Schematic representation of a stomaįIGURE 2. In the absence of any supply of water to the soil surface, evaporation decreases rapidly and may cease almost completely within a few days.įIGURE 1. Under these circumstances the limited availability of water exerts a controlling influence on soil evaporation. However, where the interval between rains and irrigation becomes large and the ability of the soil to conduct moisture to pear the surface is small, the water content in the topsoil drops and the soil surface dries out. Where the soil is able to supply water fast enough to satisfy the evaporation demand, the evaporation from the soil is determined only by the meteorological conditions. Frequent rains, irrigation and water transported upwards in a soil from a shallow water table wet the soil surface. Where the evaporating surface is the soil surface, the degree of shading of the crop canopy and the amount of water available at the evaporating surface are other factors that affect the evaporation process. Hence, solar radiation, air temperature, air humidity and wind speed are climatological parameters to consider when assessing the evaporation process. The replacement of the saturated air with drier air depends greatly on wind speed. As evaporation proceeds, the surrounding air becomes gradually saturated and the process will slow down and might stop if the wet air is not transferred to the atmosphere. The driving force to remove water vapour from the evaporating surface is the difference between the water vapour pressure at the evaporating surface and that of the surrounding atmosphere. Direct solar radiation and, to a lesser extent, the ambient temperature of the air provide this energy. Water evaporates from a variety of surfaces, such as lakes, rivers, pavements, soils and wet vegetation.Įnergy is required to change the state of the molecules of water from liquid to vapour.
EvaporationĮvaporation is the process whereby liquid water is converted to water vapour (vaporization) and removed from the evaporating surface (vapour removal). The combination of two separate processes whereby water is lost on the one hand from the soil surface by evaporation and on the other hand from the crop by transpiration is referred to as evapotranspiration (ET).
It also examines the factors that affect evapotranspiration, the units in which it is normally expressed and the way in which it can be determined. This chapter explains the concepts of and the differences between reference crop evapotranspiration (ET o) and crop evapotranspiration under standard conditions (ET c) and various management and environmental conditions (ET c adj). Chapter 1 - Introduction to evapotranspiration
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