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2.3.2 Rainfall and water availabilityRainfall is another important weather variable for the growth of crops as it moistens the soil.During photosynthesis, the water is released as a by-product through the stomata in a processcalled transpiration (Jones, 1998). A large volume of water is required by the crop during thetranspiration.The availability of water has become a growing concern in many parts of the world(Vorosmarty, et al., 2000) especially in the south Asia. Figure 2.4 shows the global variationin water availability in 1995 and the projected situation in 2025. Regions with different levelsof water stress are represented by different colours. It can be seen that the Middle East isalready suffering from water stress and by 2025 it is estimated that two-thirds of the worldpopulation may be subjected to high water stress (UNEP, 1999).Figure 2-4 World map showing water stress in 1995 and 2025. Reproduced from UNEP(1999)13