ET0 Values Research Articles

Research on the joint probability distribution of rainfall and reference crop evapotranspiration

The relationship between rainfall and reference crop evapotranspiration (ET0) can be complex in an irrigation district. To reveal such complex and possibly a nonlinear relationship, rainfall and ET0 data were collected at the Xinxiang irrigation experiment station from the years 1961 to 2010. Marginal distributions of the two variables derived from fitting the Pearson Type III distribution were incorporated for the development of the joint probability distribution using a copula model. Subsequently, the joint probability as well as the return period of given specific rainfall and ET0 values can be computed. For the analysis of the variations in drought conditions over the 50 years, the copula-based joint probability is further decomposed into several subcomponents using the empirical mode decomposition method. The variations under distinct frequencies and over different time periods were observed. Our results showed that the joint probability distribution of rainfall and ET0 can facilitate the planning for drought resistance in an irrigation district.

Developing Equations for Estimating Reference Evapotranspiration in Australia

Quantifying reference evapotranspiration (ET0) is essential in water resources management. Although, many methods have been developed with different level of accuracy, in this study, two new equations were developed and optimized for estimating ET0 using Honey-Bee Mating Optimization (HBMO) algorithm. The first eq. estimates ET0 from extraterrestrial radiation (Ra), relative humidity (RH) and mean daily temperature (Tmean), while the second uses the same parameters except that mean daily temperatures is replaced with maximum daily air temperature (Tmax). Both equations were developed using climatic data from eight weather stations in Western Australia and subsequently verified using data from ten sites across Australia. The estimated ET0 values from both equations versus the FAO56-Penman-Monteith have a coefficient of determination, R2, of larger than 0.96. Moreover, the performance of six commonly used methods of estimating ET0 including Hargreaves-Samani, Thornthwaith, Hamon, Mc Guinness-Bordne, Irmak and Jensen-Haise were assessed and the Hargreaves-Samani method performed better than others. An attempt was made to calibrate the Hargreaves-Samani equation; however, its overall performance did not improved and the two newly proposed equations are suggested to be used in Australia.

Estimating models for reference evapotranspiration with core meteorological parameters via path analysis

Accurate estimation and reliable universal performance of reference evapotranspiration (ET0) obtained from a few meteorological parameters are important for the rational planning of agricultural water resources and the effective management of water in irrigated regions. Meteorological data in southern China were used to calculate ET0 using the standard Penman–Monteith formula and determined the core decision variable (hours of sunshine, N) and the limited decision variable (relative humidity, RH) using path analysis. Estimation models using an artificial neural network and wavelet neural network were established for the Wuhan and Guangzhou meteorological stations. The statistical indices were positively correlated with the decision contribution rates to ET0. The ET0 values for other stations in southern China were all estimated by these models, which were trained for the Guangzhou station, and then made a total comparison with Hargreaves–Samani (HS) and Priestley–Taylor (PT) empirical ET0 models. Error analysis indicated that the root mean square error and the mean absolute per cent error were around 0.32 mm and 5.5%, respectively, with a high coefficient of determination and Nash–Sutcliffe efficiency over 0.9, indicating that these estimating models could be applied in more regions for universal analysis with high accuracy.

Performance of radiation-based reference evapotranspiration equation developed for Indian sub-humid conditions

In this study, asite-specific radiation based equation for estimating reference evapotranspiration (ETo) was developed and its performance was statistically analysed in comparison to widely accepted FAO Penman-Monteith (FAO-56 PM) model and four radiation-based ETo methods for sub-humid Hazaribagh region of Jharkhand state. The equation was developed with daily values of incoming solar radiation in conjunction with air temperature (minimum and maximum) by considering daily FAO-56 PM ET0 values as index with weather dataset of 15 years (1990-2004). The performance of developed equation validated with eight years (2005-2012) daily weather dataset revealed that it estimated ETo values better than other radiation-based methods. The respective higher and lower values of agreement index and root mean square error with FAO-56 PM ET0 values during validation period confirms efficacy of developed equation whose performance tested at another Indian sub-humid location (Pantnagar) confirmed its suitability as well. Considering the limitations associated with reliability and availability of weather data especially in developing countries, developed equation is recommended as practical one to estimate ETo in sub-humid climatic conditions if FAO-56 PM model cannot be used due to non-availability of required weather parameters at a location.

