d. Evaporation and humidity.
The rates of evaporation rise rapidly with higher temperatures so that considerably more rainfall is needed to maintain equivalent air humidity, than is needed in the temperate zones. The greater evaporation losses are accompanied by higher transpiration losses of water from plant leaves and stems. Even the humid wet-dry tropics, because of the extended dry season, produce substantial moisture stresses in plants.
Evaporation from free-water surfaces (lakes, ponds, streams), and from soils and the water losses directly from plants, are very high in warm dry situations. Growing plants must maintain a minimum water balance between uptake by roots and losses from leaves and stems, to remain active in growth and in storage of plant food reserves. Unless a positive balance is achieved, the plant ceases to function and either becomes dormant or dies, The length of the periods during which plants can function, is a fair measure of how much growth can be made in the course of a year, whether the growth be in foods and fiber used directly by man, or as forage for consumption by livestock. As the climate becomes more severe in terms of water balance for plants, the growing of crops tends to dwindle, and prime reliance is placed on forages (grasses, legumes and browse plants) that must support crazing livestock enterprises to yield products that man can use to meet his needs.