Variability in rainfall is the single greatest cause of variation in forage production for a given site. Current climate scenarios predict future annual rainfall to decrease at some geographic regions. The intensity of future rainfall is also predicted to increase at other regions, with the expectation of greater variability in soil moisture. The adaptation of forage species to drought is an issue that is likely to remain with us into the future.
Precise definitions are critical to water relations work, and imprecise use of terms has complicated comparisons of some studies on plant response to drought. Drought is a purely relative term, being abnormally low rainfall. Its duration and intensity will vary between locations. The intensity of drought is measured as water potential (-MPa) (relatively difficult), or water deficit (mm) (well suited to modeling) or soil water content (g/g, cm3/cm3, %). None of these measurements has a linear effect on plants. Drought resistance is a virtually meaningless term. Plant water reserves are trivial compared to the demand from the environment, and plants are virtually incapable of resisting drought. Forage plants do vary in tolerance to the intensity and duration of water deficit. Definition is further complicated by the scale of reference; at the plant-scale the plant might tolerate a level of water deficit, however tissues such as primordial are protected and do not encounter stress. At the tissue-scale certain cell components might be protected by osmotic adjustment, which allows some water loss by the plant but maintains turgor and some plant function
