A television commercial causes confusion with its claim “studies show vegetables use far less water than trees.” Is it so? No.
Questions about trees and groundwater abound of late, mainly along two themes.
The first, “Are trees (and not groundwater pumping) really the cause of central Wisconsin’s dry lakes and streams?” merits a “definitely not!” The second, “Do trees really use more water than irrigated crops,” needs a little more explanation.
The questioners’ curiosity often traces back to this claim made in a Wisconsin Potato and Vegetable Growers Association television commercial:
“Studies show vegetables use far less water than trees.”
To be fair, the commercial does not assert that trees are causing the dry-ups, though some individual growers do, perhaps the source of some confusion. Note too that the commercial states, “vegetables,” which excludes field corn, the largest irrigated crop. We’ll presume that when “vegetables” refers to irrigated vegetables, as it features video of an irrigation rig applying water on fields.
What is “water use”?
“Water use” in the commercial’s context means the water evaporating from a landscape into the atmosphere, not just groundwater being pumped from the subsurface. This is the water taken up by plant roots that exits plant leaves as water vapor, as well as water that directly evaporates from cover crops, crop residues, bare soils, and the like. An accurate accounting of “water use” requires that we count both.
Getting to the nub
In consulting with my hydrologist and biophysicist colleagues (Drs. Ken Bradbury and Michael Fienen, who did the Little Plover River re-analysis, and Profs. William Bland, Chris Kucharik and Ph.D. student Mallika Nocco), we collectively were aware of past studies (cited at the end of this article) that concluded irrigated crops use more water than trees in the Central Sands, but were completely unaware of any new studies that concluded the opposite.
So… I emailed the WPVGA and asked for the studies. They kindly provided a list of 13 references for studies mainly done overseas; China, England, Australia, South Africa, and Spain. Most had little in common with the Central Sands and its oak and maple and pine forests; crops of irrigated corn, sweet corn, potato, bean, and peas; sandy soils; and climate.
The email also contained an assertion that “…trees use water 365 days/year, while vegetable crops use water over a much shorter time period, in some cases less than 90 days.”
Trees use water 365 days a year, not so for vegetables?
Ahhhh! Perhaps we are getting close to the source of a misunderstanding! Is this true? Maybe only on a technicality, but it’s definitely faulty accounting. Here’s why: it ignores that water is “used” or evaporated not only from the actively growing vegetable crop, but also from off-season cover crops, crop residues, and bare soil! Year around. Just like evaporation continues from forests even when trees are dormant. Correct accounting demands we look at the annual water use on irrigated fields and forest, not just the periods of active growth for vegetables.
What does it take to estimate water use?
Estimating water use and water use differences among different plant and landscape types is tricky business. Side-by-side comparisons are needed of the specific plants or cover types for the exact location of interest. If we want to understand the differences in water consumption between Central Sands irrigated crops and trees, our comparisons need to be under conditions that represent Central Sands soils, trees, crops, and weather.
Because it’s difficult to measure water use directly, biophysicists more often than not estimate it through soil-plant-atmosphere models. Model parameters – things like the ability of leaves to pass water into the atmosphere, how different plants deploy their leaves, how much sun energy plants absorb or reflect – need to be accurately known and their uncertainty needs to be quantified. (Ph.D. candidate Mallika Nocco is working on making model parameters more precise for Central Wisconsin conditions.)
Evaluating the studies
Five criteria were used to evaluate the claim “Studies show (irrigated) vegetables use far less water than trees”:
- Were the study crops representative of Central Sands crops (irrigated sweet corn, potatoes, peas, beans, field corn)?
- Were the study trees representative trees (oak, maple, jackpine, white pine, red pine, pine barrens, and oak savannah)?
- Was the study climate comparable?
- Were soil types comparable?
- Was water use accounting done year-around and for multiple years?
Here’s how they stacked up:
*Only one study was done in central Wisconsin. But it was only for a single irrigated crop (potato) and for the growing season, not for trees and not year ‘round. It fails to support the claim. (Tanner 1981)
*Two studies were done in the eastern US, one in the North Carolina Appalachians and one in New Hampshire, but no irrigated crops were studied. Criteria 1-4 were failed. (O’Brien and others 2004, Swank and Douglass 1974)
*One Chinese study examined water use under apple trees, a second, how increasing forest cover might change Chinese hydrology. But no irrigated crops were studied. These fail criteria 1-4. (Liu and Huang 2002; Sun and others 2006)
*A Spanish study in a semi-arid climate compared irrigated citrus trees with irrigated melons, lettuce, and artichokes. Criteria 1-4 were failed. (Jimenez-Martinez and others 2010)
*Two citations were for South African work. One was not a study but a government licensing water use licensing procedure. The other examined water use by eucalyptus plantations, but no irrigated crops. Both fail criteria 1-4. (Albaugh 2013)
*An Australian study examined the impacts on hydrology with change in Victoria pasture, blue-gum plantations, and eucalyptus but no irrigated crops. Criteria 1-4 were failed. (Sinclair and others 2008)
*Four English studies addressed trees and other land covers (grass, heather, Corsican pine, oak) but not irrigated crops and crops generally. (I was unable to obtain a copy of one and hence had to infer from the title.) These fail or mostly fail criteria 1-4 (Calder and others 2002; Calder and others 2003; Hall and others 1996; Nisbet 2005)
So, do these “…studies show vegetables use far less water than trees”?
Eleven of the 13 cited works did not even study water use for irrigated crop. And one of those was for irrigated melons, artichokes, and lettuce grown in Spain. The other, for Wisconsin potato, was only for the growing season and without comparisons to trees. Studies involving trees were often for species and weather conditions in no way comparable to Wisconsin: blue-gum and eucalyptus in South Africa and Australia, apple orchards China, citrus trees in Spain.
Nothing I write here is meant to impugn the WPVGA as being intentionally and deliberately misleading. I take the incongruity as an attempt to do science by people who are very good at growing and marketing crops but are not necessarily expert scientists. I imagine my attempt as a scientist at large-scale crop production wouldn’t turn out the best either!
Post Script: Water use comparisons by previous studies