Evapotranspiration is the process of water loss in vapour form from a unit surface of land both directly by evaporation from the ground and by transpiration through leaf surfaces during a specific period of time. Soil moisture is defined as the water stored in or at the continental surface and available for evaporation.

The processes of evaporation and transpiration (evapotranspiration) are closely linked to the water found in soil moisture; these processes act as driving forces on water transferred in the hydrological cycle.

Soil moisture storage is dependent on a number of factors in addition to precipitation and evaporation, such as soil type, soil depth, vegetation cover and slope.

Movement through soil and vegetation is large and accounts for 62% of annual globally renewable freshwater.

In 1974, Korzun postulated that the active soil water occurs mainly in the top two metres of the soil – within the rooting depth of most vegetation. On this basis he estimated the globe’s total volume of soil moisture to be approximately 16,500 km3. This figure assumes that soil moisture is 10% of the two-metre layer, and that the area of soil containing moisture covers 55% of the land area or 82 million km2.

Evaporation from surface water bodies such as lakes, rivers, wetlands and reservoirs is also an important component of the hydrological cycle and integral to basin development and regional water management.

In the case of artificially created reservoirs, it has been estimated that the global volumes evaporating since the end of the 1960s have exceeded the volume consumed to meet both domestic and industrial needs.

From the standpoint of food production and ecosystem maintenance, soil moisture is the most important parameter to net primary productivity (NPP) and to the structure, composition and density of vegetation patterns. Near-surface soil moisture content strongly influences whether precipitation and irrigation waters either run off to surface water bodies or infiltrate into the soil column.

It is the water of evapotranspiration that mainly supports forests, rainfed cultivated and grazing land, and a variety of ecosystems. Despite a withdrawal of only 8% of total annual renewable freshwater resources, it has been estimated that 26% of annual evapotranspiration is now appropriated by humans.

Information from:
the 2nd UN World Water Development Report: ‘Water, a shared responsibility’
the Food and Agriculture Organization of the United Nations’ (FAO) publication ‘Glossary of biotechnology and genetic engineering’
the 1st United Nations World Water Development Report: Water for People, Water for Life

Source: UNESCO Water Portal, February 2007