Crop ETEstimation Models AReview And Discussion Subedi Chavez2015 (original) (raw)
Related papers
Journal of Agricultural Science, 2015
This is a review paper on existing methodologies to calculate crop evapotranspiration (ET c ). We have attempted to present all the important ET estimation procedures to date starting from the simple empirical Blaney Criddle method to the complex Shuttleworth model. The common approach to calculate ET c is to estimate a reference crop ET rate (ET ref ) using weather variables from nearby weather station, and multiplying it by an appropriate crop coefficient (K c ). Recently, there have been attempts to calculate actual crop ET (ET a ) directly without using K c . The latter method is still in the developmental phase. This study reviews the existing literature on ET estimation and identifies research needs in the current methods and technology. The extension of the Shuttleworth model for hourly time step and the validity of the Irmak and Mutibwaa model at field level for various crops would be a good milestone for the one step ET estimation. Furthermore, there are indications that the development of a new variable canopy surface resistance (r c ) model, which can be applicable for different crops at different climatic conditions, would be a good contribution in this field.
Estimation of short-term actual crop évapotranspiration
2006
A method is presented for estimating the hourly actual évapotranspiration (ET) from short natural vegetation or agricultural crops. The method consists of equating the ET flux equations based on the generalized Penman-Monteith (GPM) combination method and a humidity gradient (HG) method. By equating the GPM and HG expressions, a single unknown parameter, either the bulk surface resistance (r s) or aerodynamic resistance (r a), can be determined. In the procedure, the value of the resistance factor is adjusted until the daily ET time series curves from the two methods approximately coincide. An overview of the technical approach and the results of a comparison between the new method and an eddy covariance system at the University of Florida at Gainesville are provided. To illustrate the utility of the method an example is presented in which the average daily ET was determined for a growing season of common bean {Phaseolus vulgaris L.) at Juana Diaz, Puerto Rico. In this example the surface resistance was measured (i.e., stomatal resistance and leaf area) and estimated using the proposed method. A third method was also evaluated in which the surface resistance was estimated using the equation of Ortega-Farias and Fuentes (1999). All three methods were in close agreement.
Comparison of various methods for estimating reference crop evapotranspiration
The reference crop evapotranspiration (ETo) was estimated for three locations of Bangladesh using the standard Penman-Monteith (P-M) method as recommended by the Food and Agriculture Organization of the United Nations (FAO). The same was also estimated using four other emperical methods and compared with the standard method. The regression equations developed were evaluated with independent data sets. The superiority order were found as: FAO temperature, Radiation, Hargreaves and pan evaporation, respectively. lt is revealed from the study that the regression equation developed herein with FAO temperature method can be used to estimate ETo more accurately than the original FAO temperature method, as well as regression equations with other methods. lntroduction Management practices for optimal utilization of water have been increasingly emphasized because of unevenly distributed rainfall, high evapotranspiration and excessive depletion of groundwater resources. Thus practical methods for the accurate estimation of water use in irrigated agriculture are essential. The estimation of crop water requirement is one of the principal steps in the planning, design and operation of irrigation and water resources systems. Crop water requirements vary with crop characteristics and local condition. Relationships between the evapotranspiration of a pre-selected crop (the reference crop), which is referred to as reference evapotranspiration (ETo), and other crops are established by multiplying ETo by crop coefficients. The ETs depends on local meteorological conditions, whereas the actual evapotranspiration (ET) of a crop depends on its characteristics, time of planting or sowing and stage of crop development.
Evaluation of Reference Crop Evapotranspiration Equations In Various Climates
Water Resources Management
Evapotranspiration is one of the most important elements for quantifying available water since it generally constitutes the largest component of the terrestrial water cycle. This study evaluated four models (Makkink, Turc, Priestley–Taylor and Hargreaves) commonly used to estimate monthly reference crop evapotranspiration (ETo) values. The main aim of this study was to determine the model used to estimate ETo with small data requirements and high accuracy for twelve synoptic stations in four climates of Iran. The results showed that the Turc model was the best suited model in estimating ETo for cold humid and arid climates. The Hargreaves model turned out to be the most precise model under warm humid and semi-arid climatic conditions. In contrast, the Makkink model presented the poorest estimates in all of the climates exception for cold humid environment. In cold humid climate, the Hargreaves model was the least accurate model in estimating ETo. In general, the results obtained from this study revealed very clearly that the Makkink and Priestley–Taylor models estimated ETo values less accurately than Turc and Hargreaves models for the all climates.
Calculating crop evapotranspiration using a dual crop coefficient - part 1 : technical
2009
The series of technical articles covers topics related to the planning and design of irrigation systems. In previous articles, we started off by looking at the irrigation planning process, which is aimed at combining climatic, soil, crop, irrigation system and management information in order to identify a suitable irrigation emitter for the system being planned, for an appropriate irrigation cycle length and standing time at the peak irrigation period expected during the growing season of a specific crop. Other topics covered in the second, third and fourth articles of the series, included the calculation of reference evapotranspiration using the Penman-Monteith (PM) equation, the collection of climatic data for the PM equation using weather stations, and the assessment of soils' suitability for irrigation. In the next 3 issues, we look further at the development of crop coefficients to calculate crop water requirements for irrigation planning and management.
