Estimation of actual evapotranspiration of pistachio plants using the SEBAL algorithm and Landsat 8 images: A case study of Abarkooh desert margin in Yazd Province

Iran, along with the Mediterranean countries, is known as one of the main habitats of pistachio (Pistacia vera L.) in the world. This plant plays an important role in the economies of arid and semi-arid countries such as Iran, Turkey and Syria. Although pistachio crop can produce a medium-quality yield with low water consumption, proper irrigation management can help to increase yield with higher quality. Evapotranspiration, which involves the evaporation of water from the soil surface and transpiration from vegetation, represents the fundamental process of a hydrological cycle. Monitoring the spatial and temporal changes of evapotranspiration is very important for irrigation and water management, especially in arid areas. Evapotranspiration on a homogeneous surface can be measured using conventional techniques such as Bowen ratio, Eddie covariance, water balance, and on field surfaces using a lysimeter system. These methods are usually costly and time-consuming and also do not have the ability to be generalized at a large heterogeneous level. As a result, it is more desirable to use remote sensing methods that take these heterogeneities and changes into account. Several algorithms have been developed to determine evapotranspiration using satellite imagery. In this regard, the SEBAL algorithm is one of the most widely used methods to determine the actual evapotranspiration by remote sensing. The aim of the present study was to determine the actual evapotranspiration using the SEBAL algorithm during different phenological periods and the growing season in pistachio orchards on the edge of Abarkooh desert in Yazd Province. The results of this study can be used to effectively manage the water consumption and prevention of drought stress in pistachio gardens.

The study area is located in Yazd Province, 20 km south of Dehshir district. The total study area is 13971 hectares, of which approximately 3160 hectares are covered with pistachio gardens. In the present study, 16 images of Landsat 8 with 16-day periods from 03/16/2015 to 11/11/2015 were used after the atmospheric, geometric and radiometric errors were corrected. The reason for choosing the mentioned time period is that this it is the active growing season of pistachios. In the next step, the input parameters of the SEBAL algorithm were prepared, and the actual daily evapotranspiration rates at the transit dates of the Landsat 8 were calculated. Then, the rates of evapotranspiration were obtained in 15-day phenological periods, including the four main stages of phenology and the entire one-year growth period of pistachios. Also, in order to evaluate the results, the daily evapotranspiration of pistachio trees in the passing days of the satellite was calculated according to FAO 56 guidelines and at six different points on pistachio lands. In the next step, the correlation coefficient and the RMSE of daily evapotranspiration were obtained by the SEBAL algorithm and the FAO 56 instructions.

A comparison of the daily rates of evapotranspiration obtained by SEBAL algorithm and the FAO 56 showed that the two models are well matched; the average correlation and RMSE at six points were 0.77 and 1.24 mm / day respectively. Based on the results, the average and maximum evapotranspiration of pistachios in a one-year growth period in 2015 in the study area were 1015 and 1650 mm respectively. Also, in 83% of the study area, the actual evapotranspiration rate in a one-year growing season was 700 to 1300 mm. The maximum rate of evapotranspiration was observed in June and July with a rate change between 80 to 100 mm during 15-day phenological stages. According to the results, 50% of evapotranspiration occurs in the period from June 1st to August 31st. The rates of pistachio evapotranspiration are different according to the climatic conditions, the water management system and the area. In some studies done outside Iran, the yearly rate of pistachio evapotranspiration have been found to be 800, 600 and 1018 mm. In two different studies in Ardakan region of Yazd, the actual evapotranspiration of pistachio trees was estimated 1133 and 1275 mm using the SEBAL algorithm. Another study in Yazd Province showed that more than 60% of Marvast pistachio gardens have a seasonal water consumption of 410 to 680 mm, while the cumulative evapotranspiration of the reference plant and the standard evapotranspiration and transpiration of pistachios in the same period were 1558 and 920 mm respectively. In other words, the pistachio trees in this area are under drought stress, which will reduce the crop yield.

Although valid terrestrial evapotranspiration data such as lysimetric data were not available in the study area to compare with the results of the Sebal algorithm, comparing these results with those of other studies in this field shows that the findings of the present study are accurate and dependable. Also, the comparison of the rates of daily evaporation obtained through the SEBAL algorithm and the FAO 56 method showed that the results of the two models were well matched. In general, the results of the present study suggest the capability of remote sensing techniques to calculate evapotranspiration and their usefulness in crop irrigation management.