THEME 9: CALCULATION OF
EVAPOTRANSPIRATION
9.1 INTRODUCTION
EVAPORATION
Evaporation is the process whereby liquid water is converted to water vapour
(vaporisation) and removed from the evaporating surface (vapour removal).
Water evaporates from “free” water surfaces, such as oceans, lakes, rivers, soils
and wet vegetation.
TRANSPIRATION
Transpiration consists of the vaporisation of liquid water contained in plant
tissues and the vapour removal to the atmosphere. Crops predominately lose
their water through stomata. These are small openings on the plant leaf through
which gases and water vapour pass (Figure 9.1). The water, together with some
nutrients, is taken up by the roots and transported through the plant. The
vaporisation occurs within the leaf, namely in the intercellular spaces, and the
vapour exchange with the atmosphere is controlled by the stomatal aperture.
Nearly all water taken up is lost by transpiration and only a tiny fraction is used
within the plant.
Figure 9.1 Schematic representation of a stoma (Allen et al., 1998).
203
, EVAPOTRANSPIRATION
Since the forcing mechanisms behind evaporation and transpiration are similar
and the end result of these two processes is the same, it is possible to treat them
together as a combined process called evapotranspiration. Evapotranspiration
is therefore the term given to the total movement of moisture into the atmosphere
(evaporation plus transpiration) and can be seen as the process by which liquid
water is converted to water vapour and removed from the evaporative surface
(Buckle, 1996).
Evapotranspiration is an important aspect of the hydrological cycle and forms a
fundamental process in the energy balance of the Earth-atmosphere system. It is
a continuous 24-hour process, although it reaches a maximum by day. In the
process heat energy is removed from an evaporating surface causing it to cool.
This heat is absorbed by water vapour where it remains as latent heat. This
latent hat does not change the air temperature; rather it is a store of potential
energy that is later released when the vapour condenses into clouds. Such
vapour is sometimes advected thousands of kilometres from its source before
condensing and releasing the stored heat.
Apart from the water availability in the topsoil, the evaporation from a cropped
soil is mainly determined by the fraction of the solar radiation reaching the soil
surface. This fraction decreases over the growing period as the crop develops
and the crop canopy shades more and more of the ground area. When the crop
is small, water is predominately lost by soil evaporation, but once the crop is well
developed and completely covers the soil, transpiration becomes the main
process. In Figure 9.2 the partitioning of evapotranspiration into evaporation and
transpiration is plotted in correspondence to leaf area per unit surface of soil
below it. At sowing nearly 100% of ET comes from evaporation, while at full crop
cover more than 90% of ET comes from transpiration.
204
EVAPOTRANSPIRATION
9.1 INTRODUCTION
EVAPORATION
Evaporation is the process whereby liquid water is converted to water vapour
(vaporisation) and removed from the evaporating surface (vapour removal).
Water evaporates from “free” water surfaces, such as oceans, lakes, rivers, soils
and wet vegetation.
TRANSPIRATION
Transpiration consists of the vaporisation of liquid water contained in plant
tissues and the vapour removal to the atmosphere. Crops predominately lose
their water through stomata. These are small openings on the plant leaf through
which gases and water vapour pass (Figure 9.1). The water, together with some
nutrients, is taken up by the roots and transported through the plant. The
vaporisation occurs within the leaf, namely in the intercellular spaces, and the
vapour exchange with the atmosphere is controlled by the stomatal aperture.
Nearly all water taken up is lost by transpiration and only a tiny fraction is used
within the plant.
Figure 9.1 Schematic representation of a stoma (Allen et al., 1998).
203
, EVAPOTRANSPIRATION
Since the forcing mechanisms behind evaporation and transpiration are similar
and the end result of these two processes is the same, it is possible to treat them
together as a combined process called evapotranspiration. Evapotranspiration
is therefore the term given to the total movement of moisture into the atmosphere
(evaporation plus transpiration) and can be seen as the process by which liquid
water is converted to water vapour and removed from the evaporative surface
(Buckle, 1996).
Evapotranspiration is an important aspect of the hydrological cycle and forms a
fundamental process in the energy balance of the Earth-atmosphere system. It is
a continuous 24-hour process, although it reaches a maximum by day. In the
process heat energy is removed from an evaporating surface causing it to cool.
This heat is absorbed by water vapour where it remains as latent heat. This
latent hat does not change the air temperature; rather it is a store of potential
energy that is later released when the vapour condenses into clouds. Such
vapour is sometimes advected thousands of kilometres from its source before
condensing and releasing the stored heat.
Apart from the water availability in the topsoil, the evaporation from a cropped
soil is mainly determined by the fraction of the solar radiation reaching the soil
surface. This fraction decreases over the growing period as the crop develops
and the crop canopy shades more and more of the ground area. When the crop
is small, water is predominately lost by soil evaporation, but once the crop is well
developed and completely covers the soil, transpiration becomes the main
process. In Figure 9.2 the partitioning of evapotranspiration into evaporation and
transpiration is plotted in correspondence to leaf area per unit surface of soil
below it. At sowing nearly 100% of ET comes from evaporation, while at full crop
cover more than 90% of ET comes from transpiration.
204
