In order to determine consumptive use of water for plants, management of irrigation and other agricultural studies potential evapotranspiration is requird. However, commonly used prediction methods have not been corrected for regions. For determining the best model for culculation of potential evapotranspiration in Ahar station, evaporation pan potential evapotranspiration values were compared applying different methods.results showed that Turc, Thornth-waite, Christianseen-Hargreaves (RA) and Blaney-Criddle methods have best correlation with pan potential evapotranspiration values than other, applicable technigus.

Introduction Rice provides more than 80 percent of the calories consumed in a large part of Asia. Rice, in addition to the food, the dependence of the water has unique characteristics. Supplying water requirements of rice, as a plant with high nutritional and economic value is important in many paddy lands. Evapotranspiration for each product such as rice is the ratio of the reference crop evapotranspiration. This ratio is called the crop coefficient that is variable due to the physiological and morphological differences between plant species, and climatic characteristics of the region. With increasing duration of growth, Crop coefficient increases, so that, at crop development, to the maximum and then decreased. In this study, the evapotranspiration of rice plants based on climatic data using pan evaporation and Penman Mantys, compared with the values lysimeter evapotranspiration, was estimated. Methodology This study was conducted during two years of field research and development center to promote technology Haraz at the km 10 Amol - Mahmoud Abad road. In terms of climate, long-term average maximum temperature 32.7 oC, minimum temperature 7.8 oC and mean annual precipitation 882.6 mm that 63 percent of precipitationoccurs in the month of September to December. Two-year field study was performed to investigate paddy rice crop evapotranspiration (ETc) wfor two types of paddy rice variant, Khazar and Tarem, by direct method (lysimeter) and calculated method of FAO in Amol, Mazandaran state. To determine the water requirement and crop coefficient of rice plants, the improved cultivars (Tarom and Khazar) as high yielding varieties during the growing season and crops were compared. All measurements of the water requirements of rice, in machine-based stations Laysymtr Ghyrvzny length 1 m, width 1 m and height was 65 cm. simultaneously reference evapotranspiration (ETo) was estimated by daily measured A class evaporation pan data after calibration of pan coefficient. Calibrated pan coefficients were calculated by different equations such as Cuenca, Allen, Pruitt, Snyder, modified Snyder, Orang and FAO. To determine the accuracy of calculating evapotranspiration rice, measurements of Laysymtry were used. Laysymtry values were used as reference data for comparing the calculated values. Results and Discussion Obtained results by lysimeter showed, rice evapotranspiration in the first and second year of 578.5 (Tarom variety) and 481.6 mm (Khazar variety), respectively. This is due to a higher being during growthof Tarom variety in the first year than Khazar variety in the second year. However, a Tarom variety is long-term with tall plants. Increased during growth, and tall plant height, is effective in the amount of water. The maximum amount of KC in the third stage of rice plant growth, for both years was 1.2. The minimum in the fourth stage, for both first and second years was 0.9. In the present study for both farming year, the value of Kc in the initial stage of growth, was1.09 and 1.13, respectively. Two-year results showed that FAO method overestimate calculated crop evapotranspiration values by 4-5% in comparison of lysimeter measurements. Results showed that Eshnaider method is suitable for ETo estimation of pan coefficient and evaporation pan estimated ETo well in the study region. Estimation of ETc on the basis of pan evaporation data, Eshnyder pan coefficient and FAO modified crop coefficient showed only on the average 1.5% underestimation in comparison with actual ET (ETc). Therefore this method is suitable and practical in the study region. Overall, theresults showed taht using appropriate methods for estimating reference crop evapotranspiration and the crop coefficient, evapotranspiration of rice can be calculated with good accuracy. Applications and is suitable for different regions. Conclusion In this study, due to economic importance and nutritional value of rice in Iran, two-year study to determine the water requirement and select the appropriate method for estimating the evapotranspiration of rice was done. Obtained results by lysimeter showed, rice evapotranspiration in the first and second year of 578.5 and 481.6 mm, respectively. Comparison of rice plant evapotranspiration estimates showed that with an estimate of reference crop evapotranspiration and crop coefficients, the results are closer to the actual value. Among the various methods of estimating pan coefficient, Schneider was more appropriate than other methods. It revealed that, using appropriate methods to estimate reference crop evapotranspiration and crop coefficients applied, can calculated rice evapotranspiration with good accuracy that is applicable and appropriate for different regions.

