Not good: Research shows irrigation has limited ability to save crop yields from drought

I’ve written before about my view that we need to get away from food production that relies on seasonal weather patterns, and some new research has been published that reinforces that opinion. In particular, the capacity for irrigation to mitigate the effects of drought:

“Plants have to balance water supply and demand. Both are extremely critical, but people overlook the demand side of the equation, especially in the U.S. Corn Belt,” says Kaiyu Guan, principal investigator on two new studies, Blue Waters professor in the Department of Natural Resources and Environmental Sciences and the National Center for Supercomputing Applications at Illinois.

The demand Guan refers to is atmospheric dryness, often expressed as vapor pressure deficit (VPD). The drier the air, the more moisture is sucked out of pores, or stomata, in plant leaves. Plants have to open stomata to take in carbon dioxide as their food, but if they sense the atmosphere is too dry, they’ll close pores to avoid drying out. Keeping stomata closed too long leads to reductions in photosynthesis, plant growth, and grain yield.

The kicker? Plants shut down stomata due to atmospheric dryness even when there’s an adequate supply of moisture in the soil.

“If you only consider rainfall and soil moisture, which is how most people think about drought, that’s mostly describing the supply side. Of course if you have low soil moisture, plants will be stressed by how much water they get. But the supply is often pretty sufficient, especially here in the U.S. Corn Belt,” Guan says. “However, the demand side from the atmosphere can also severely stress plants. We need to pay more attention to that drought signal.”

Guan’s two recent studies used multiple technological approaches, including field measurements, various sources of satellite data, hydrological model simulations, and government crop yield statistics. The first study, published in Agricultural and Forest Meteorology, used data from seven sites across the Corn Belt to conclude VPD accounts for nearly 90% of the changes in crop stomatal conductance, a proxy for drought stress, and approximately 85% of changes in gross primary productivity, a measure of productivity.

“By comparison, soil moisture typically accounts for 6-13% of these measures for corn and soybean, and up to 35% when considering time lag effects,” says Hyungsuk Kimm, doctoral student in Guan’s group and the study’s lead author.

In the other study, published in the Journal of Hydrology, Guan’s team focused on grain yield. Yield depends on many factors related to water cycles, but the researchers found that VPD explains the biggest proportion of variability in crop yield and also provides the earliest warning for yield loss when comparing with other water cycle metrics and traditional drought indices.

“This led us to build a new drought index integrating VPD, soil moisture, and measures of evapotranspiration, which can account for more than 70% of yield variation. Our index outperforms all the existing drought indices,” says Wang Zhou, postdoctoral researcher in Guan’s group and the study’s lead author.

Guan adds, “In these two studies, we tried to understand the demand side of drought from two major angles, one using eddy covariance data which measures landscape water and carbon use very accurately — the gold standard — and the other leveraging satellite data and model-simulated hydrological variables correlated with regional yield,” Guan says. “In both, we demonstrate VPD is more important than soil moisture to explain the crop drought response in the U.S. Midwest.”

The researchers are continuing to look into things like how to breed more drought-resistant crops, but I honestly feel that that is too little, too late. There are limits to how drought-resistant you can make any given plant, and from what I can tell there are not predictable limits on how severe droughts and heat waves may be getting in the coming decades. If we want to avoid food shortages, we should be working on indoor food production. That could be the various techniques mentioned in my recent post on climate change and agriculture, or it could be something like seawater greenhouses, but in any case, there is zero question that the world’s food production is vulnerable to droughts, and there is zero question that droughts and heat waves are going to continue getting worse.

In my opinion, there is no greater threat to the United States in the coming decades than the degree to which our food production relies on the stability of a climate that is now so unstable that it’s warming at a rate that may be unprecedented in the history of life on this planet.

I continue to believe that if we take urgent action, we can survive what’s coming, and even build a better human society in the process, but we do need to act, and the longer we continue to delay, the higher the death toll will become.

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  1. says

    One of the things that I think its exceptionally interesting is the new research on outdoor agriculture beneath elevated photovoltaic panels. They call it “agrivoltaics”.

