Contrary to a common perception, water-use efficiency and drought tolerance are not one and the same. Photo: Supplied
Originally published on February 6, 2019 in Country Guide.
Should you be more concerned about drought tolerance, or water-use efficiency?
It’s a term that’s often loosely used, and it isn’t well understood at any level. Yet it may become one of the dominant yield limitations in your crop production within the next few years.
Water-use efficiency has been part of agronomy’s lexicon for decades, yet as yields have increased, its definition and how it functions have been blurred and confused. At its simplest, water-use efficiency can be defined as the amount of water it takes to make a bushel of grain. Other definitions rank it as the amount of yield per unit of water applied, or the yield per unit of soil-water used.
For Dr. Hugh Earl, the more scientific rendering is: Water-use efficiency equals total crop biomass divided by the volume of water the crop transpires in its life.
In other words, it can seem pretty remote from anything a crop manager would need to know on a day-to-day basis. But it isn’t.
The total biomass is the dry-weight of the crop with the water removed. In a greenhouse lab, it’s a fairly simple computation: just weigh the pots every day to determine how much water the plants have transpired, then harvest the entire plant, dry it down and measure the dry-weight.
On a field scale, measuring water-use efficiency does become more of a challenge. Anything that affects biomass accumulation will affect water-use efficiency, as will anything that affects water use.
“There are a lot of subtleties involved here — when I talk water use, I mean water used by the plant itself,” says Earl, an associate professor in the department of plant agriculture at the University of Guelph. “But a significant fraction of water use in the field is from soil evaporation, directly. From an agronomic standpoint, you care about both — you care about the soil-water evaporation and also the transpiration by the crop.”
In humid areas, water-use efficiency will be naturally high because the plants don’t transpire as much water. In a drier environment, it will be lower.
Overall, according to Earl, water-use efficiency in agriculture has been increasing steadily since the Industrial Revolution. A lot of different factors are involved, but one reason is related to the fact that atmospheric CO2 concentrations have increased, which has directly increased water-use efficiency.
In a short-term scenario, higher CO2 levels are actually a good thing for crops because CO2 is good for photosynthesis. That means higher CO2 concentrations are a net benefit to water-use efficiency, for now. When we factor in any temperature increase from higher CO2, not surprisingly, the picture isn’t so benign.