Crops use water for both growth and cooling. Plants are a sitting target in sun-baked soil with no relief from the evaporating heat and solar rays. Crop plants use water to combat the dehydrating effects of heat that affect their yield and, if prolonged, shorten their lives. Supplying enough water to cool crops is vital. The amount of crop water in a season is affected by soil moisture, rain, humidity, temperature, solar radiation and wind.
Crop water is called evapotranspiration. It is the combination of evaporation from the soil and plant leaf surfaces and the water used by plants to grow and transpire, which is the process plants use to cool themselves. A steady supply of water is vitally important for both growth and for cooling crop plants.
Crops manage hot, dry summer conditions with a process of self-cooling called transpiration. Plants release moisture in the form of water vapor through pores, called stomata, found on the surface of their leaves. This creates a pocket of moist air around the plant, which cools it and reduces the stresses of dehydration and heat.
Evaporation occurs after natural or artificial precipitation adds moisture to soil and foliage. The process of evaporation cools the plant as moisture rises from the soil. Once the dampness has evaporated from the top 1 to 1 1/2 inches (2 to 4 centimeters) of soil, evaporation ceases and plants suffer heat stress. Plants respond by closing their stomata and wilting in the mid-day heat. When temperatures cool and solar radiation lessens in the afternoon, plants reopen their stomata, resume transpiration and become rigid again. Crops that can't cool themselves in drought conditions lose older leaves, stop transpiring and growth slows to the point of death.
As the leaf canopy forms a complete cover over the soil, evaporation is reduced. Crops can draw water from the soil through their roots and cool themselves through transpiration. In drought conditions, soil moisture dries up. Transpiration decreases and plants suffer severe heat stress.
Crops don't need as much water in a cool spring, when transpiration is less, than in a warm, dry one. When the temperature reaches 85 degrees Fahrenheit (30 degrees Celsius), it is important that cereal crops get 2 inches (5 centimeters) of moisture a day. That rises to 3 inches (8 centimeters) from July to August when crops are forming heads. Any lack of water at this stage affects growth, causes heat stress and reduces crop production. When plants with enough moisture wilt in high heat, overhead irrigation can physically cool the plants and reduce stress by delivering cool moisture to the leaf surfaces. Irrigating flowering crops during warm spells in the winter is important. It keeps them from flowering too early and freezing the buds later. Water initiates transpiration, cooling the plants to prevent budding.