Monitoring the spatial and temporal variations of evapotranspiration is crucial for irrigation management and the crop water requirement, especially in arid and semi-arid areas. The purpose of the present study is to estimate the actual evapotranspiration using the SEBAL algorithm and compare it with the FAO 56 standard evapotranspiration to determine pistachio orchards under drought stress in Yazd province. To do so, Landsat 8 satellite images time series with 15 images in 2015 were used. At first, actual evapotranspiration was calculated in 15 days of pistachio phenology and then by summation of evapotranspiration in 15 days, total evapotranspiration was determined in four main stages of pistachio phenology covering the whole period of annual growth. The FAO 56 standard evapotranspiration was also obtained by using the KC-NDVI relationship as the standard for comparing with actual evapotranspiration. Based on the results, SEBAL algorithm has an acceptable capability to determine the evapotranspiration rate in the study area. However, due to lack of valid Lysimeter data in the study area, It was not possible to validate the results of the SEBAL algorithm. But comparing the results with the FAO 56 standard method showed that the two methods are in good agreement with each other. In average, the coefficient determination, RMSE and MAE between the results of SEBAL algorithm and FAO KC-NDVI approach were 0.8, 16.7 mm and 14.5 mm, respectively, for the 15-day of pistachio phenology stages. The average of actual and standard evapotranspiration rates during a pistachio growing season at the study area were 950 and 1086 mm, respectively. Comparison of actual and standard evapotranspiration shows that in most of the study area actual evapotranspiration is lower than standard conditions.

In water systems, precipitation is considered as input and evaporation as the output of the system. Water availability can be estemated from the relationship between these two factors. Therefore, evapotranspiration is the most important factor after precipitation in hydrological cycle. Evapotranspiration is influenced by climatic parameters such as temperature, wind, humidity and sunshine hours. In this research, changes in PET and effective climatic parameters, influencing on PET changes including temperature, wind, humidity, and solar radiation were investigated. For this purpose, PET in 14 weather stations of Yazd province were calculated using the FAO-Penman-Monteith method. Due to the lack of sunshine hours data in some stations, regeneration of the incomplete data was done by using regression method. Due to the lack of wind speed data at some stations, their reconstruction by using data from other stations was done by applying three methods of Inverse Distance Weighted, Kriging and Cokriging. After calculating potential evapotranspiration, PET data were zoned and their monthly and annual trends tested by Mann-Kendall test. Despite occurrence of climate change and increasing of temperature in 13 stations out of the 14 stations, it is expected an increase in potential evapotranspiration in past few decades, while, there is a decreasing trend in PET. Investigating on the effective parameters in potential evapotranspiration showed that wind speed has declined in the last few decades, and despite of an increase in temperature, potential evapotranspiration rate reduces in 64.3% of the stations. General trend of evapotranspiration was -0.86 in this period, which indicates a decrease in evapotranspiration in the Yazd province.

One of the most fundamental processes, which influence climate and weather, both global and local scales that a fact which gives it the status of agriculture, is Evapotranspiration (ET). In irrigation and water resources management, ecosystem modelers, environmental assessment and solar energy system, accurate assessment of evapotranspiration is essential. Potential evapotranspiration (ET) has commonly applied to calculate the actual evapotranspiration, which was difficult to estimate by lysimeter measurement and water balance approach under field conditions. Until now, many methods have reported to estimating ET, however, due to the availability of the observed data, it is difficult to choose the optimal method. In this study, to determination of the best potential evapotranspiration method for the Khorasan Razavi Province, three temperature-based methods, Hargreaves–Samani (HS), Hamon (HAM) and Linacre (LIN) and five radiation-based methods, Jensen-Haise (JH), Makkink (MAK), McGuinness and Bordne (MB), Abtew (ABT), and Priestley–Taylor (PT), were compared with PM at yearly scale, using long-term (11-67 years) data from 13 meteorology stations. Indicators, viz. The correlation coefficient (CC), Relative bias (BIAS), normalized root mean squared error (NRMSE) and Relative error(Re) were used to evaluate the performance of ET estimations by the above-mentioned eight methods. The results showed that the performance of the methods in ET estimation varied among regions; ETLIN overestimated ET, while others underestimated. In Dargaz and Golmakan, ETHS yielded similar estimations to that of ETPM, while, in Torbate-jam and Khaf, ETLIN showed better performances. Also in Mashhad and Gonabad, ETHAM, in Neyshabour, ETJH and in Torbat-e Heydarieh, ETABT showed better performances. But in Fariman, Kashmar, Sabzevar, Sarakhs and Quchan, all methods showed poor performance. It indicated that ETPM is acceptable for ET simulation for the yearly timescale in these areas.    ​

