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).EvaporationEvaporation 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.FIGURE 1. Schematic representation of a stomaFIGURE 2. The partitioning of evapotranspiration into evaporation and transpiration over the growing period for an annual field cropTranspirationTranspiration 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.UnitsThe evapotranspiration rate is normally expressed in millimetres (mm) per unit time. The rate expresses the amount of water lost from a cropped surface in units of water depth. The time unit can be an hour, day, decade, month or even an entire growing period or year.As one hectare has a surface of 10000 m2 and 1 mm is equal to 0.001 m, a loss of 1 mm of water corresponds to a loss of 10 m3 of water per hectare. In other words, 1 mm day-1 is equivalent to 10 m3 ha-1 day-l.Water depths can also be expressed in terms of energy received per unit area. The energy refers to the energy or heat required to vaporize free water. This energy, known as the latent heat of vaporization (l), is a function of the water temperature. For example, at 20C, l is about 2.45 MJ kg-1. In other words, 2.45 MJ are needed to vaporize 1 kg or 0.001 m3 of water. Hence, an energy input of 2.45 MJ per m2 is able to vaporize 0.001 m or 1 mm of water, and therefore 1 mm of water is equivalent to 2.45 MJ m-2. The evapotranspiration rate expressed in units of MJ m-2 day-1 is represented by l ET, the latent heat flux.Table 1 summarizes the units used to express the evapotranspiration rate and the conversion factors.TABLE 1. Conversion factors for evapotranspiration depth volume per unit area energy per unit area * mm day-1 m3 ha-1 day-1 l s-1 ha-1 MJ m-2 day-1 1 mm day-1 1 10 0.116 2.45 1 m3 ha-1 day-1 0.1 1 0.012 0.245 1 l s-1 ha-1 8.640 86.40 1 21.17 1 MJ m-2 day-1 0.408 4.082 0.047 1 * For water with a density of 1000 kg m-3 and at 20C.EXAMPLE 1. Converting evaporation from one unit to anotherOn a summer day, net solar energy received at a lake reaches 15 MJ per square metre per day. If 80% of the energy is used to vaporize water, how large could the depth of evaporation be?From Table 1:1 MJ m-2 day-1 =0.408mm day-1Therefore:0.8 x 15 MJ m-2 day-1 = 0.8 x 15 x 0.408 mm d-1 =4.9mm day-1The evaporation rate could be 4.9 mm/dayFIGURE 3. Factors affecting evapotranspiration with reference to related ET conceptsFactors affecting evapotranspiration Weather parameters Crop factors Management and environmental conditions
Distinctions are made (Figure 4) between reference crop evapotranspiration (ETo), crop evapotranspiration under standard conditions (ETc) and crop evapotranspiration under non-standard conditions (ETc adj). ETo is a climatic parameter expressing the evaporation power of the atmosphere. ETc refers to the evapotranspiration from excellently managed, large, well-watered fields that achieve full production under the given climatic conditions. Due to sub-optimal crop management and environmental constraints that affect crop growth and limit evapotranspiration, ETc under non-standard conditions generally requires a correction.Reference crop evapotranspiration (ETo)The evapotranspiration rate from a reference surface, not short of water, is called the reference crop evapotranspiration or reference evapotranspiration and is denoted as ETo. The reference surface is a hypothetical grass reference crop with specific characteristics. The use of other denominations such as potential ET is strongly discouraged due to ambiguities in their definitions.The concept of the reference evapotranspiration was introduced to study the evaporative demand of the atmosphere independently of crop type, crop development and management practices. As water is abundantly available at the reference evapotranspiring surface, soil factors do not affect ET. Relating ET to a specific surface provides a reference to which ET from other surfaces can be related. It obviates the need to define a separate ET level for each crop and stage of growth. ETo values measured or calculated at different locations or in different seasons are comparable as they refer to the ET from the same reference surface.The only factors affecting ETo are climatic parameters. Consequently, ETo is a climatic parameter and can be computed from weather data. ETo expresses the evaporating power of the atmosphere at a specific location and time of the year and does not consider the crop characteristics and soil factors. The FAO Penman-Monteith method is recommended as the sole method for determining ETo. The method has been selected because it closely approximates grass ETo at the location evaluated, is physically based, and explicitly incorporates both physiological and aerodynamic parameters. Moreover, procedures have been developed for estimating missing climatic parameters.Typical ranges for ETo values for different agroclimatic regions are given in Table 2. These values are intended to familiarize inexperienced users with typical ranges, and are not intended for direct application. The calculation of the reference crop evapotranspiration is discussed in Part A of this handbook (Box 1).Crop evapotranspiration under standard conditions (ETc)The crop evapotranspiration under standard conditions, denoted as ETc, is the evapotranspiration from disease-free, well-fertilized crops, grown in large fields, under optimum soil water conditions, and achieving full production under the given climatic conditions.TABLE 2. Average ETo for different agroclimatic