Irrigation scheduling requires knowledge of two crop characteristics: (1) how much water a tree needs and (2) when it should be applied. The amount of water a tree crop uses is called evapotranspiration (ET) and is based on both the atmospheric demand for water and the ability of the soil to supply it. Potential ET refers to the maximum ET rate from a large area covered completely and uniformly by actively growing vegetation with adequate moisture at all times. Potential ET is generally determined using computer models that utilize weather data consisting of solar radiation, temperature, wind speed, and relative humidity. Potential ET can also be estimated with a class A evaporation pan or a potometer. Based on tree age, height, species, and crop load, crop coefficients are used to adjust the potential ET to the actual ET that must be applied in irrigation. For example, a young apple orchard with an incomplete canopy within the row might have a crop coefficient of 0.70, whereas a fully mature orchard may have a crop coefficient of 1.25. Crop coefficients are time and locale specific, so local extension and other agriculture resources should be consulted for explicit information. Another approach to determining how much water an orchard needs is to measure water use from the soil and replace the same amount through irrigation. Soil moisture sensors and sensor access tubes can be installed in the root zone of the orchard and monitored periodically. These sensors will determine how much water has been removed from the root zone through actual ET, and the same amount is replaced through irrigation.
Irrigation timing depends on both the plant requirements for water and the capacity of the irrigation system to supply water. In general, deciduous fruit trees can tolerate a reduction of 50 percent of the available water in the root zone before economic stress levels occur. Available soil water is the amount of water retained in the soil between field capacity and the permanent wilting point. Field capacity can be estimated as the amount of water in the soil one to three days after a full irrigation or a prolonged period of rain. The permanent wilting point is the amount of water that remains in the soil when plants are no longer able to transpire. The permanent wilting point is difficult to measure in the field; however, for a wide range of soil types, it is approximately 50 to 75 percent of the field capacity value. On shallow or sandy soils, 50 percent depletion of available water can occur in less than five days, while on deep silt loam soils, the water-holding capacity of the soil can provide adequate water for up to 14 days in many climates. Young trees planted on any soil type have a limited root zone and will require frequent irrigation in the absence of frequent and effective precipitation.
Irrigation systems are generally designed to provide water, in rotation, to numerous sections of an orchard. Less water reserves are needed for a single irrigation event, and a smaller pump using less energy can be used. The irrigation pump size and the number of sections in the orchard are initially designed to ensure that when the orchard is mature, sufficient water can be supplied to all sections under the maximum ET demand for the region. Irrigation design is a complex discipline.
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