The water management, more efficient use of water in important factor limiting the plant growth is lack of the agricultural sector should be achieved for the water beneficial to the crop [3]. Under no circumstances, this is necessary for In the research area, the Mediterranean sustainable water management so as to meet climate is dominant. Summers are hot and dry; additional water demand of other sectors.
On the winters are moderately warm and rainy. Long-term other hand, it has been experienced lately that about 60 years mean temperature was recorded as farmers in the Lower Seyhan Plain ASO have August is the hottest month incontrovertibly faced irrigation water shortages with However, the coldest month is due to the increasing demand for irrigation water.
January by 9. All of the precipitation nearly The ASO, one of the largest irrigation area in falls during the winter months in the form of rain. Turkey, has been partially irrigated since the The average annual precipitation is As a the distribution of precipitation during the year is result of irrigation applications in the region, not homogeneous.
This research was aimed at investigating A field survey was performed to determine irrigation performance in a large-scale irrigation subsurface lateral flows from the Ceyhan river, catchment located in the Lower Seyhan Plain. Flow rate monitoring stations FGSs were installed on irrigation and drainage channels to The research was conducted on the command measure incoming and outgoing waters in the study area of Akarsu Irrigation Association, located in the area Figure 1.
The incoming irrigation waters south of Adana province, Turkey. In the research area, irrigation has been season. L2 and L4 are drainage flow gauging statins practiced for more than 60 years. Irrigation for conducting water balance work. Limnigraphs were set rooting depth of 0. In this study, for the sake of up such that flow rates were recorded on hourly simplicity, it was assumed that there was no deep base by averaging 6 readings with ten minutes percolation and capillary rise in the study area.
In order to carry out the mass balance studies Likely subsurface lateral flows seepage in- for the water elements in Figure 2, it was necessary and out-flows along right-hand bank of Ceyhan to measure or estimate the main inputs and outputs River were assessed once a week, at each in the study area.
In situ originated from outside areas, likely lateral seepage horizontal hydraulic conductivities of the soil Dsi along Ceyhan River flood levee; and outputs profile along Ceyhan River were determined by were crop evapotranspiration ETc , irrigation auger-hole technique, following the principles outflows Iout discharging from YS2 main irrigation given in Oosterbaan and Nijland [14].
Then, the canal, drainage outflows, i. Assessing cropping pattern data led us to was winter wheat [16]. Corn was the dominant conclude that there was a gradual increase in citrus- irrigated field crop in summer, planted either as a planted areas. Monoculture should be recorded at ten-minute intervals at each cropping pattern development affects adversely FGS.
Assessing limnigraph data showed that total irrigation performance in the district even if there is amount of irrigation water diverted into the study nocturnal irrigation in the area. There is no doubt area was around mm, while a total of mm that introduction of nocturnal irrigations will precipitation was recorded throughout the year.
In our case, diurnal irrigation in the irrigation season IS Table 1. It implies practice was not well enough to increase irrigation that the irrigation water diverted for the study area management performance due to the fact that field was recklessly wasted as drainage outflows during crops, in particular, have been irrigated by surface the irrigation season.
This was due to the fact that irrigation methods. However, water diverted to the study area has been wasted by irrigation canals should have been operated based drainage.
To learn more, view our Privacy Policy. To browse Academia. Log in with Facebook Log in with Google. Remember me on this computer. Enter the email address you signed up with and we'll email you a reset link. Need an account? Click here to sign up. Download Free PDF. Peter Salvatierra. A short summary of this paper. Download Download PDF. Luis S. The FAO Penman method was found to frequently overestimate ETo while the other FAO recommended equations, namely the radiation, the Blaney- Criddle, and the pan evaporation methods, showed variable adherence to the grass reference crop evapotranspiration.
In May , FAO organized a consultation of experts and researchers in collaboration with the International Commission for Irrigation and Drainage and with the World Meteorological Organization, to review the FAO methodologies on crop water requirements and to advise on the revision and update of procedures.
The panel of experts recommended the adoption of the Penman-Monteith combination method as a new standard for reference evapotranspiration and advised on procedures for calculating the various parameters. The FAO Penman-Monteith method was developed by defining the reference crop as a hypothetical crop with an assumed height of 0. The method overcomes the shortcomings of the previous FAO Penman method and provides values that are more consistent with actual crop water use data worldwide.
