Validation of Valiantzas Reference Evapotranspiration Equation under Different Climatic Conditions (original) (raw)
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Current Research in Agricultural Sciences, 2020
The estimation of reference evapotranspiration (ETo) is required for effective development and management of agriculture water systems. In order to define the most accurate method to estimate ETo in semi-arid climatic environment of Faisalabad, Lahore and Peshawar. Penman and Thornwait ETo methods are compared with standard Penman-Monteith (PM) ETo method. The statistical results show that Penman ETo method overestimates PM ETo method in all the semi-arid climatic regions of Faisalabad, Lahore and Peshawar by 34.91%, 39.51% and 30.75%, respectively. The R2 were 0.98, 0.98 and 0.99 at Faisalabad, Lahore and Peshawar weather stations, respectively. The RMSE were 2.47 mm/day, 2.64 mm/day and 2.19 mm/day at Faisalabad, Lahore and Peshawar weather station, respectively. The MBE of-2.41 mm/day,-2.58 mm/day and-2.13 mm/day were noted at Faisalabad, Lahore and Peshawar weather stations, respectively. The statistical result of Thornwait (Th) ETo method as compared with PM ETo method indicates underestimation of ETo in winter season and overestimation of ETo in summer season by 13.81%, 22.43% and 14.54% at Faisalabad, Lahore and Peshawar stations, respectively. The R2 of 0.92, 0.89 and 0.95 were noted at Faisalabad, Lahore and Peshawar weather stations, respectively. The RMSE were 2.14 mm/day, 2.36 mm/day and 1.16 mm/day at Faisalabad, Lahore and Peshawar weather stations, respectively. The MBE were-0.68 mm/day,-1.12 mm/day and 0.61 mm/day at Faisalabad, Lahore and Peshawar weather stations, respectively. Overall, Thornwait method gave better estimation of ETo than Penman ETo method at all the weather stations. Contribution/Originality: The main objective of this research is to compare the performance of Penman and Thornwait ETo methods against standard PM ETo method under semi-arid climatic conditions of Lahore, Faisalabad and Peshawar, Pakistan.
Current Research in Agricultural Sciences
There are various equations for calculation of reference evapotranspiration (ETo), but the Penman-Monteith (PM FAO-56) equation has been considered as the standard ETo equation. The key problem of PM FAO-56 equation is that it uses large number of weather parameters like air temperature, wind velocity, humidity and sun radiation as input. These weather parameters are not accessible at all weather stations of the world especially in developing countries. So, ablatives ETo equations like Hargreaves (HG) and Blaney-Criddle (BC) equations are used for estimation of ETo which required very small number of weather parameters that are readily available at most of the weather stations of the world. A research is conducted to compare HG and BC ETo equations for estimation of monthly ETo under semi-arid climatic regions of Lahore, Faisalabad and Peshawar, Pakistan. The PM FAO-56 ETo equation is considered as reference ETo equation for the assessment of HG ETo and BC ETo equation. The statistical results indicate that HG ETo equation overestimates PM FAO-56 ETo method by 7.91% at Lahore weather station, 5.59 % at Faisalabad weather station and 11.95% at Peshawar weather station. The BC ETo equation overestimates PM FAO-56 ETo equation by 34.345% at Lahore weather station, 28.637% at Faisalabad weather station and 21.44% at Peshawar weather station. The variation of HG ETo equation with PM FAO-56 ETo equation with RMSE of 0.487 mm/day at Lahore weather station, 0.521 mm/day at Faisalabad weather station and 0.985 at Peshawar weather station is noted. The variation of BC ETo equation with PM FAO-56 ETo equation having RMSE of 3.03 at Lahore weather station, 2.58 at Faisalabad weather station and 1.96 at Peshawar weather station is noted. Contribution/Originality: The objectives of this study is to compare the HG ETo and BC) ETo equations against FAO-56 PM ETo equation in semi-arid climatic conditions. 1. INTRODUCTION Pakistan is under the problem of water shortage and the demand of water for irrigation is also increased due to mounting demand of food and fiber [1]. Pakistan is in between the arid to semi-arid region [2]. The knowledge of