Modifying Hargreaves–Samani equation with meteorological variables for estimation of reference evapotranspiration in Turkey

The Food and Agriculture Organization advocates the Penman–Monteith (FAO-56 PM) equation as the standard model for estimation of the reference evapotranspiration (ET0) because it is considered to have better accuracy. However, in regions where meteorological variables such as solar radiation, wind speed, and relative humidity are not gauged, the Hargreaves–Samani (HS) equation is resorted to as an alternative simply because it needs minimum and maximum air temperatures only as the explanatory variables. In this study, first the HS equation is applied to the monthly means of measured temperature data recorded at 275 meteorology stations in Turkey. Next, the coefficients of the HS equation are calibrated using the ET0 values given by the FAO-56 PM equation at all these stations. Next, the HS equation is modified by adding the wind speed as an extra explanatory variable, separately in each one of seven geographical regions of Turkey, which is observed to yield smaller error statistics as compared to the original HS equation. It is concluded that for estimation of the ET0 in regions where meteorological measurements are scarce, the HS equation modified in a similar manner can be used with better precision.

Actual evapotranspiration for a reference crop within measured and future changing climate periods in the Mediterranean region

The study compares two formulas for calculating the daily evapotranspiration ET0 for a reference crop. The first formula was proposed by Allen et al. (AL), while the second one was proposed by Katerji and Perrier with the addition of the carbon dioxide (CO2) effect on evapotranspiration (KP). The study analyses the impact of the calculation by the two formulas on the irrigation requirement (IR). Both formulas are based on the Penman-Monteith equation but adopt different approaches for parameterising the canopy resistance r c . In the AL formula, r c is assumed constant and not sensitive to climate change, whereas in the KP formula, r c is first parameterised as a function of climatic variables, then ET0 is corrected for the air CO2 concentration. The two formulas were compared in two periods. The first period involves data from two sites in the Mediterranean region within a measured climate change period (1981–2006) when all the input climatic variables were measured. The second period (2070–2100) involves data from a future climate change period at one site when the input climatic variables were forecasted for two future climate scenarios (A2 and B2). The annual cumulated values of ET0 calculated by the AL formula are systematically lower than those determined by the KP formula. The differences between the ET0 estimation with the AL and KP formulas have a strong impact on the determination of the IR for the reference crop. In fact, for the two periods, the annual values of IR when ET0 is calculated by the AL formula are systematically lower than those calculated by the KP formula. For the actual measured climate change period, this reduction varied from 26 to 28 %, while for the future climate change period, it varied based on the scenario from 16 % (A2) to 20 % (B2).

Calibration of Valiantzas’ reference evapotranspiration equations for the Pilbara region, Western Australia

Reference evapotranspiration (ET0) is a critical component of water resources management and planning. Different methods have been developed to estimate ET0 with various required data. In this study, Hargreaves, Turc, Oudin, Copais, Abtew methods and three forms of Valiantzas’ formulas, developed in recent years, were used to estimate ET0 for the Pilbara region of Western Australia. The estimated ET0 values from these methods were compared with those from the FAO-56 Penman-Monteith (PM) method. The results showed that the Copais methods and two of Valiantzas’ equations, in their original forms, are suitable for estimating ET0 for the study area. A modification of Honey-Bee Mating Optimization (MHBMO) algorithm was further implemented, and three Valiantzas’ equations for a region located in the southern hemisphere were calibrated.

Comparison of Different Methods to Estimate Mean Daily Evapotranspiration from Weekly Data at Patna, India

Estimation of evapotranspiration is necessary for efficient water management and crop planning. Fifteen different methods of ET0 estimation were employed to compute daily reference evapotranspiration for the period 2010 to 2014. In the absence of reliable open pan evaporimeter data, FAO-56 Penman Monteith method was considered as one of the reliable method of ET0 estimation. The results showed that mean weekly evapotranspiration values obtained from Penman-Monteith method were very closer to FAO-56 Penman-Monteith method and values from all the other methods except FAO- 24 Pan, Christiansen Pan and Hargreaves methods generally predicted higher values of mean weekly daily ET0 in comparison to FAO-56 Penman-Monteith method. The analysis shows that mean weekly daily ET0 estimates of combination methods resulted better ET0 estimates than radiation, and temperature and evaporation methods. Weekly ET0 values estimated by FAO-56 Penman-Monteith method were found to vary in the range of 1.3 mm/day to 6.7 mm/day. Average annual reference evapotranspiration was found as 1517.1 mm.