With respect to that water resources shortage in recent years in Iran has become a major challenge and the major water losses are in the agriculture section and due to evapotranspiration, sustainable management of water resources requires identification and correct estimation of evapotranspiration. Among the different methods to estimate evapotranspiration, the remote sensing technique is an appropriate option for evapotranspiration estimating because of appropriate coverage of place and time in regional scale and using this technique, it is possible to appropriately estimate the regional evapotranspiration without requiring the historical information about soil, crop, and type of farm management. Among the various algorithms based on remote sensing, in this research, relatively new METRIC algorithm has been used for estimating rice evapotranspiration in the Rasht city.
To estimating the rice evapotranspiration in the Rasht city, meteorological data of agricultural weather station of Rasht including variables of air temperature, relative humidity, sunshine hours, rainfall and wind speed in the 3-hour time scale, MODIS Satellite images and Lysimeter data were used for 8 non-clouds days of the August, 2014. Using the mentioned data and images and by adopting the energy balance based algorithms of METRIC, The rice crop evapotranspiration was estimated and for validation the results of the METRIC algorithm, the Lysimeter measured data at the Rasht Rice Research Institute in the summer of 2014 were used.
The results of this study showed that by an implementation of the METRIC algorithm and by using MODIS satellite images, it is possible to estimate the actual rice daily evapotranspiration with a reasonable accuracy. Comparing between the estimated amounts of rice evapotranspiration by the METRIC algorithm and the measured Lysimeter values showed that the amounts of three applied error indices including RMSE, MAE, and MBE were respectively 1.21, 1.14 and -1.17 mm/day. These extents represent an acceptable accuracy of the METRIC algorithm for estimating rice evapotranspiration, but in general, this algorithm tends to underestimate the amount of rice evapotranspiration so that the mean of evapotranspiration at the 8 mentioned days using Lysimetery data and the METRIC algorithm were 9.2 and 8 mm, respectively.
In general, use of remote sensing technique and METRIC algorithm as a new method for estimating evapotranspiration have an appropriate and acceptable accuracy and the source of errors between calculated and measured values of evapotranspiration and underestimating of METRIC algorithm as well can be attributed to some factors such as low spatial resolution of MODIS images and selecting procedure of hot and cold pixels.

Changes in evapotranspiration values of rice plants in non-northern regions of the country were estimated for three different planting dates (early, on time and late planting) and with four different probabilities of 75%, 50%, 25%, and 10% (years with low, medium, high, and very high evapotranspiration, respectively), using the Penman-Monteith Equation and meteorological data of 15 stations with a statistical period of 30 years (1990-2020). The crop coefficient of rice in different stages of growth were calculated as an average in 10-day periods based on the Weibull Model. The results showed that, in all non-northern climatic regions, adopting an early planting strategy leads to a reduction in water requirement in all evapotranspiration scenarios. Adopting early planting strategy in years with very high evapotranspiration reduces the water requirement of rice in hot, cold and temperate regions by 125.9, 113.5, and 115 mm, respectively, during the growing season (on average 0.7 to 9. 0 mm per day) compared to late planting. However, in cold and temperate climates, there was no big difference between early planting and timely planting and even late planting. So, it is better to use late planting to avoid the possibility of late frosts. Comparison of the results of evapotranspiration rate of rice plant in all planting dates and different probability levels in cold, moderate, hot and very hot climate regions showed that the highest evapotranspiration rate is related to the hot region at the rate of 1021, and moderate and cold regions have much lower evapotranspiration rate than hot regions with evaporation rates of 784.8 and 729 mm, respectively. Therefore, considering the lack of water resources and the negative effects of climate change in arid or semi-arid regions such as Iran, it is better to avoid rice cultivation in hot regions, especially in years with high evapotranspiration. Also, it is better that in cold and temperate regions, the type of irrigation management and the design of structures and water systems should be adapted to the calculations of years with high evapotranspiration.

