Water use reductions of 20 percent today would cut agricultural production to the levels of 15 to 20 years ago. However, the time of peak agricultural production would extend from the 2040's to the 2070's, and production beyond 2070 would significantly exceed that projected without reduced pumping.
Our findings substantiate that saving more water today would result in increased net production over the long-term due to projected future increases in crop water use efficiencies. Society has an opportunity now to make changes with tremendous implications for future sustainability and livability.
Steve Welch, Plant Modeling Agronomist
David Steward, Civil Engineering