Evapotranspiration Estimation using Six Different Multi-layer Perceptron Algorithms

Evapotranspiration has a vital importance in water resources planning and management. In this study, the applicability of six different multi-layer perceptron (MLP) algorithms, Quasi-Newton, Conjugate Gradient, Levenberg- Marquardt, One Step Secant, Resilient Back propagation and Scaled Conjugate Gradient algorithms, in modeling reference evapotranspiration (ET0) is investigated. Daily climatic data of solar radiation, air temperature, relative humidity and wind speed from Antalya City are used as inputs to the MLP models to estimate daily ET0 values obtained using FAO 56 Penman Monteith empirical method. The results of the MLP algorithms are compared with those of the multiple linear regression models with respect to root mean square error (RMSE), mean absolute error (MAE), Willmott index of agreement (d) and determination coefficient (R2). The comparison results indicate that the Levenberg-Marquardt is faster and has a better accuracy than the other five training algorithms in modeling ET0. The Levenberg-Marquardt with RMSE = 0.083 mm, MAE = 0.006 mm, d = 0.999 and R2 = 0.999 in test period was found to be superior in modeling daily ET0 than the other algorithms, respectively.

Using scintillometry to assess reference evapotranspiration methods and their impact on the water balance of olive groves

Reference evapotranspiration (ET0) is widely used for irrigation scheduling, to promote an efficient use of water resources for a sustainable agro-ecosystem productivity, as well as to manage water quality and to face other environmental concerns. As suggested by ASCE-EWRI and FAO, standard Penman–Monteith methods are generally applied for an accurate estimation of ET0 from hourly to daily scale. In absence of detailed meteorological information several simplified equations, using a limited number of variables, have been proposed as alternative. In this paper, the performance of different reference evapotranspiration methods, at hourly (Penman–Monteith, Pristley–Taylor, Makkink and Turc) and daily scale (Penman–Monteith, Blaney and Criddle, Hargreaves, Pristley–Taylor, Makkink and Turc), was evaluated against scintillometer measurements collected during six month in 2005 in an experimental plot maintained under “reference” conditions (alfalfa crop). The daily values of ET0 obtained with the examined methodologies were then used as input in the FAO-56 agro-hydrological model, in order to evaluate, for an olive grove in a Mediterranean environment, the impact on simulated actual evapotranspiration. The experiment was carried out in South-West Sicily, in an area where olive groves are the major crop. The comparison between estimated and measured fluxes confirmed that FAO-56 Penman–Monteith (PM) standardized equation is characterized by the lowest mean bias error (−0.15mmd−1 and 0.06mmd−1 using daily or hourly data, respectively). Additionally, the analysis also highlighted that the Pristley–Taylor equation can be considered a valid alternative for an accurate estimation of ET0 (mean bias error of 0.35mmd−1 and 0.43mmd−1 using daily or hourly data, respectively). The application of the FAO-56 water balance model on the investigated olive grove evidenced that the best estimations of actual evapotranspiration are obtained when the Pristley–Taylor ET0 data are used as input, confirming that this approach can be considered a valid alternative to the standard Penman–Monteith.

Atmospheric conditions controlling extreme summertime evapotranspiration in Poland (central Europe)

Extremely high evapotranspiration may constitute a threat to agriculture and to water storage, supply, management and quality. To better understand the occurrence of high evapotranspiration, our goals were to (1) document the synoptic conditions favorable for extreme evapotranspiration and (2) determine the anomalies in the factors that directly affect extremely high evapotranspiration on a daily scale. We calculated the daily reference evapotranspiration (ET0; June–August, 1971–2010) for 31 stations in Poland using the FAO-56 Penman–Monteith method and identified the days with the highest ET0 values (>90th percentile). For these days, we calculated the sea level pressure (SLP), the 500 hPa geopotential heights (z500), the air temperature at 850 hPa (t850) and the amount of precipitable water. We detected a correlation between the air pressure field and high ET0 values (r > 0.4, p < 0.001) in northeast Poland; positive anomalies in SLP, z500 and t850 were documented on days with high ET0. Substantial anomalies were also observed in the factors that directly affect evapotranspiration, i.e., sunshine duration and air temperature (positive) and relative humidity (negative). The conditions favorable for high ET0 events are characteristic of anticyclonic weather, so when an anticyclone center is near northeast Poland, high evapotranspiration can be expected.

