Measuring the effect of water stress on wheat evapotranspiration

[Introduction]: The focus of irrigated agriculture has traditionally been to irrigate crops to meet crop water requirements during the entire growing season, aiming at maximising crop yield. As water becomes scarce in many areas, full irrigation of crops to meet their water requirements is not a viable option for many growers. Therefore, they have to allow some level of crop water stress, accepting reduced yield (Payero et al., 2008; Payero et al., 2009). The challenge is to know how much and when to stress crops to minimise the impact on yield and profits. This isa difficult question for growers to answer, especially when the stochastic nature of rainfall is considered. Crop growth and irrigation scheduling models can provide some assistance if relationships between water stress and crop evapotranspiration (ETc) (and its relationship to yield) are known for specific crops. The FAO-56 model (Allen et al., 1998), perhaps the most popular approach used in irrigation scheduling, uses a generic relationship to explain the impact of stress on Etc. This approach does not reduce Etc when soil water depletion is less than a maximum allowable value at which the crop is presumed to have no stress, and when this depletion value is exceeded, ETc is linearly decreased until the soil water reaches the permanent wilting point level in soil . The objective of this study was to test the FAO-56 model under a controlled environment for wheat. This information is critical for developing reliable models to predict the impact of water stress on crop development and yield