Agricultural research, 2015
Evapotranspiration (ET) is one of the largest components of hydrological cycle, and its accurate quantification is needed in water allocation, irrigation management, and to protect surface and ground water quantity and quality. So there is a need of improved techniques for accurate quantification of ET to enhance efficient use of water resources and sustainability of agro-ecosystem productive. A number of methods have been developed till now for ET o estimation, but most of them are only applicable in areas where they have been developed. Till now, only the Penman method has been accepted worldwide which is acceptable in almost all climatic conditions, but the only major drawback of this method is the large number of data requirement. Therefore, in this study, we have used four reference ET o estimation methods which include two radiation methods (Turc and Priestley-Taylor), one temperature (Hargreaves), and one combined method (Penman method) of ET o estimation. The weekly average meteorological data for the period 1975-2005 were used here to estimate ET o. This study has attempted to select reliable reference ET estimation method other than Penman where less input variables are required. Here, Penman-derived ET o has been selected as the standard for evaluating the performance of other methods of ET o estimation. This study has further attempted to demonstrate some of the significant applications of estimated ET o. The execution of all radiation-and temperature-based methods shows that outcome of Turc-derived ET o is comparable with Penman-derived ET o , and thus this can be used for ET o estimation for this region other than Penman method.
Modeling Earth Systems and Environment, 2018
The objectives of this study were to assess the accuracy of FAO Penman Monteith equation (FAO PM) under limited data conditions and to perform sensitivity analysis to determine approximately the change in reference evapotranspiration (ETref) expected for a known change in one of the independent variables and derive the sensitivity coefficient. Meteorological data were collected from 8 weather stations under humid and semiarid condition in Côte d'Ivoire. The results showed good performance of FAO PM equation under missing solar radiation (Rs) in semiarid condition and under missing wind speed data (U 2) and relative humidity (RH) in all locations with coefficient of determination (R 2) ranging from 0.70 to 0.99 and regression slope from 0.99 to 1.05. Under missing Rs, RMSE varied from 0.45 to 0.48 mm/day and AME from 0.30 to 0.35 mm/day. The RMSE and AME vary respectively from 0.12 to 0.51 mm/day and from 0.09 to 0.30 mm/day under missing RH data, and respectively from 0.11 to 0.60 mm/day and 0.04 to 0.34 mm/day under missing wind speed data. The poor performance of FAO PM method to estimate ETo was observed when three climatic variables were missing with regression slope from 0.93 to 1.06 and R 2 from − 0.06 to 0.26. The response of ETo to changes in all climate variables was linear, with high R 2 values (≥ 0.99) in most cases. Any error in Rs, maximum temperature (Tmax) data would have contributed to significant change in ETo estimate. The effect of Rs on change in ETo estimates had the greatest slope (≥ 2.80) in Bouake, Daloa, Korhogo, Man, Seguela whereas it had the lowest slope in Ferkessedougou (slope = 2.74), Odienne (slope = 2.73), Yamoussoukro (slope = 2.77). The effect of Tmax in change in ETo was also important in all location except Daloa and Man with low regression slope values of 1.63, 1.74, respectively. All sensitivity coefficients showed a large degree of daily and seasonal fluctuations and revealed significant differences in northern and central study locations. The sensitivity coefficients of U 2 and Tmax were greatest under semiarid condition while the one of Rs were very high in humid condition. Accurate measuring of U 2 , Tmax and Rs in estimating reference evapotranspiration using Penman-Monteith equation is required.
