Gradually and then suddenly

Learn how aquifer depletion connects to the loss of drought resilience.
By Nick Brozović, PhD
Photos by Rachel Williss, Daugherty Water for Food Global Institute

Almost 100 years ago, a character in Ernest Hemingway’s “The Sun Also Rises” memorably describes how he went bankrupt: “Two ways. Gradually, then suddenly.” Not only does this quote apply to the pains of financial loss, it also captures the similar experience of aquifer depletion.

It’s not terribly noticeable, until it is.

While I didn’t grow up in a farming community (suburban London is not rural by any measure), I’ve worked on water use in agriculture for more than 20 years. I’ve been lucky to visit with growers all over the world, both small and large, and learn from their insights and experiences. Here, I’ll share a couple of anecdotes that serve as bookends to some of my current thinking and research on aquifer depletion.

What really matters for aquifer depletion?

Early in my career, I presented some of my academic work on groundwater management to a group in Kansas. I met with several growers who (correctly) told me I didn’t know what I was talking about. The highly technical analysis I’d just presented didn’t include well yield or measurements of crop water needs expressed in precise units such as gallons per minute per acre. So, they explained, it would be hard for me to say anything relevant about aquifer depletion as I wasn’t considering the things that were relevant to the lived experience of irrigating during droughts and with declining water tables.

Of course, those Kansas growers were right. What really matters is the ability to meet crop water needs throughout the growing season, especially on the hottest days. This is a function of well yield and irrigated area, as well as weather. Depending on how these variables change over time, irrigated agriculture gets more or less risky. Understanding the importance and implications of such changes is one of the things that I, and my various collaborators, have worked on over the years since.

The other bookend is more recent. A few weeks ago, I talked with a grower in Oklahoma. He shared an anecdote about rural electrification. A rural electric provider in Oklahoma had declined to extend electric service to an area that requested it. In the provider’s estimation, the aquifers in the potential new service area were too thin to allow growers to sustain irrigation in the long term, meaning that they would be unable to recover the costs of providing a grid connection over the long term. This is the rural development impact of declining aquifers writ large.

How might we take an anecdotal understanding of the role that drought, aquifer thickness and well yield play in determining the riskiness of irrigated agriculture and put it on a rigorous scientific footing? In a recent paper, “Aquifer depletion exacerbates agricultural drought losses in the US High Plains,” we’ve tried to do just that. The paper was published in Nature Water and supported by funding from USDA-Office of the Chief Economist, the Daugherty Water for Food Global Institute and Innovate UK. Taro Mieno, an associate professor at the University of Nebraska-Lincoln, is the lead author of the paper. Tim Foster, senior lecturer at the University of Manchester, and Shunkei Kakimoto, currently a doctoral candidate at the University of Minnesota, are co-authors. I’ve been fortunate to collaborate with Taro and Tim on multiple projects over the last decade.

How aquifer depletion exacerbates agricultural drought losses

In our paper, we looked at 30 years of historical crop yield and production data for a portion of the High Plains region, together with relevant hydrologic and climate data. Our statistical analysis demonstrated that over a large area, and over time, aquifer conditions fundamentally affect growers’ ability to reliably buffer crops against the impact of drought. Our results show that thick aquifers (hundreds of feet or more) can effectively and consistently mitigate the impacts of drought on crop yields. During droughts, crop yields for non-irrigated land above thick aquifers drop considerably relative to wet years, as we’d expect. Those fields have no access to water, after all. On the other hand, crop yields don’t drop at all between wet and drought years for irrigated land above thick aquifers. For thin aquifers, of around 100 feet thickness or less, yield reductions during droughts occur on irrigated land even for small crop water deficits. The available water simply can’t keep up with crop water needs.

Overall, our results show that the depletion of groundwater increases the physical and economic vulnerability of agricultural production to drought risks, now and in the future.

Overall, our results show that the depletion of groundwater increases the physical and economic vulnerability of agricultural production to drought risks, now and in the future. Moreover, the impact of thinning an aquifer on growers’ ability to mitigate drought risk is nonlinear. As aquifers thin, agricultural production becomes progressively more and more risky, leading expected crop yields to drop first gradually, and then suddenly.

What’s happening from an economic perspective? There are two effects to consider. First, as an aquifer thins, the lift required to bring water to the surface will increase. This means that energy requirements and production costs will increase and irrigation will become less profitable as aquifers deplete. This effect is roughly linear: if the lift required to bring water to surface doubles, then the energy needed to do so will also more or less double. All by itself, this effect should reduce the sale and rental values of irrigated land over time, as that land will now be less profitable due to increased pumping costs.

This means that energy requirements and production costs will increase and irrigation will become less profitable as aquifers deplete.

The second effect is around well yield. This effect is nonlinear. As aquifers thin, well yield will also go down, but the impact will be nonlinear because of the impact on the ability to meet crop water needs. For example, a reduction in aquifer thickness from 500 to 450 feet will have a very small impact on well yield. The same reduction in thickness, of 50 feet, for a 100-foot-thick aquifer will have a catastrophic effect on a growers’ ability to meet crop water needs in a drought. This means that a constant rate of aquifer depletion will have larger and larger negative impacts on growers’ profitability over time. To keep irrigating, many growers will reduce the area they irrigate as a risk management strategy. Our work shows this effect taking place in the High Plains.

So what?

The kinds of effects that we’ve documented are not just confined to the High Plains region. Similar relationships between thinning aquifers, declining well yields and increasing drought risks to agriculture are observed globally. So, what can we take away from this to inform management decisions and policy?

First, it’s important to understand the economics. I’m an economist, so arguably I’m biased here. There’s a through line from declining well yields in one field to increasing revenue risks from crop production, to a grower’s decision to cut back on irrigated acreage, to declining regional agricultural land values, and to decisions to make new investments elsewhere (think back to the electric provider that declined to extend its service). Understanding the economic ramifications of aquifer depletion and their underlying drivers is necessary for individuals and communities to be able to discuss what meaningful actions could be taken.

Second, it’s clear that well yield is a key indicator of resilience and of the ability of irrigation to maintain crop yields during drought. Yet, from a policy perspective, well yield in production wells is rarely measured and never managed. I would love to see some serious consideration of incentivizing the maintenance of well yields as a tool for groundwater management, in addition to other policy tools that are in use. If policies were able to provide incentives around well yield, it might be possible to stop the creeping progression from “gradually” to “suddenly” water bankruptcy.

Nick Brozović, PhD, is director of policy for the Daugherty Water for Food Global Institute and also a professor at the University of Nebraska-Lincoln.



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