Furthermore, recommendations have been developed using the FAO Penman-Monteith method with limited climatic data, thereby largely eliminating the need for any other reference evapotranspiration methods and creating a consistent and transparent basis for a globally valid standard for crop water requirement calculations.
The FAO Penman-Monteith method uses standard climatic data that can be easily measured or derived from commonly measured data. All calculation procedures have been standardized according to the available weather data and the time scale of computation. The calculation methods, as well as the procedures for estimating missing climatic data, are presented in this publication.
In the 'Kc-ETo' approach, differences in the crop canopy and aerodynamic resistance relative to the reference crop are accounted for within the crop coefficient. The Kc coefficient serves as an aggregation of the physical and physiological differences between crops. The first approach integrates the relationships between evapotranspiration of the crop and the reference surface into a single Kc coefficient. In the second approach, Kc is split into two factors that separately describe the evaporation Ke and transpiration Kcb components.
The selection of the Kc approach depends on the purpose of the calculation and the time step on which the calculations are to be executed. The final chapters present procedures that can be used to make adjustments to crop coefficients to account for deviations from standard conditions, such as water and salinity stress, low plant density, environmental factors and management practices. Examples demonstrate the various calculation procedures throughout the publication.
Most of the computations, namely all those required for the reference evapotranspiration and the single crop coefficient approach, can be performed using a pocket calculator, calculation sheets and the numerous tables given in the publication.
The user may also build computer algorithms, either using a spreadsheet or any programming language. These guidelines are intended to provide guidance to project managers, consultants, irrigation engineers, hydrologists, agronomists, meteorologists and students for the calculation of reference and crop evapotranspiration. They can be used for computing crop water requirements for both irrigated and rainfed agriculture, and for computing water consumption by agricultural and natural vegetation.
Crop evapotranspiration v Acknowledgements These guidelines constitute the efforts of eight years of deliberations and consultations by the authors, who together formed the working group to pursue the recommendations of the FAO expert consultation that was held in May in Rome. Doorenbos and W. The conceptual framework for the revised methodologies introduced in this publication came forth out of the advice of the group of eminent experts congregated in the meetings and who have importantly contributed to the development of the further studies conducted in the framework of the publication.
Fleming of Australia, Dr A. Perrier of France, Drs L. Cavazza and L. Tombesi from Italy, Drs R. Feddes and J. Doorenbos of the Netherlands, Dr L. Pereira of Portugal, Drs J. Monteith and H. Gunston from the United Kingdom, Drs R. Allen, M. Jensen and W. Many other experts and persons from different organizations and institutes have provided, in varying degrees and at different stages, important advice and contributions.
Special acknowledgements for this are due in particular to Prof. Pruitt retired of the University of California, Davis and J. Monteith whose unique work set the scientific basis for the ETo review. Pruitt, despite his emeritus status, has consistently contributed in making essential data available and in advising on critical concepts.
Dr James L. Wright of the USDA, Kimberly, Idaho, further contributed in providing data from the precision lysimeter for several crops. Further important contributions or reviews at critical stages of the publication were received from Drs M. Jensen, G. Hargreaves and C. The authors thank their respective institutions, Utah State University, Instituto Superior de Agronomia of Lisbon, Katholieke Universiteit Leuven and FAO for the generous support of faculty time and staff services during the development of this publication.
The authors wish to express their gratitude to Mr H. Wolter, Director of the Land and Water Development Division for his encouragement in the preparation of the guidelines and to FAO colleagues and others who have reviewed the document and made valuable comments. Special thanks are due to Ms Chrissi Redfern for her patience and valuable assistance in the preparation and formatting of the text.
Mr Julian Plummer further contributed in editing the final document. Schematic representation of a stoma 2 2. The partitioning of evapotranspiration into evaporation and transpiration over the growing period for an annual field crop 2 3. Factors affecting evapotranspiration with reference to related ET concepts 4 4.