2009
El método FAO Penman (PM), es el más confiable para estimar la evapotranspiración de referencia (ETo) y es recomendado por la FAO como estándar para verificar otros métodos empíricos. En su utilización es necesaria información de cuatro parámetros meteorológicos: temperatura del aire, humedad relativa, velocidad del viento y radiación neta. La poca disponibilidad de estos parámetros limita el uso del método en muchos lugares; por lo que los modelos Thornthwaite (TM) y Hargreaves (HM) son usados con frecuencia, ya que únicamente se basan en medidas de temperatura del aire, medidas comunes en muchas estaciones meteorológicas en todo el mundo, por lo que son una opción para estimar la ETo. Sin embargo, con el objetivo de obtener resultados apropiados de ETo, los modelos HM y TM deben ser calibrados de acuerdo con las condiciones locales. En el presente estudio, los coeficientes originales de TM y HM son modificados para una calibración local en climas semi-áridos y tropicales sub-húmedos del estado de Yucatán, México, usando como estándar la ecuación Penman-de FAO. Se usaron datos meteorológicos de dos estaciones en el estado de Yucatán, México que corresponden a un clima costero semi-árido (Progreso) y a un clima tropical sub-húmedo tierra adentro (Mérida). En la comparación se analizaron los índices de concordancia (D), confianza (C), correlación (R) y regresión (R 2 ), así como indicadores del sesgo medio del error (MBE), raíz cuadrada media del error (RMSE), error relativo (RE) y el cociente entre ambas estimaciones promedio de ETo ( r). Usando HM sin ajuste se obtuvieron buenas estimaciones de ETo en Mérida y Progreso, con valores de C de 0.825 y 0.816, respectivamente. No se recomienda el uso de TM sin ajuste en ninguna de las estaciones meteorológicas estudiadas. Sin embargo, en ambas estaciones, el modelo TM estima mejor la Eto durante los meses lluviosos (de junio a octubre). En ambas estaciones meteorológicas, costera y de tierra adentro, se obtuvieron mejores estimaciones anuales de ETo con el uso de HM sin ajuste (valores de C de 0.906 y 0.917, respectivamente).
Performance Evaluation of Reference Evapotranspiration Equations across a Range of Indian Climates
Journal of Irrigation and Drainage Engineering, 2006
Five ETo estimation methods viz. Hargreaves (HAR), improved Hargreaves (IHA), FAO-Radiation (RAD), reduced-set Penman-Monteith (RPM), combination method FAO-Penman with wind functions of Watts & Hancock (W&H) were evaluated at eight semi-arid stations in the Indus Basin Irrigation Area (IBIA) of Pakistan. Penman-Monteith (P_M) method was used as base method for evaluation/calibration of those methods. At all eight stations except two (Lahore and Bahawalpur), four out of five methods namely W&H, RAD, IHG, and HAR exhibited strong tendency to overestimate ETo estimates; at Lahore, RAD method under-estimated ETo by 27%, and at Bahawalpur HAR by 6%. Overestimation ranges observed were: W&H (9-25%), RAD (5-42%), IHG (14-73%) & HAR (11-45%). At all eight stations except Jhelum and D.I.Khan, RPM method underestimated ETo by 1-12%; overestimation at D.I.Khan was 1% against Jhelum where it was 0%. This aspect highlighted the variant nature of ETo methods in terms of daily ETo estimates under different climatic conditions. At Sialkot, D.I.Khan and Khanpur, RPM method showed excellent performance with top rank; and being ahead of W&H combination method at those stations indicated
Journal of Irrigation and Drainage Engineering, 2017
The unavailability of some meteorological variables, especially solar radiation and wind speed, is the main constraint for reference evapotranspiration (ETo) estimation using the standard United Nations Food and Agriculture Organization (FAO) Penman-Monteith (FAO-PM) equation in most developing countries. The application of ETo methods with fewer input requirements is necessary under limited climatic data conditions. The FAO-PM method under limited data conditions and nine of Valiantzas' equations were evaluated for daily ETo estimation in a humid climate in Uganda. The FAO-PM method with missing relative humidity data performed very well across Uganda, whereas using the long-term local wind speed average values in place of missing wind speed data resulted in inaccurate ETo estimates. Under missing solar radiation measurements, the FAO-PM method showed different performances relative to the locations. When more than one climatic variable is missing, the FAO-PM method yielded poor ETo estimates compared to the FAO-PM method with full climatic data. The performance of Valiantzas' equations depends on data requirements: the more meteorological inputs, the higher the ETo accuracy.
