Evapotranspiration and irrigation water requirements

Basic factors controlling the evapotranspiration process for irrigated crops are similar to those for other plant communities, except that the water requirement is largely satisfied by irrigation rather than by precipitation. A natural consequence of crop growth is the withdrawal of soil water from the crop root zone, accompanying the evaporative loss of water from exposed plant and soil surfaces, with the water vapor subsequently being carried away in the atmospheric air flow. We have traditionally come to speak of this evaporative water loss as evapotranspiration, or ET for short, though the process is strictly evaporation, whether from plant or soil surfaces (McIlroy, 1984). The aim of efficient and effective irrigation management is to provide sufficient water to a growing crop to replenish depleted soil water in time to avoid physiological water stress in the growing plants. Meeting this objective requires knowing when to irrigate and for how long or how much water to apply. The determination of irrigation requirements is thus of major importance in providing desirable irrigation management in arid and semiarid climates, or humid or subhumid climates where irrigation supplements precipitation