Schematic presentation of the diurnal variation of the components of the energy balance above a well-watered transpiring surface on a cloudless day 10 6. Soil water balance of the root zone 12 7. Simplified representation of the bulk surface and aerodynamic resistances for water vapour flow 19 8.
Typical presentation of the variation in the green Leaf Area Index over the growing season for a maize crop 22 9. Characteristics of the hypothetical reference crop 24 Illustration of the effect of wind speed on evapotranspiration in hot-dry and humid- warm weather conditions 30 Variation of the relative humidity over 24 hours for a constant actual vapour pressure of 2.
Various components of radiation 44 Conversion factor to convert wind speed measured at a certain height above ground level to wind speed at the standard height 2 m 56 Two cases of evaporation pan siting and their environment 79 Typical Kc for different types of full grown crops 92 Extreme ranges expected in Kc for full grown crops as climate and weather change 92 The effect of evaporation on Kc. The horizontal line represents Kc when the soil surface is kept continuously wet.
The curved line corresponds to Kc when the soil surface is kept dry but the crop receives sufficient water to sustain full transpiration 94 Typical ranges expected in Kc for the four growth stages 97 Generalized crop coefficient curve for the single crop coefficient approach General procedure for calculating ETc Variation in the length of the growing period of rice cultivar: Jaya sown during various months of the year at different locations along the Senegal River Africa Average Kc ini as related to the level of ETo and the interval between irrigations greater than or equal to 40 mm per wetting event, during the initial growth stage for: a coarse textured soils; b medium and fine textured soils Partial wetting by irrigation Adjustment additive to the Kc mid values from Table 12 for different crop heights and mean daily wind speeds u2 for different humidity conditions Ranges expected for Kc end Crop coefficient curve Constructed curve for Kc for alfalfa hay in southern Idaho, the United States using values from Tables 11 and 12 and adjusted using Equations 62 and 65 Kc curve and ten-day values for Kc and ETc derived from the graph for the dry bean crop example Box 15 Constructed basal crop coefficient Kcb curve for a dry bean crop Example 28 using growth stage lengths of 25, 25, 30 and 20 days Soil evaporation reduction coefficient, Kr Determination of variable few as a function of the fraction of ground surface coverage fc and the fraction of the surface wetted fw Water balance of the topsoil layer Depletion factor for different levels of crop evapotranspiration Water stress coefficient, Ks Water balance of the root zone The effect of soil salinity on the water stress coefficient Ks Different situations of intercropping Conversion factors for evapotranspiration 4 2.
Conversion factors for radiation 45 4. General classes of monthly wind speed data 63 5. Pan coefficients Kp for Class A pan for different pan siting and environment and different levels of mean relative humidity and wind speed 81 6. Pan coefficients Kp for Colorado sunken pan for different pan siting and environment and different levels of mean relative humidity and wind speed 81 7.
Pan coefficients Kp : regression equations derived from Tables 5 and 6 82 8. Ratios between the evaporation from sunken pans and a Colorado sunken pan for different climatic conditions and environments 83 9. Approximate values for Kc ini for medium wetting events mm and a medium textured soil 95 General selection criteria for the single and dual crop coefficient approaches 98 Lengths of crop development stages for various planting periods and climatic regions Classification of rainfall depths Kc ini for rice for various climatic conditions Empirical estimates of monthly wind speed data Typical values for RHmin compared with RHmean for general climatic classifications General guidelines to derive Kcb from the Kc values listed in Table 12 Typical soil water characteristics for different soil types Common values of fraction fw of soil surface wetted by irrigation or precipitation Common values of fractions covered by vegetation fc and exposed to sunlight 1- f c Ranges of maximum effective rooting depth Zr , and soil water depletion fraction for no stress p , for common crops Salt tolerance of common agricultural crops as a function of the electrical conductivity of the soil saturation extract at the threshold when crop yield first reduces below the full yield potential ECe, threshold and when crop yields becomes zero ECe, no yield.
Chapters concerning the calculation of the reference crop evapotranspiration ETo 8 2. Chapters concerning the calculation of crop evapotranspiration under standard conditions ETc 8 3.