The analysis of trend variations of reference evapotranspiration via eliminating the significance effect of all autocorrelation coefficients

Reference evapotranspiration (ET0) is considered a key parameter for evaluating the climatic changes as well as spatial and temporal patterns of parameters influencing the eco-hydrological processes. The analysis of trend variations of this index can be used to determine appropriate strategies in planning and management of water resources. In this paper, the trend variations of monthly and annual ET0 in Urmia Lake basin, located in the northwest of Iran, have been analyzed using data from 14 synoptic stations in the study area. Regarding the significant effect of autocorrelation coefficients with different lags on trend variations of ET0, this paper has resorted to modified Mann–Kendall test via eliminating the significance effect of autocorrelation coefficients with different lags to analyze the trend variations. Furthermore, Theil–Sen estimator has been used to determine the slope of trend line of ET0. The results indicated an increasing trend in ET0 values at all the studied stations. Having used the modified Mann–Kendall test, the values of significant increasing (positive) trend, which were estimated using common Mann–Kendall test, dramatically decreased. As such, the values of only 7 stations have been significant at 95 % level. The results confirmed the need for eliminating the significance effect of autocorrelation coefficients with different lags to determine and evaluate the trend of hydrological variables.

English

Water scarcity is a global concern, as the demand for water is increasing tremendously and poor management of water resources will accelerates dramatically the depletion of available water. The precise prediction of evapotranspiration (ET), that consumes almost 100% of the supplied irrigation water, is one of the goals that should be adopted in order to avoid more squandering of water especially in arid and semiarid regions. The capabilities of feedforward backpropagation neural networks (FFBP) in predicting reference evapotranspiration (ET0) are evaluated in this paper in comparison with the empirical FAO Penman-Monteith (P-M) equation, later a model of FFBP+Genetic Algorithm (GA) is implemented for the same evaluation purpose. The study location is the main station in Iraq, namely Baghdad Station. Records of weather variables from the related meteorological station, including monthly mean records of maximum air temperature (Tmax), minimum air temperature (Tmin), sunshine hours (Rn), relative humidity (Rh) and wind speed (U2), from the related meteorological station are used in the prediction of ET0 values. The performance of both simulation models were evaluated using statistical coefficients such as the root of mean squared error (RMSE), mean absolute error (MAE) and coefficient of determination (R2). The results of both models are promising, however the hybrid model shows higher efficiency in predicting ET0 and could be recommended for modeling of ET0 in arid and semiarid regions.

Satellite-based crop coefficient and regional water use estimates for Hawaiian sugarcane

Water availability is a major limiting factor for sustainable production of potential biofuel crops in Maui, Hawaii, USA. It is essential to improve regional, near-real time estimates of crop water use to facilitate optimal water management. Satellite remote-sensing offers multiple methods to estimate water management, however, most approaches assume a spatially homogeneous regional meteorology, which does not apply in Hawaii due to strong orographically-induced variations in climate. Canopy ground cover (fc) and spectral reflectance were measured over two sugarcane fields on the satellite Landsat 7 overpass dates. A strong linear relationship between satellite-based Normalized Difference Vegetation Index (NDVI) values and fc was found (R2=0.97). Canopy ground cover was also found to be highly correlated to crop coefficient (Kc) which was calculated from reference evapotranspiration (ET0) and sugarcane crop evapotranspiration (ETc) observations from the Eddy Covariance towers. Spatial ET0 for the entire sugarcane plantation was determined from an automated weather station network previously installed by the commercial grower of the plantation. Regressions were used to convert the satellite NDVI to Kc maps, and then the satellite-based ETc maps were created from the satellite-based Kc maps and ET0 values computed and interpolated from the weather station network. To compare with the satellite-based crop coefficient/reference ET method, ETc and Kc were also estimated using a satellite-based surface energy balance technique, the METRIC model. These two satellite-based methods were correlated fairly well (R2=0.84 for Kc and 0.81 for ETc). Compared to the METRIC model, the satellite-based crop coefficient/reference ET method may be more practicable for estimating crop water use in Maui, where ground-based ET0 measurements are available through the on-site automated weather station network with right choice of reference ET equation.