regions in mm/day Regions Mean daily temperature (C) Cool 10C Moderate 20C Warm > 30C Tropics and subtropics - humid and sub-humid 2 - 3 3 - 5 5 - 7 -arid and semi-arid 2 - 4 4 - 6 6 - 8 Temperate region - humid and sub-humid 1 - 2 2 - 4 4 - 7 -arid and semi-arid 1 - 3 4 - 7 6 - 9 BOX 1. Chapters concerning the calculation of the reference crop evapotranspiration (ETo)PART A ----Chapter 2 - FAO Penman-Monteith equation:This chapter introduces the user to the need to standardize one method to compute ETo from meteorological data. The FAO Penman-Monteith method is recommended as the method for determining reference ETo. The method and the corresponding definition of the reference surface are described.Chapter 3 - Meteorological data:The FAO Penman-Monteith method requires radiation, air temperature, air humidity and wind speed data. Calculation procedures to derive climatic parameters from the meteorological data are presented. Procedures to estimate missing meteorological variables required for calculating ETo are outlined. This allows for estimation of ETo with the FAO Penman-Monteith method under all circumstances, even in the case of missing climatic data.Chapter 4 - Determination of ETo:The calculation of ETo by means of the FAO Penman-Monteith equation, with different time steps, from the principal weather parameters and with missing data is described. The determination of ETo from pan evaporation is also presented.BOX 2. Chapters concerning the calculation of crop evapotranspiration under standard conditions (ETc)PART B ----Chapter 5 - Introduction to crop evapotranspiration:This chapter introduces the user to the 'Kc ETo' approach for calculating crop evapotranspiration. The effects of characteristics that distinguish field crops from the reference grass crop are integrated into the crop coefficient Kc. Depending on the purpose of the calculation, the required accuracy, the available climatic data and the time step with which the calculations have to be executed, a distinction is made between two calculation methods.Chapter 6 - ETc - Single crop coefficient (Kc):This chapter presents the first calculation method for crop evapotranspiration whereby the difference in evapotranspiration between the cropped and reference grass surface is combined into a single crop coefficient (Kc).Chapter 7 - ETc - Dual crop coefficient (Kc = Kcb + Ke):This chapter presents the other calculation method for crop evapotranspiration. Kc is split into two separate coefficients, one for crop transpiration (i.e., the basal crop coefficient Kcb) and one for soil evaporation (Ke).The amount of water required to compensate the evapotranspiration loss from the cropped field is defined as crop water requirement. Although the values for crop evapotranspiration and crop water requirement are identical, crop water requirement refers to the amount of water that needs to be supplied, while crop evapotranspiration refers to the amount of water that is lost through evapotranspiration. The irrigation water requirement basically represents the difference between the crop water requirement and effective precipitation. The irrigation water requirement also includes additional water for leaching of salts and to compensate for non-uniformity of water application. Calculation of the irrigation water requirement is not covered in this publication, but will be the topic of a future Irrigation and Drainage Paper.Crop evapotranspiration can be calculated from climatic data and by integrating directly the crop resistance, albedo and air resistance factors in the Penman-Monteith approach. As there is still a considerable lack of information for different crops, the Penman-Monteith method is used for the estimation of the standard reference crop to determine its evapotranspiration rate, i.e., ETo. Experimentally determined ratios of ETc/ETo, called crop coefficients (Kc), are used to relate ETc to ETo or ETc = Kc ETo.Differences in leaf anatomy, stomatal characteristics, aerodynamic properties and even albedo cause the crop evapotranspiration to differ from the reference crop evapotranspiration under the same climatic conditions. Due to variations in the crop characteristics throughout its growing season, Kc for a given crop changes from sowing till harvest. The calculation of crop evapotranspiration under standard conditions (ETc) is discussed in Part B of this handbook (Box 2).Crop evapotranspiration under non-standard conditions (ETc adj)The crop evapotranspiration under non-standard conditions (ETc adj) is the evapotranspiration from crops grown under management and environmental conditions that differ from the standard conditions. When cultivating crops in fields, the real crop evapotranspiration may deviate from ETc due to non-optimal conditions such as the presence of pests and diseases, soil salinity, low soil fertility, water shortage or waterlogging. This may result in scanty plant growth, low plant density and may reduce the evapotranspiration rate below ETc.The crop evapotranspiration under non-standard conditions is calculated by using a water stress coefficient Ks and/or by adjusting Kc for all kinds of other stresses and environmental constraints on crop evapotranspiration. The adjustment to ETc for water stress, management and environmental constraints is discussed in Part C of this handbook (Box 3).Determining evapotranspiration ET measurement ET computed from meteorological data ET estimated from pan evaporation
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