Prediction of Potential Evaporation and Transpiration under Radiative Forcing Scenarios)Case Study: Tabriz(

Considering the phenomenon of climate change and subsequent changes in plant water requirements, it is crucial to recognize and estimate climate change in the coming decades with the aim of proper environmental planning to adapt to future climate conditions and reduce its effects. The main factor in the consumption of water resources in arid and semi-arid areas is agriculture and, accordingly, evaporation-transpiration, therefore, awareness of the process of changes and its prediction plays an effective role in planning, development, and water resources management. Since evaporation-transpiration accounts for an important part of the water balance of arid and semi-arid regions, its correct estimation is very important in the optimal preservation of water resources. On the other hand, knowledge of evaporation-transpiration process is necessary to estimate plant water consumption and design irrigation systems. According to the conducted research, estimation of evapotranspiration in the present and future periods is one of the basic needs of development managers, therefore, this research aims to evaluate the evapotranspiration potential in the base period (1991-2005), and predict temperature changes, using 3 scenarios (RCP2.6, RCP4.5, and RCP8.5) and evapotranspiration based on CMIP5 models (1.1 BCC-CSM and CCSM4) in the 75 year period (2025-2100) was carried out in Tabriz city.

For this purpose, the Penman-Monteith method was used as a standard algorithm to estimate potential evaporation and transpiration in the basic period (1991-2005). Then, potential evaporation and transpiration in the period of 2025 to 2100 were estimated using 2.6 RCP, 4.5 RCP, and 8.5 RCP scenarios and the LARS-WG6 model. Finally, the predicted values of evaporation and transpiration in future periods were predicted using the statistical indicators and the calculated evaporation and transpiration values for the base period (1991-2005). The ROC skill score curve also was used in order to general assessment of the quality of the estimations.

The results showed an increase in the predicted potential evaporation and transpiration under the RCP scenarios. In all three scenarios, the highest amount of evaporation-transpiration was obtained in July and the lowest amount was obtained in December. Moreover, an increase in the predicted transpiration evaporation was observed in July, August, January and February, compared to the base period. In addition, a decrease in the predicted transpiration evaporation was obtained in November and December, compared to the base period. In the RCP 8.5 scenario, the difference was much higher than in the base period. The evaluation of the model performance showed that for the hot months of the year, the model had a better ability to estimate the amount of potential evaporation-transpiration compared to the cold months, and the lowest RMSE error was in the hot months, so that in January with a value of 0.15 mm, the lowest value Dara showed the error. A comparison between the months indicated the two months of February and August as the best estimation for the estimation of annual evapotranspiration values. Likewise, the results showed that in all future periods and under all scenarios, the average reference evaporation and transpiration in annual scales will increase significantly at the level of 0.01 compared to the base period. The verification results also showed an acceptable ability for the predictions of the potential transpiration evaporation model.

This research amed at investigation of the amount of reference evapotranspiration changes based on RCP radiative forcing scenarios and climate models from 2025 to 2100 in Tabriz. The obtained results indicate the increase of reference evapotranspiration in all RCP scenarios for each of the future periods. In addition, the highest percentage of the reference evapotranspiration changes in the 8.5 RCP scenario is more than other scenarios because this scenario predicts the worst climate conditions and obvious changes in evapotranspiration will occur. The results of this research can be beneficial to solve the challenges of managers and relevant officials in the future periods. Considering this, the water, environment and health sector planners should adopt the necessary solutions for adaptation and reducing the consequences. Reasonable use of water resources is inevitable under the conditions of warming weather in Tabriz. Increasing evaporation and creating important changes in the quality and quantity of water resources, consequently changes in the quantity and quality of agricultural products. This situation determines the necessity of planning changes in the exploitation of water resources and agriculture. The future plans should be such that the upcoming changes have less harmful effects on the water and agriculture sector in this region. It is necessary to consider measures to improve the irrigation system, reduce evaporation, reuse wastewater and improve the cultivation pattern.