Long-term monthly evapotranspiration modeling by several data-driven methods without climatic data

In this study, the ability of four different data-driven methods, multilayer perceptron artificial neural networks (ANN), adaptive neuro-fuzzy inference system (ANFIS) with grid partition (GP), ANFIS with subtractive clustering (SC) and gene expression programming (GEP), was investigated in predicting long-term monthly reference evapotranspiration (ET0) by using data from 50 stations in Iran. The periodicity component, station latitude, longitude and altitude values were used as inputs to the applied models to predict the long-term monthly ET0 values. The overall accuracies of the multilayer perceptron ANN, ANFIS-GP and ANFIS-SC models were found to be similar to each other. The GEP model provided the worst estimates. The maximum determination coefficient (R2) values were found to be 0.997, 998 and 0.994 for the ANN, ANFIS-GP and ANFIS-SC models in Karaj station, respectively. The highest R2 value (0.978) of GEP model was found for the Qom station. The minimum R2 values were respectively found as 0.959 and 0.935 for the ANN and ANFIS-GP models in Bandar Abbas station while the ANFIS-SC and GEP models gave the minimum R2 values of 0.937 and 0.677 in the Tabriz and Kerman stations, respectively. The results indicated that the long-term monthly reference evapotranspiration of any site can be successfully estimated by data-driven methods applied in this study without climatic measurements. The interpolated maps of ET0 were also obtained by using the optimal ANFIS-GP model and evaluated in the study. The ET0 maps showed that the highest amounts of reference evapotranspiration occurred in the southern and especially southeastern parts of the Iran.

The Determination of Reference Evapotranspiration for Spatial Distribution Mapping Using Geostatistics

The reference evapotranspiration (ET0) is an essential variable in the agrohydrological systems and its estimation on a regional scale is limited to its spatial variability. This study compares two approaches for preparation of spatial distribution maps of ET0 in Mazandaran province of Iran. In the first approach, ET0 was calculated using climatic data and Hargreaves-Samani equation in weather stations locations and then were interpolated. In the second approach, the components of the Hargreaves-Samani equation were interpolated and then ET0 maps were prepared by applying the Hargreaves-Samani equation and suitable commands in GIS. The 10-year climatic data for 51 weather stations (46 stations for preparing ET0 maps and 5 stations as validation station) were gathered over Mazandaran province. Semivariograms were calculated and the best semivariogram model was selected on the basis of the least value of Residual Sums of Squares (RSS). The spatial correlation of the data was compared on the basis of Nugget to Sill ratio. The data were interpolated using Ordinary Kriging method and the interpolation error was computed by cross validation technique based on Root Mean Square Standardized Error (RMSSE). The predicted ET0 values were compared to the computed ET0 in validation stations and sensitivity analysis was conducted. Results show the second approach had better spatial correlation and lower interpolation error and the difference between these two approaches were not significant. Therefore, the accuracy of the ET0 maps is more related to the method of computing ET0 than the type of climatic data is being interpolated.

Spatiotemporal analysis of potential evapotranspiration in the Changwu tableland from 1957 to 2012

The Changwu tableland is located in the semi-humid to semi-arid transition area of the Loess Plateau, China. In the present study, the daily potential evapotranspiration (ET0) of the Changwu tableland was calculated using the FAO Penman–Monteith (FPM) equation for the years 1957–2012, and the temporal variation and frequency distribution characteristics were analysed. The results indicate that the annual average ET0 of the Changwu tableland was 949.3 mm, varying from 789.2 to 1093.3 mm; a slight downward trend with a rate of −0.48 mm year−1 was found. An abrupt increase occurred in 1994. The ET0 values that correspond to frequencies of 5, 10, 25, 50, 75, 90 and 95% were 1076.8, 1047.6, 999.7, 947.8, 897.3, 853.0 and 827.0 mm, respectively. The recurrence intervals of maximum and minimum ET0 were both more than 50 years. The contributions of Tmax, Tmin, vapour pressure, wind speed and sunshine hours to changes in ET0 were 0.74, 0.07, −0.51, −0.57 and −0.23, respectively. The change in ET0 and the meteorological factors in the Changwu tableland are worthy of attention; measures to reduce the negative impacts of climate change are necessary.