Methods to estimate irrigated reference crop evapotranspiration – a review
Water Science & Technology, 2012
Efficient water management of crops requires accurate irrigation scheduling which, in turn, requires the accurate measurement of crop water requirement. Irrigation is applied to replenish depleted moisture for optimum plant growth. Reference evapotranspiration plays an important role for the determination of water requirements for crops and irrigation scheduling. Various models/approaches varying from empirical to physically base distributed are available for the estimation of reference evapotranspiration. Mathematical models are useful tools to estimate the evapotranspiration and water requirement of crops, which is essential information required to design or choose best water management practices. In this paper the most commonly used models/approaches, which are suitable for the estimation of daily water requirement for agricultural crops grown in different agro-climatic regions, are reviewed. Further, an effort has been made to compare the accuracy of various widely used methods ...
Water Resources Management, 2011
The study at first recalls the concept of “potential evapotranspiration” (PET), originally considered equal to the evaporation climatic demand; then, it reminds the steps of its progressive evolution toward the concept of “reference crop evapotranspiration” (ET0) determined on irrigated grass. A physical analysis conducted on the evaporation process is subsequently reported to help clarifying the links between ET0 and evaporation climatic demand. This analysis clearly demonstrates that the equivalence of ET0 to evaporation climatic demand is not correct, although still common assumption in recent scientific literature, particularly in hydrology. The study also identifies two processes acting in opposite directions in the dynamics of ET0: (1) the climatic variables determining the evaporation demand, and (2) the canopy resistance which slows down the response of irrigated grass to such demand. The analysis of the respective impact of these two processes on ET0 dynamics shows that the available energy is the dominant process. This variable takes into account the 60–70% of the variation of ET0, both at hourly and daily scales, while canopy resistance only explains 10–20% of ET0 variation of irrigated grass. The study regards different climatic situations. Possible effects on practical applications were also discussed in the conclusions, together with comments on the correct canopy resistance modelling.
Journal of Irrigation and Drainage Engineering, 2005
The Imperial Irrigation District is a large irrigation project in the western United States having a unique hydrogeologic structure such that only small amounts of deep percolation leave the project directly as subsurface flows. This structure is conducive to relatively accurate application of a surface water balance to the district, enabling the determination of crop evapotranspiration ͑ET c ͒ as a residual of inflows and outflows. The ability to calculate ET c from discharge measurements provides the opportunity to assess the accuracy and consistency of an independently applied crop coefficient-reference evapotranspiration ͑K c ET 0 ͒ procedure integrated over the project. The accuracy of the annual crop evapotranspiration via water balance estimates was ±6% at the 95% confidence level. Calculations using K c and ET 0 were based on the FAO-56 dual crop coefficient approach and included separate calculation of evaporation from precipitation and irrigation events. Grass reference ET 0 was computed using the CIMIS Penman equation and ET c was computed for over 30 crop types. On average, K c -based ET computations exceeded ET c determined by water balance (referred to as ET c WB ) by 8% on an annual basis over a 7 year period. The 8% overprediction was concluded to stem primarily from use of K c that represents potential and ideal growing conditions, whereas crops in the study area were not always in full pristine condition due to various water and agronomic stresses. A 6% reduction to calculated K c -based ET was applied to all crops, and a further 2% reduction was applied to lower value crops to bring the project-wide ET predicted by K c -based ET into agreement with ET c WB . The standard error of estimate (SEE) for annual ET c for the entire project based on K c , following the reduction adjustment, was 3.4% of total annual ET c , which is considered to be quite good. The SEE for the average monthly ET c was 15% of average monthly ET c . A sensitivity analysis of the computational procedure for K c showed that relaxation from using the FAO-56 dual K c method to the more simple mean (i.e., single) K c curve and relaxation of specificity of planting and harvest dates did not substantially increase the projectwide prediction error The use of the mean K c curves, where effects of evaporation from wet soil are included as general averages, predicted 5% lower than the dual method for monthly estimates and 8% lower on an annual basis, so that no adjustment was required to match annual ET derived from water balance. About one half of the reduction in estimates when applying the single (or mean) K c method rather than the dual K c method was caused by the lack of accounting for evaporation from special irrigations during the off season (i.e., in between crops).
Current frameworks for reference ET and crop coefficient calculation
6th Decennial National Irrigation Symposium, 6-8, December 2021, San Diego, California, 2021
This article describes current and likely near-term future frameworks for calculating evapotranspiration. These include structures for estimating crop coefficients (Kc) primarily centered on the FAO-56 dual Kc approach, with example applications. Emphasis is placed on estimation of parameters and special cases to be considered. Newer, and often preferred, bases for establishing Kcb curves include thermal units and vegetation indices. Also described and discussed are the application of reference ET calculations using hourly vs. 24-hour timesteps, the use of and conditioning of gridded weather data sets, and the likelihood of movement toward multi-layer and multi-source resistance models for ET estimation. Complementing this is satellite-based determination of ET using both vegetation indices and surface energy balance.