Temperature And Water Use By Crops
DR. D.B. ELGI
LEXINGTON, KENTUCKY
Summer is when farmers stress about the weather – when will it rain, when will it cool off? This obsession is not surprising - rain is the key to high crop yields (unless you can irrigate) and high temperatures increase water use, making rain less effective.
Crops use enormous amounts of water – a well-watered corn or soybean crop can use 0.25 inches (6788 gallons per acre) or more in a day. That is an inch every 4 days that must be supplied by rain (by irrigation) or by water stored in the soil to avoid stress. High temperatures make this challenging situation worse.
Let’s review the processes that control water use by crops to help us understand the effect of temperature. Transpiration - the movement of water vapor out of leaves through stomata (tiny pores in the leaf) - accounts for most of the water used by crops. Water is also lost by evaporation from the soil, which is usually less than transpiration, especially when the soil surface is dry or when crop leaves completely cover the soil. The combined loss is called evapotranspiration (ET).
Transpiration occurs when water in the leaf evaporates, and the vapor moves out of the leaf by diffusion. The rate of diffusion depends upon the amount of water vapor in the air inside the leaf vs. the amount in the air surrounding the leaf. Diffusion occurs only when there is a gradient in water vapor concentration between the air inside the leaf and the outside air.
Air inside the leaf is saturated with water vapor, but the atmosphere is usually not saturated (relative humidity < 100%) providing the gradient that drives transpiration. The larger the gradient, the higher the rate of transpiration. Transpiration will be higher if the air is dry (low relative humidity - larger gradient) than if the relative humidity is high (smaller gradient).
Temperature affects transpiration by changing the gradient from inside the leaf to the atmosphere. Increasing temperature increases the gradient and transpiration. The same logic applies to evaporation from the soil.
The temperature effect is significant – increasing the temperature from 68 to 86°F increases the gradient by 1.8 to 5.4 times depending on changes in relative humidity of the air surrounding the leaf. A further Increase in temperature to 104°F increases the gradient by 1.7 to 2.4 times over the gradient at 86°F. Increasing the temperature from 86, a fairly normal summer temperature, to 104°F would roughly double the gradient and significantly increase the rate of transpiration if plenty of water is available to the crop.
Wind also affects transpiration by influencing the water vapor gradient between the leaf and the air. In still air, the water vapor that diffuses out of the leaf increases the water vapor content of the air next to the leaf which reduces the gradient and reduces transpiration. Wind sweeps the water vapor away from the leaf, maintaining the gradient and the rate of transpiration.
It takes a lot of energy to evaporate water (585 calories per g) – which is why transpiration is so effective in cooling the plant. When a lack of water limits ET, some of the energy that would have been used to evaporate water heats the plant and the air. Air and plant temperatures are usually higher during a drought. Plants in a desert can actually be cooled below air temperature by high transpiration rates resulting from the dry air and the large gradient.
Climate change and the resulting higher temperatures will increase water use by crops which will, in turn, cause a more rapid depletion of the soil moisture reservoir causing stress. High temperatures increase ET, deplete the soil water reservoir faster, and the lack of water makes it hotter. Isn’t that a kick in the head?
The size of the soil moisture reservoir plays a critical role in matching the intermittent supply of water (rain + irrigation) with the relentless daily demand from ET. It is not surprising that soils that store large amounts of water often produce the highest yields. The increasing temperatures associated with climate change will increase ET making the size of the soil moisture reservoir even more important.
“Human vanity can best be served by a reminder that, whatever his accomplishments , his sophistication, man owes his very existence to a six-inch layer of topsoil – and the fact that it rains.” (Richard L. Evans, 1906 – 1971, author and radio personality). ∆
DR. D.B. ELGI: University of Kentucky