Evapotranspiration concepts Reference crop evapotranspiration ET o Crop evapotranspiration under standard conditions ET c Crop evapotranspiration under non-standard conditions ET c adj. Distinctions are made Figure 4 between reference crop evapotranspiration ET o , crop evapotranspiration under standard conditions ET c and crop evapotranspiration under non-standard conditions ET c adj. ET o is a climatic parameter expressing the evaporation power of the atmosphere. ET c 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, ET c under non-standard conditions generally requires a correction. Reference crop evapotranspiration ET o The evapotranspiration rate from a reference surface, not short of water, is called the reference crop evapotranspiration or reference evapotranspiration and is denoted as ET o.
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. ET o 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 ET o are climatic parameters. Consequently, ET o is a climatic parameter and can be computed from weather data. ET o 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 method has been selected because it closely approximates grass ET o 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 ET o 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 ET c The crop evapotranspiration under standard conditions, denoted as ET c , 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. The method and the corresponding definition of the reference surface are described. Calculation procedures to derive climatic parameters from the meteorological data are presented. Procedures to estimate missing meteorological variables required for calculating ET o are outlined.
This allows for estimation of ET o with the FAO Penman-Monteith method under all circumstances, even in the case of missing climatic data. Chapter 4 - Determination of ET o : The calculation of ET o 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 ET o from pan evaporation is also presented.
BOX 2. Chapters concerning the calculation of crop evapotranspiration under standard conditions ET c PART B Chapter 5 - Introduction to crop evapotranspiration: This chapter introduces the user to the 'K c ET o ' approach for calculating crop evapotranspiration. The effects of characteristics that distinguish field crops from the reference grass crop are integrated into the crop coefficient K c.
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 - ET c - Single crop coefficient K c : 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 K c.
K c is split into two separate coefficients, one for crop transpiration i. 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. 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, K c for a given crop changes from sowing till harvest. The calculation of crop evapotranspiration under standard conditions ET c is discussed in Part B of this handbook Box 2.
Crop evapotranspiration under non-standard conditions ET c adj The crop evapotranspiration under non-standard conditions ET c 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 ET c 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 ET c. The adjustment to ET c for water stress, management and environmental constraints is discussed in Part C of this handbook Box 3. ET measurement Evapotranspiration is not easy to measure. Specific devices and accurate measurements of various physical parameters or the soil water balance in lysimeters are required to determine evapotranspiration. The methods are often expensive, demanding in terms of accuracy of measurement and can only be fully exploited by well-trained research personnel.
Although the methods are inappropriate for routine measurements, they remain important for the evaluation of ET estimates obtained by more indirect methods.
BOX 3. Chapters concerning the calculation of crop evapotranspiration under non-standard conditions ET c adj PART C Chapter 8 - ET c under soil water stress conditions: This chapter discusses the reduction in transpiration induced by soil moisture stress or soil water salinity. The resulting evapotranspiration will deviate from the crop evapotranspiration under standard conditions. The evapotranspiration is computed by using a water stress coefficient, K s , describing the effect of water stress on crop transpiration.
Chapter 9 - ET c for natural, non-typical and non-pristine vegetation: Procedures that can be used to make adjustments to the K c to account for less than perfect growing conditions or stand characteristics are discussed. The procedures can also be used to determine K c for agricultural crops not listed in the tables of Part B.
Chapter 10 - ET c under various management practices: This chapter discusses various types of management practices that may cause the values for K c and ET c to deviate from the standard conditions described in Part B. Adjustment procedures for K c to account for surface mulches, intercropping, small areas of vegetation and management induced stress are presented. Chapter 11 - ET c during non-growing periods: This chapter describes procedures for predicting ET c during non-growing periods under various types of surface conditions.
Schematic presentation of the diurnal variation of the components of the energy balance above a well-watered transpiring surface on a cloudless day Energy balance and microclimatological methods Evaporation of water requires relatively large amounts of energy, either in the form of sensible heat or radiant energy. Therefore the evapotranspiration process is governed by energy exchange at the vegetation surface and is limited by the amount of energy available. Because of this limitation, it is possible to predict the evapotranspiration rate by applying the principle of energy conservation.
The energy arriving at the surface must equal the energy leaving the surface for the same time period. All fluxes of energy should be considered when deriving an energy balance equation. The various terms can be either positive or negative. In Equation 1 only vertical fluxes are considered and the net rate at which energy is being transferred horizontally, by advection, is ignored. Therefore the equation is to be applied to large, extensive surfaces of homogeneous vegetation only. Other energy terms, such as heat stored or released in the plant, or the energy used in metabolic activities, are not considered These terms account for only a small fraction of the daily net radiation and can be considered negligible when compared with the other four components.
The latent heat flux l ET representing the evapotranspiration fraction can be derived from the energy balance equation if all other components are known. Net radiation R n and soil heat fluxes G can be measured or estimated from climatic parameters. Measurements of the sensible heat H are however complex and cannot be easily obtained.
H requires accurate measurement of temperature gradients above the surface.
Another method of estimating evapotranspiration is the mass transfer method. This approach considers the vertical movement of small parcels of air eddies above a large homogeneous surface. The eddies transport material water vapour and energy heat, momentum from and towards the evaporating surface. By assuming steady state conditions and that the eddy transfer coefficients for water vapour are proportional to those for heat and momentum, the evapotranspiration rate can be computed from the vertical gradients of air temperature and water vapour via the Bowen ratio. Other direct measurement methods use gradients of wind speed and water vapour.
These methods and other methods such as eddy covariance, require accurate measurement of vapour pressure, and air temperature or wind speed at different levels above the surface.
Therefore, their application is restricted to primarily research situations. Soil water balance Evapotranspiration can also be determined by measuring the various components of the soil water balance. The method consists of assessing the incoming and outgoing water flux into the crop root zone over some time period Figure 6. Irrigation I and rainfall P add water to the root zone. Part of I and P might be lost by surface runoff RO and by deep percolation DP that will eventually recharge the water table.
Water might also be transported upward by capillary rise CR from a shallow water table towards the root zone or even transferred horizontally by subsurface flow in SF in or out of SF out the root zone. In many situations, however, except under conditions with large slopes, SF in and SF out are minor and can be ignored. Soil evaporation and crop transpiration deplete water from the root zone. The soil water balance method can usually only give ET estimates over long time periods of the order of week-long or ten-day periods.
Soil water balance of the root zone Lysimeters By isolating the crop root zone from its environment and controlling the processes that are difficult to measure, the different terms in the soil water balance equation can be determined with greater accuracy. How can we make sure that cities become more inclusive, with a smaller environmental footprint, and leave no-one behind? Vesna Blazhevska T 02 Jul Vesna Blazhevska T 02 Jul Press material. Goal 1: End poverty in all its forms everywhere.
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