Water-Use Efficiency Under Changing Climatic Conditions

Water-use efficiency is an important subject in agriculture in semiarid regions, because of the increasing areas under irrigation and the high water requirements of crops. The scarcity of water resources is leading to increasing controversy about the use of water resources by agriculture and industry, for direct human consumption, and for other purposes. Such controversy could be alleviated by increasing the crop water-use efficiency, so that improving water-use efficiency of crops is becoming a main agriculture and food security goal. Moreover, climate change predictions show clear increases in temperatures (and a concomitant increase in potential evapotranspiration) and more frequent episodes of climatic anomalies, such as droughts and heat waves. In general terms, the efficiency of one process is the ratio between the obtained product (the numerator) and the energy or resource invested in the process (denominator). In the context of water-use efficiency the “product” is the assimilated carbon and the “inversion” is the used water (the resource). The numerator and denominator of this ratio may be considered at several levels, and consequently, different definitions of water-use efficiency can be made. The water issue is crucial for environmental sustainability of agriculture, because 60% of agriculture is located in semiarid areas and regular water applications are necessary to complete the growth cycle of crops. Crops in semiarid regions grow and mature during the driest months, making irrigation scheduling and timing critical. Consequently, scientific interest in research on crop water-use efficiency has focused on the evaluation of new irrigation techniques and on genetic variation in water-use efficiency in rootstocks or cultivars and reflect the social interest and necessity of optimizing water use in viticulture. Evapotranspiration, grain yield, biomass, water-use efficiency, and the harvest index of post rainy season crops were all affected by controlled ranges of soil water content during growing seasons. Grain yield response to irrigation varied considerably due to differences in soil moisture content and irrigation scheduling between seasons. Evapotranspiration was highest under continuous high soil moisture conditions, as was aboveground biomass. However, grain yield was not the highest in these conditions.