Crop Water Use
DR. DENNIS B. EGLI
PRINCETON, KENTUCKY
We all know that water is the key to high grain yields. It’s been said that “….. man owes his very existence to a thin layer of topsoil – and the fact that it rains”. The fundamental problem in maintaining well-watered crops is that crops use water every day while it only rains occasionally. A second problem is that crops use huge amounts of water. On a warm sunny day in mid-summer when the leaves completely cover the ground, a well-watered crop can use between 0.25 and 0.30 inches (6788 to 8146 gallons/acre); that’s an inch every three or four days! It’s a rare growing season when we get an inch of rain every three or four days. Total seasonal use can be 20 inches (543,048 gallons/acre) or more. That’s a lot of water. Let’s look at the fundamentals of water use to help us better manage this conflict between an intermittent supply and a constant demand. We must consider the atmosphere, the plant and the soil – all are important.
All plants have small pores in their leaves (stomates) that allow CO2 to move into the leaf for photosynthesis. These pores also allow water vapor to move out of the leaves (transpiration), accounting for most of the water used by a crop. When the plant doesn’t have enough water, stress develops and stomates close. Unfortunately, now CO2 can’t move into the leaf and photosynthesis stops. Photosynthesis, plant growth and water use are intimately linked - plants can’t grow without water!
The total water use by a crop has two components – transpiration from the leaves and evaporation from the soil surface, which are combined into evapotranspiration (ET). ET is low when crop plants are small (not much leaf area), but it increases as the plants grow and produce more leaves, eventually reaching a maximum when there are enough leaves to cover the soil (assuming plenty of water is available). Soil evaporation is low when the soil surface is dry or when the leaves completely cover the soil.
Water use starts with evaporation, whether it occurs at the soil surface or inside the leaf (transpiration). Evaporation requires a lot of energy (585 calories /g of water) from sunlight, so the amount of sunlight plays a role in determining ET. The high energy use by ET tends to moderate leaf and air temperatures. If water limits ET, the energy that would have been used to evaporate water ends up heating the air and the soil. In fact, one way of determining if plants are suffering from water stress is to measure the temperature of the leaves. Under drought conditions, the leaves will be warmer than the air.
After evaporation, the water vapor must be moved away from the crop or the soil surface into the atmosphere. The rate of movement is determined by air temperature, water vapor content of the atmosphere (dewpoint temperature or the vapor pressure deficit) and wind speed. Maximum rates of ET occur on warm sunny days, with relatively dry air and a nice wind (assuming there is adequate soil moisture available).
These environmental characteristics (temperature, sunshine, water vapor content of the air, and wind speed) set the maximum level of ET which is called potential evapotranspiration (PET) or reference evapotranspiration (ETO). If water is not limiting and the crop leaves completely cover the soil, ET will be close to PET. The concept of PET implies that the atmosphere is more important in determining maximum ET than the crop. In fact, with no limitations (plenty of water and complete ground cover, the ET of a corn field will be about the same as a soybean field. The characteristics of the crop (rooting depth, for example) are important when water is limiting (ET is less than PET). Since PET provides an estimate of maximum water use, it can be used to schedule irrigation. Crops growing in high PET environments (deserts) will use more water than in humid climates (lower PET). Making the desert bloom may not be the most efficient use of water.
Soil water storage represents the final component of the water use system. Water stored in the soil represents the reservoir that matches the intermittent rainfall with the constant ET. The size of the reservoir is determined by the soil texture (silt loam holds the most water per unit depth) and the rooting depth. No-till and conservation tillage practices that leave crop residues on the surface increase infiltration of rainfall, thereby maximizing recharge of the soil reservoir. Water that doesn’t infiltrate is lost, actually its worse than lost - it causes erosion. The high rainfed corn and soybean yields in the central Corn Belt are, in large part, the result of deep soils that hold a lot of water. Fragipan soils with restricted rooting depth require frequent rains to make high yields.
Successful rainfed crop production depends on having enough water, from rain and soil storage, to meet the atmospheric demand (PET).The producer that is blessed with deep soils doesn’t watch the weather forecast nearly as closely as the one cursed with shallow soils. Crop production may be more difficult in the future as increasing temperatures increase water use (ET) and lower more erratic rainfall decreases supply. ∆
DR. DENNIS B. EGLI: University of Kentucky