    I bring it up now because an article I read on this practice recently mentioned that the atmospheric relative humidity was increased in the area under the panels. If that turns out to be a generalizable result, then agrivoltaics would provide a certain amount of protection from the drought effects you’re talking about.

  2. says

    Thanks for that!

    I’ve seen some stuff on things like that over the years, and it always seemed like a good use of that land, but it’s been a while since I looked into research on it. I’ll be sure to read the link!

  3. John Morales says

    I don’t like equivocation.

    The demand Guan refers to is atmospheric dryness, often expressed as vapor pressure deficit (VPD).

    But that’s not drought, is it? Drought is a shortage of rainfall, not a dryness of the air.

    re agrivoltaics: whatever sunlight goes to the panels does not go to the plants. And sunlight is what they use for photosynthesis.

    (Also, a major point is that the panels work best at lower temperatures, and the plants’ transpiration provides cooling — it’s a cost-benefit thing)

  4. R Simons says

    Twenty years ago, a commercial grower just outside Windhoek, Namibia, was growing tomatoes, lettuce and other crops in a greenhouse in a hydroponic system using the greenhouse to increase the humidity rather than to control the temperature.

    Crip Dyke: I suspect that grass growing under solar panels performs better than out in the open in places like Spain. The reduced temperature caused by the shading, reducing the water demand for evaporative cooling, could well more than compensate for any lost photosynthesis resulting from the lower light level.

  5. says

    @ John – in general droughts come with hotter, drier air, I believe.

    @R Simon Hydroponics of various forms have been around for a while. It’s unclear how well they could scale up, giving the amount of food needed to feed several billion people. Grain, in particular, seems like it might be tricky to grow indoors at scale.

    But yeah – we have the tools we need to make the changes we need. We’re just lacking the political will/power to make it happen.

  6. says

    @R Simons:

    It’s not just those factors, at least by my imperfect understanding. As I understand it – and again, this should be taken as a point of information with which to start, not as any statement of what’s true – the solar panels & the structure that supports them holds a certain amount of atmospheric moisture near the ground, so in addition to the reduced temperature, the relative humidity is also different under the structure than it is just a few feet away. That relative humidity increase prevents the plants’ stomata from closing, as the research above discusses.

    That’s the difference (more than just shade temperatures) that made me think it was relevant to bring up agrivoltaics.

    Of course, the pilot projects undertaken so far haven’t used the tall structures over a hundred acres or so that would be necessary to sow, raise, & harvest grain using the mechanical apparatuses that make modern agriculture efficient enough to feed billions.

    Also, I seriously wonder how they would do over rice patties, but that may not be an issue, since rice grows in water so the relative humidity might be high in rice fields anyway.

  7. says

    @John Morales:

    re agrivoltaics: whatever sunlight goes to the panels does not go to the plants. And sunlight is what they use for photosynthesis.

    Understood, and I’m not saying I understand why it works, but the studies that have been done show that it does work for certain crops. They haven’t done a big grain test yet, but certain fruits including tomatoes and peppers do – if the published research is accurate – do better under the panels than they do without the panels under direct sun.

    Weird… but unless the researchers are not to be trusted, true.

  8. Dunc says

    whatever sunlight goes to the panels does not go to the plants. And sunlight is what they use for photosynthesis.

    But that does not necessarily mean that more sunlight is always better, or that reduced sunlight is necessarily a problem. There’s Liebig’s law of the minimum to consider, and sunlight is not usually the limiting factor – at least, not anywhere where it’s worthwhile putting lots of PV panels.

    Even up here in Scotland, I grow my tomatoes in a shaded greenhouse, because heat stress is significant problem during the summer (a greenhouse can get very hot with 18 hours of sunlight a day), whereas reducing the incident sunlight by 50% does not impose any significant limit. Many crops actually benefit from being shaded from the worst heat of the day.

  9. R. Simons says

    The fields of solar panels I saw in Spain were being grazed by sheep, solving the problems of dealing with an annual crop.

  10. says

    Honestly, if we’re going to keep having highways as a thing, it might be a good idea to “roof” them with solar panels, and have “green walls” along the sides to help soak up some of the pollution generated by tires on asphalt.

    It’d probably also lower air pollution by reducing heat from the sun hitting the road.

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