In this study, potential and actual evapotranspiration were estimated by Penman- Matis using climatic data of Saveh Station, and plant and soil characteristics were estimated by Cropwat 8.0 software. Our results showed that potential evapotranspiration in growing season was 6.16 times greater than the average of actual evapotranspiration. Actual evapotranspiration was 1.18 times greater than the average precipitation of growing season. It means that plant species have used the stored moisture for actual evapotranspiration. According to the estimation of actual evapotranspiration in a ten-year period, a model was provided for forage production as Ya= 74.30+ 2.698 (ETact). Accordingly, average forage production of the studied site was estimated as 257 kg/hec. It could be said that actual evapotranspiration as an index for climate yield is one of the fundamental factors in improving water use efficiency. Application of the mentioned climate index in different estimation models of long term forage production could be considered in order to determine the rangeland grazing capacity and developing rangeland insurance as a replacement for the usual methods of production.

Evapotranspiration is one of the critical parameters in an interdisciplinary topic between agriculture and hydrology. Thus, the accurate estimate of evapotranspiration can reduce the casualties of water resources and cause proper management in planning. In this study, the standard FAO penman- monteith method is used to estimate the evapotranspiration of the refrence crop (ET0) in Tehran province for selected synoptic stations. Some studies show that in case of high temperature, the wind speed performs an important factor in increasing values of evapotranspiration. The lowest annual reference crop evapotranspiration in synoptic station north of Tehran with an average temperature of 15.6 degrees celsius high it is estimated the average 1160 mm. While Abali stations and Firouzkooh respectively 8.6 and 9 ° C mean annual temperature, 1180 and 1220 mm annual evapotranspiration values experience. The annual evapotranspiration reference plant in Doushan Tappeh station with average annual temperature of 18 °C, About1425 mm. The most important factor in reducing evaporation and transpiration synoptic station in northern Tehran and Doushan Tappeh with average annual temperature is much higher, Wind speed in this station. The maximum value of evapotranspiration estimated around 1700 and 1680 mm in geophysics station and then Mehrabad Airport stations respectively. Approximately, 75% of the annual reference crop evapotranspiration has been occurred from April to September. Also, the maximumvalue of evapotranspiration is observed between June and September, and the minimum is seen from November to Februaey in alla station.

Evapotranspiration (ET) plays a crucial role in arid and semi-arid regions, and its precise estimation of ET is essential for effective irrigation planning and management. Semnan province faces challenges due to a scarcity of synoptic and evaporation stations, making spatial estimation of ET difficult. This study utilized the evapotranspiration product from the ERA5-Land reanalysis dataset, in conjunction with auxiliary variables such as elevation and temperature, to estimate ET in the study area. The Random Forest (RF) model was employed to establish the relationship between auxiliary variables and ET data, resulting in the creation of an ET map using the RF model. The accuracy of the RF model in estimating ET was assessed against observational data using four statistical criteria: R², BIAS, RMSE, and KGE. The validation results demonstrated the high efficiency of the RF model (R² = 0.95, BIAS = -4.1, RMSE = 98.6, and KGE = 0.92). It was observed that the RF model's performance in ET estimation is influenced by the relationship between model error (BIAS) and topography, with elevation playing a significant role in ET estimation accuracy. This study underscores the effectiveness of utilizing data mining and processing techniques within the R programming environment to accurately estimate ET in regions with limited weather stations, particularly in arid and semi-arid areas. By leveraging these methods, it becomes possible to enhance the estimation of evapotranspiration in such challenging environments.