EVAPOTRANSPIRATION

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).

Evaporation

Evaporation is the process whereby liquid water is converted to water vapour (vaporization) and removed from the evaporating surface vapour removal. Water evaporates from a variety of surfaces, such as lakes, rivers, pavements, soils and wet vegetation.

Energy is required to change the state of the molecules of water from liquid to vapour. Direct solar radiation and, to a lesser extent, the ambient temperature of the air provide this energy. 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. 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 replacement of the saturated air with drier air depends greatly on wind speed. Hence, solar radiation, air temperature, air humidity and wind speed are climatological parameters to consider when assessing the evaporation process.

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. Frequent rains, irrigation and water transported upwards in a soil from a shallow water table wet the soil surface. 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. 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. Under these circumstances the limited availability of water exerts a controlling influence on soil evaporation. In the absence of any supply of water to the soil surface, evaporation decreases rapidly and may cease almost completely within a few days.

Transpiration

Transpiration consists of the vaporization 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 1). The water, together with some nutrients, is taken up by the roots and transported through the plant. The vaporization 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.

Transpiration, like direct evaporation, depends on the energy supply, vapour pressure gradient and wind. Hence, radiation, air temperature, air humidity and wind terms should be considered when assessing transpiration. The soil water content and the ability of the soil to conduct water to the roots also determine the transpiration rate, as do waterlogging and soil water salinity. The transpiration rate is also influenced by crop characteristics, environmental aspects and cultivation practices. Different kinds of plants may have different transpiration rates. Not only the type of crop, but also the crop development, environment and management should be considered when assessing transpiration.

EVAPOTRANSPIRATION (ET)

Evaporation and transpiration occur simultaneously and there is no easy way of distinguishing between the two processes. 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 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.

Factors affecting evapotranspiration

      Weather parameters

The principal weather parameters affecting evapotranspiration are radiation, air temperature, humidity and wind speed. Several procedures have been developed to assess the evaporation rate from these parameters. The evaporation power of the atmosphere is expressed by the reference crop evapotranspiration (ETo). The reference crop evapotranspiration represents the evapotranspiration from a standardized vegetated surface. 

      Crop factors

The crop type, variety and development stage should be considered when assessing the evapotranspiration from crops grown in large, well-managed fields. Differences in resistance to transpiration, crop height, crop roughness, reflection, ground cover and crop rooting characteristics result in different ET levels in different types of crops under identical environmental conditions. Crop evapotranspiration under standard conditions (ETc) refers to the evaporating demand from crops that are grown in large fields under optimum soil water, excellent management and environmental conditions, and achieve full production under the given climatic conditions.

      Management and environmental conditions

Factors such as soil salinity, poor land fertility, and limited application of fertilizers, the presence of hard or impenetrable soil horizons, the absence of control of diseases and pests and poor soil management may limit the crop development and reduce the evapotranspiration. Other factors to be considered when assessing ET are ground cover, plant density and the soil water content. The effect of soil water content on ET is conditioned primarily by the magnitude of the water deficit and the type of soil. On the other hand, too much water will result in waterlogging which might damage the root and limit root water uptake by inhibiting respiration.

When assessing the ET rate, additional consideration should be given to the range of management practices that act on the climatic and crop factors affecting the ET process. Cultivation practices and the type of irrigation method can alter the microclimate, affect the crop characteristics or affect the wetting of the soil and crop surface. A windbreak reduces wind velocities and decreases the ET rate of the field directly beyond the barrier. The effect can be significant especially in windy, warm and dry conditions although evapotranspiration from the trees themselves may offset any reduction in the field. Soil evaporation in a young orchard, where trees are widely spaced, can be reduced by using a well-designed drip or trickle irrigation system. The drippers apply water directly to the soil near trees, thereby leaving the major part of the soil surface dry, and limiting the evaporation losses. The use of mulches, especially when the crop is small, is another way of substantially reducing soil evaporation. Anti-transpirants, such as stomata-closing, film-forming or reflecting material, reduce the water losses from the crop and hence the transpiration rate.

ESTIMATING EVAPOTRANSPIRATION

A value of the actual evapotranspiration (ET) over a catchment is more often obtained by first calculating the potential evapotranspiration (PE) assuming an unrestricted availability of water and then modifying the answer by accounting for the actual soil moisture content.

There are several formulae for calculating potential evaporation based on theoretical or empirical models, but the most commonly used are the following ones:

v  Penman equation:

This equation directly results from the basic formula which allowed estimating evaporation from an open water surface. Then:

Where:

QET = QS*(1 - r) - Ql

Ql =0.95*[8.64*107/(ρ*λ)]*σ*(273.16+Ta)4*(0.53+0.065*(ed-1.0)1/2)*(0.10+0.90*(n/N))

Eat = 0.3*(1+0.5*u2)*(ea-ed)

ü  Δ (mb/C) is the the slope of the saturation vapour pressure curve with respect to temperature.

ü  γ is the hygrometric constant (=0.65 mb/C).

ü  Ql is long wave radiation from the water body.

ü  r is a coefficient relating to vegetation cover (r = 0.25 for a short grassed surface).

ü  Ta is air temperature (C).

ü  n/N is the ratio of actual/possible sunshine hours of bright sunshine.

ü  ρ is the density of water (kg/m3).

ü  λ is the latent heat of evaporization of water (J/kg).

ü  σ is Stefan Bolzman's constant (= 5.7*10-8 W/(m2*grad4)).

ü  u2 is wind velocity (m/s).

ü  ea is the saturation vapour pressure for the measured air temperature (mb).

ü  ed is the actual vapour pressure of the air (mb).

ü  OBS: QET, Qs, Ql, Eat are all expressed in mm/day.

Example calculation:

The set of equations presented in this section are intended to calculate reference crop ET (height = 0.12 m, canopy resistance = 70 s m^-1 or 0.00081 d m^-1) for a full-day (24-h) period using the Penman-Monteith formulation.

1. The slope of the saturation vapor pressure function of temperature is given by
   

2. The atmospheric density in kg m^-3 is calculated as
   

3. Where P is the atmospheric pressure in kPa, and T_kv is the virtual temperature in K, T_kv = 1.01 (T +273). The psychrometric constant is given by
   

4. Where C_p is the specific heat capacity of moist air, and C_p = 0.001013 MJ kg^-1    C^-1, as given earlier. λ is the latent heat of vaporization as previously defined. The atmospheric pressure in kPa can be estimated as follows :
   

5. Where z is the elevation above the sea level in m. The latent heat of vaporization 8 in MJ kg^-1 is given by
   

v  Thornthwaite's formula:

This formula is based mainly on temperature with an adjustment being made for the number of daylight hours. An estimate of the potential evapotranspiration, calculated on a monthly basis, is given by:

 where m is the months 1, 2, 3…12, Nm is the monthly adjustment factor related to hours of daylight, Tm is the monthly mean temperature (C), I is the heat index for the year,

Given the monthly mean temperatures from the measurements at a climatology station, an estimate of the potential evaporation for each month of the year can be calculated. This method has been used widely throughout the world, but strictly is not valid for climates other than those similar to that area where it was developed.

Compared to the Penman formula, Thornthwaite values tend to exaggerate the potential evaporation. This is particularly marked in the summer months with the high temperatures having a dominant effect in the Tornthwaite computation, whereas the Penman estimate takes into consideration other meteorological factors.