Irrigation water requirement is the quantity of water which needs to be applied with the irrigation system. The irrigation water requirement considers a plant’s evapotranspiration (ET), irrigation losses, rain, and leaching requirements. Typically only about 65 percent of the total irrigation water delivered is available for plant growth, the balance is lost to leaching or held too tightly by soil particles to be absorbed by plant roots. The portion of the precipitation available to plants depends on the timing and amount.
Weather and Microclimate Evapotranspiration is the combination of evaporation of water from the soil and transpiration from the plants. Evapotranspiration is necessary for plant growth (photosysthensis); it maintains a healthy plant temperature and provides for the transportation of nutrients to and through the plant. Evapotranspiration requires energy. Energy comes from radiation and advection. Radiation usually comes from direct sunlight, and advection comes from heated air surrounding the plant. In addition to the energy required for evaporation, the air above the plant needs to be able to accept more water. The drier (lower humidity) and hotter the air, the more water the air can hold. If energy is being added to the plant and no evaporation is taking place, the temperature of the plant will increase.