Erratum to: Evaluation and calibration of Blaney–Criddle equation for estimating reference evapotranspiration in semiarid and arid regions

Penman–Monteith (FAO-56 PM) equation is suggested as the standard method for estimating evapotranspiration by the International Irrigation and Drainage Committee and FAO. On the other hand, the Blaney–Criddle (BC) temperature-based equation is an alternative and simple method compared with the FAO-56 PM equation. In the present study, the original coefficients BC equation (a and b) were calculated and calibrated spatial and temporal calibration at each station for each month based on the FAO-56 PM method for estimating reference evapotranspiration (ET0) from 15 meteorological stations in central Iran (about 170,000 km2) under semi-arid and arid conditions. The values of a and b in BC equation were negative and positive for all months of any station, respectively. Highest and lowest a values were obtained in December and August, respectively. December showed the lowest b values while August showed the highest. Therefore, the values of a and b were greater in cold and warm months of the year, respectively. After calibration, the root mean square error, mean bias error and percentage error values were obtained lower than 0.50, 0.015 mm day−1 and 10 % for the whole stations and months, respectively. The calibrated b values (b cal) were proportional and inversely to the calibrated a values (a cal ). The ET0 values based on the calibrated Blaney–Criddle equation were better than the results of the BC equation when compared to the FAO-56 PM equation as the reference model.

Comparison of hourly and daily reference crop evapotranspiration equations across seasons and climate zones in Australia

Estimates from the FAO Penman–Monteith (FAO-PM) and the standardized ASCE Penman–Monteith (ASCE-PM) hourly and daily reference evapotranspiration (ET0) equations were compared at daily scale, based on the hourly climate data collected from forty (40) geographically and climatologically diverse Automatic Weather Stations (AWS) across the Australian continent. These locations represent 23 agricultural irrigation areas in tropical, arid and temperate climates. The aims of this paper are to: compare the effects of different methods of estimating Clear-sky-radiation—(Rso); compare sum-of-hourly and daily ET0; compare the results of aggregation of hourly ET0 over 24h compared with daylight hours; and examine the impact of seasonality and climate type. At selected AWS locations, the hourly ET0 was calculated using the hourly FAO-PM and the ASCE-PM equations and then summed to derive daily ET0 (reported as ET0,soh). This was compared against the daily ET0 values, calculated using the corresponding daily equation (reported as ET0,daily). Using Rso calculated following the “complex” approach improves the agreement between ET0,soh and ET0,daily of both hourly equations, compared with the “simple” approach. Better agreement between ET0,soh and ET0,daily estimates for the FAO-PM and ASCE-PM were found, when the hourly values are aggregated over 24h rather than over daylight hours. The average ratio between ET0,soh and ET0,daily for the FAO-PM and ASCE-PM equations is 0.95 and 1.00, respectively. The range of the former is 0.90–0.98 and that of the latter is 0.96–1.04. There was very strong correlation between the two hourly equations at the daily time step: on average 0.997, with a range of 0.993–0.998. The results imply that the ASCE-PM hourly equation's daily ET0 values are higher than those of FAO-PM, which can be explained by the difference in the treatment of surface resistances. Better agreements between ET0,soh and ET0,daily values for winter, spring and autumn were found for the FAO-PM version, while during summer, the ASCE-PM version showed better agreement. The best agreement between the hourly and daily results for the FAO-PM version was found in temperate climates and the ASCE-PM version showed best agreement in the tropical and arid climates.

Usefulness of class A Pan coefficient models for computation of reference evapotranspiration for a semi-arid region

The reliability of estimates of reference evapotranspiration (ET0) using pan evaporation (Epan) depends on the accurate determination of pan coefficients (Kpan). Six ET0 models were evaluated for their usefulness using 33-year climatological dataset of a semi-arid region of the Gujarat state of India. The equations compared include Cuenca (1989), Allen and Pruitt (1991), Snyder (1992), Modified Snyder (Grismer et al., 2002), Orang (1998), and Pereira et al. (1995). The ET0 data, calculated using daily Kpan values from these equations, were compared to the Food and Agricultural Organization (FAO)-Penman-Monteith (FAO56-PM) method as a reference. Based on the visual comparison as well as from the statistical criteria, ET0 values computed using Modified Snyder and Orang model have very close agreement with the FAO56-PM method for daily, monthly, and annual estimates as compared to other approaches. The sequential performances of the explored models was found as: Modified Snyder (Eqn. 5) &gt; Orang (Eqn. 6) &gt; Cuenca Eqn. (2) &gt; Allen &amp; Pruitt (Eqn. 3) &gt; Snyder (Eqn. 4) &gt; Pereira et al. (Eqn. 7) model. Therefore, the Modified Snyder model (Grismer et al., 2002) could be recommended as the best model for ET0 computations under these prevailing climatic conditions for a semi arid region.