Transpiration

Transpiration is an ecologically important process. In areas where forests are common, evapotranspiration almost entirely occurs as transpiration, and this process can account for a substantial part of the manner by which the landscape deals with water inputs through rain and snow. For example, in typical forested landscapes of northeastern North America, evapotranspiration accounts for about 15-40% of the annual inputs of water with precipitation, the remainder draining to ground-water, or flushing from the system as streamflow.

In any regions with a seasonal climate, rates of evapotranspiration vary greatly during the year. Consider, for example, the case of a landscape in a temperate climate, covered with a seasonally deciduous, angiosperm forest. During winter, very little transpiration occurs because plant tissues are frozen. However, there can still be some physical evaporation of water from the surface of snow and ice, occurring by the direct vaporization of solid water, a process known as sublimation. During the springtime, unfrozen water is abundant, but the trees do not yet have foliage and this greatly reduces the rates of transpiration. During the growing season, air temperatures are warm and the trees are fully foliated, so transpiration occurs in large rates. During this time of the year, so much water is pumped into the atmosphere through foliage that the rate of evapotranspiration typically exceeds water inputs by rainfall. As a result, the soil is dried by the demands of plant roots for water, to the extent that streams may cease to flow by late summer. Once the trees drop their leaves in the autumn, transpiration rates decrease greatly, the water-depleted soil becomes recharged by rainfall, and streams again flow.