Chapters concerning the calculation of crop evapotranspiration under non-standard conditions ETc adj 10 4. The aerodynamic resistance for a grass reference surface 21 5. The bulk surface resistance for a grass reference crop 22 6. Calculation sheet for vapour pressure deficit es - ea 40 8. Conversion from energy values to equivalent evaporation 44 9. Calculation sheet for extraterrestrial radiation Ra and daylight hours N 49 Calculation sheet for net radiation Rn 53 Description of Class A pan 84 Description of Colorado sunken pan 85 Demonstration of effect of climate on Kc mid for tomato crop grown in field Case study of a dry bean crop at Kimberly, Idaho, the United States single crop coefficient Case study of dry bean crop at Kimberly, Idaho, the United States dual crop coefficient Measuring and estimating LAI Converting evaporation from one unit to another 4 2.
Determination of atmospheric parameters 32 3. Determination of saturation vapour pressure 36 4. Determination of actual vapour pressure from psychrometric readings 38 5.
Determination of actual vapour pressure from relative humidity 39 6. Determination of vapour pressure deficit 39 7. Conversion of latitude in degrees and minutes to radians 46 8. Determination of extraterrestrial radiation 47 9. Determination of daylight hours 49 Determination of solar radiation from measured duration of sunshine 50 Determination of net longwave radiation 52 Determination of net radiation 53 Determination of soil heat flux for monthly periods 55 Adjusting wind speed data to standard height 56 Determination of solar radiation from temperature data 61 Determination of net radiation in the absence of radiation data 62 Determination of ETo with mean monthly data 70 Determination of ETo with daily data 72 Determination of ETo with hourly data 75 Determination of ETo with missing data 77 Determination of ETo from pan evaporation using tables 83 Determination of ETo from pan evaporation using equations 86 Estimation of interval between wetting events Graphical determination of Kc ini Interpolation between light and heavy wetting events Determination of Kc ini for partial wetting of the soil surface Determination of Kc mid Numerical determination of Kc Selection and adjustment of basal crop coefficients, Kcb Determination of the evapotranspiration from a bare soil Estimation of crop evapotranspiration with the dual crop coefficient approach Determination of readily available soil water for various crops and soil types Effect of water stress on crop evapotranspiration Irrigation scheduling to avoid crop water stress Effect of soil salinity on crop evapotranspiration First approximation of the crop coefficient for the mid-season stage for sparse vegetation Estimation of mid-season crop coefficient Estimation of mid-season crop coefficient for reduced ground cover Estimation of Kcb mid from ground cover with reduction for stomatal control Effects of surface mulch Intercropped maize and beans Energy balance equation 11 2.
Soil water balance 12 3. Penman-Monteith form of the combination equation 19 4. Aerodynamic resistance ra 20 5. Bulk surface resistance rs 21 6. FAO Penman-Monteith equation for daily, ten-day and monthly time steps 24 7. Atmospheric pressure P 31 8. Mean air temperature Tmean 33 Relative humidity RH 35 Saturation vapour pressure es 36 Actual vapour pressure derived from dewpoint temperature ea 37 Actual vapour pressure derived from psychrometric data ea 37 Actual vapour pressure derived from RHmax and RHmin ea 38 Actual vapour pressure derived from RHmax ea 39 Actual vapour pressure derived from RHmean ea 39 Conversion form energy to equivalent evaporation 44 Extraterrestrial radiation for daily periods Ra 46 Conversion from decimal degrees to radians 46 Inverse relative distance Earth-Sun dr 46 Parameter X of Equation 26 47 Extraterrestrial radiation for hourly or shorter periods Ra 47 Seasonal correction for solar time Sc 48 Parameter b of Equation 32 48 Daylight hours N 48 Solar radiation Rs 50 Clear-sky radiation near sea level Rso 51 Clear-sky radiation at higher elevations Rso 51 Net solar or net shortwave radiation Rns 51 Net longwave radiation Rnl 52 Net radiation Rn 53 Soil heat flux G 54 Soil heat flux for day and ten-day periods Gday 54 Soil heat flux for monthly periods Gmonth 54 Soil heat flux for hourly or shorter periods during daytime Ghr 55 Soil heat flux for hourly or shorter periods during nighttime Ghr 55 Adjustment of wind speed to standard height u2 56 Estimating actual vapour pressure from Tmin ea 58 Importing solar radiation from a nearby weather station Rs 59 Estimating solar radiation for island locations Rs 62 FAO Penman-Monteith equation for hourly time step 74 Actual vapour pressure for hourly time step 74 Deriving ETo from pan evaporation 79 Crop evapotranspiration ETc 90 Dual crop coefficient 98 Crop evapotranspiration - single crop coefficient ETc Interpolation for infiltration depths between 10 and 40 mm Adjustment of Kc ini for partial wetting by irrigation Irrigation depth for the part of the surface that is wetted Iw Climatic adjustment for Kc end Interpolation of Kc for crop development stage and late season stage Relation between grass-based and alfalfa-based crop coefficients Ratio between grass-based and alfalfa-based Kc for Kimberly, Idaho Crop evapotranspiration - dual crop coefficient ETc Climatic adjustment for Kcb Soil evaporation coefficient Ke Upper limit on evaporation and transpiration from any cropped surface Kc max Maximum depth of water that can be evaporated from the topsoil TEW Evaporation reduction coefficient Kr Exposed and wetted soil fraction few Effective fraction of soil surface that is covered by vegetation fc Daily soil water balance for the exposed and wetted soil fraction Limits on soil water depletion by evaporation De Drainage out of topsoil DPe Crop evapotranspiration adjusted for water stress - dual crop coefficient Crop evapotranspiration adjusted for water stress - single crop coefficient Total available soil water in the root zone TAW Readily available soil water in the root zone RAW Water stress coefficient Ks Limits on root zone depletion by evapotranspiration Dr Initial depletion Dr,i-1 Deep percolation DP Water stress coefficient Ks under saline conditions Water stress coefficient Ks under saline and water stress conditions Adjustment coefficient from LAI Adjustment coefficient from fc K cb mid adj from Leaf Area Index K cb mid adj from effective ground cover Kcb full for agricultural crops Kcb full for natural vegetation Kcb h for full cover vegetation Adjustment for stomatal control Fr Water stress coefficient Ks estimated from yield response to water function Crop coefficient estimate for intercropped field Kc field 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.
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 near 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. 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. 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 10 m2 and 1 mm is equal to 0. In other words, 1 mm day-1 is equivalent to 10 m3 ha-1 day 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. In other words, 2. Hence, an energy input of 2. The related ET concepts presented in Figure 3 are discussed in the section on evapotranspiration concepts.
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. The ETo is described in detail later in this Chapter and in Chapters 2 and 4. It moves rapidly out of well drained soil and is not considered to be available to plants.
This means that the tension measured in bars is increasing as the soil dries out. The water is held so tightly by the soil that it can not be taken up by roots. This means clay will contain much more of this type of water than sands because of surface area differences. However, the force of gravity is counteracted by forces of attraction between water molecules and soil particles and by the attraction of water molecules to each other.
At the water table reference, the pressure potential is set equal to zero. This negative pressure in unsaturated soil is termed matric, tension or suction pressure potential so as not to confuse it with positive pressures. Water content and matric potential are strongly related to soil texture through the effects of pore size and shape. Therefore the pore size distribution is determined to a large extent by the grain size distribution.
Most soils are a mixture of grain sizes. When the soil becomes dry and plants cannot take up water anymore the soil is at W. The amount of water held by a soil in the root zone between F. For sand, loam and clay the values are 6, 20 and 17 volume percent respectively. Note that 1 vol. This means that well irrigated crops with a water use of 5 mm a day, have to be irrigated every 6, 20 or 17 days for respectively sand, loam and clay.
Some examples of available water are given. The soil is at field capacity. At the field capacity the matric forces just exceeds the force of gravity. Afterward, if there is no more rainfall the capillary water is slowly redistributed and drawn towards the surface where it is removed by plants and by evaporation.
Eventually, almost all the capillary water is lost, the soil is at the wilting point plants cannot obtain the remaining moisture and wilt. Most of the water left is referred to hydroscopic water. After one to three days, this rapid downward movement becomes negligible and matric forces begin to play a greater role in water movement. The soil is then at field capacity. In this condition, water has moved out of macropores and air has moved in. Micropores are still filled with water and continue to supply plants.
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