2006
The performance of two temperature-based empirical models for computing reference evapotranspiration (ETo): the Hargreaves (HG) and the Jensen-Haise (J-H) models, were evaluated for the three: Upper, Middle, and Lower zones of the Mkoji sub-catchment of the Great Ruaha River Basin in Tanzania. Climatic data from the Mbeya, Igurusi, and Kapunga weather stations, representing the Upper, Middle, and Lower Mkoji respectively, were used to compute daily ETo in accordance with the two temperature-based models. A third model: the FAO-Penman-Monteith (F-P-M) model was used as a reference model for assessing the performance of the temperature-based models. The F-P-M model was used as a reference because it has been recommended as a universal model for computing ETo. The daily ETo calculated based on the temperature-based models were compared statistically with those of the F-P-M model. The results showed that daily ETo of the HG and J-H models were significantly different from the F-P-M model at P<0.05 in the three zones. In the Upper Mkoji, the ETo based on the HG and J-H models were significant different (P<0.01) from the F-P-M model for each month of the year. The absolute mean difference between the daily ETo by the F-P-M and the HG models was 0.32 mm/day, and between the F-P-M and J-H model was 0.38 mm/day. In the Middle Mkoji, ETo based on the HG and J-H models were also significant different (P<0.01) from the F-P-M model for each month of the year. The mean difference between the F-P-M and HG was 0.62 mm/day, and 0.60 mm/day between the F-P-M and J-H. In the Lower Mkoji, the ETo of the HG and J-H were significant different (P<0.01) from the F-P-M in the months of December to August, but in the other months of the year, the temperature-based models were not significantly different from the F-P-M model. The mean difference between the daily ETo of the F-P-M and the HG model, and between the F-P-M and J-H model were: 0.50 mm/day, and 0.62 mm/day, respectively. The coefficients of determination (r 2 ) of the linear regression equations between the F-P-M and the HG models, and between the F-P-M and J-H models for the three zones of the sub-catchment were found to be good (> 0.80 in the Upper and Middle zone, and >0.60 in the Lower zone). The equations can therefore be used to convert daily ETo from the temperature-based models to their equivalent in the F-P-M model.
Theoretical and Applied Climatology, 2018
Reference evapotranspiration (ETo) is an important parameter in hydrological, agricultural, and environmental studies. Accurate estimation of ETo helps to improve water management and increase water productivity and efficiency. While the Penman-Monteith ETo equation enjoys worldwide adoption as the most accurate ETo equation, the number of requested climatic variables makes its application very questionable under limited data conditions. The objective of this study was to evaluate the Penman-Monteith ETo equation under limited climatic data and 34 simple ETo equations that request few climatic variables. Five weather stations were considered under the semiarid and dry climate across New Mexico for the period of 2009-2017. The Penman-Monteith ETo equation showed good performance under missing solar radiation, relative humidity, and wind speed and could still be adapted under limited data conditions across New Mexico. However, it tended to underestimate daily ETo when more than one climatic variable data is missing. Among the simple ETo equations, four of the Valiantzas equations, along with the Makkink, Calibrated Hargreaves, Abtew, Jensen-Haise, and Caprio equations, were the best performing ones compared to the Penman-Monteith equation and could be the best alternative ETo estimation methods. These alternative equations could be used by irrigation managers, producers, engineers, and university researchers to improve water management across the dry semiarid and arid zone across New Mexico, as well as other semiarid areas where water is the most limiting factor to food and fiber production.