Trend Analysis of pan coefficient and projection of pan evaporation in selected stations of Iran

The pan evaporation is one of the major components of hydrological cycle. It is quite important in agricultural water management and water balance estimations. The current research was performed with two main goals. First, to study the trend of pan coefficient during baseline period of 1993-2018 and second, projection of pan evaporation during three future periods under RCP climate change scenarios in 5 selected stations across Iran, namely Mashhad, Bushehr, Ahvaz, Kerman. In part one; the monthly trend of Kp values were studied using Man-Kendal and Sen’s slope estimator in warm season (Spring and Summer).During baseline period a significant decreasing trend in pan evaporation was observed Mashhad,Bushehr and Ahvaz stations.In second part, for projection of pan evaporation under RCP scenarios, the PenPan model, a modified form of P-M equation, was used. The required projected climate data were retrieved from CNRM-C5 model outputs under RCP 4.5 and RCP 8.5 scenarios. The trend analysis of Kp values using Man-Kendal test during the baseline period showed a significant decreasing trend except for the Yazd station with the least coefficient of -1.88 mm. The greatest decreasing value based on Sen’slope estimator was observed in Bushehr station. The results of Man-Kendal test revealed a decreasing trend in pan evaporation in Ahvaz, Mashhad, Bushehr and increasing trend in Kerman stations. For future periods of 2020-2049, 2050 to 2079 and 2080 to 2100, an increasing trend of pan evaporation in Kerman station and decreasing trend in 3 other stations was detected. According to Sens’s slope estimator test, during the baseline period the most increasing trend was observed in Kerman station. For future period the most increasing trend was observed in Mashhad station (+0.64). It is projected that mean pan evaporation values during near, middle and far future periods would decrease 4.7, 6.2 and 8.8%, respectively. Despite of projected increase of air temperature in Mashhad, Bushehr and Ahvaz stations, a reduction in pan evaporation was observed which might be attributed to reduced received radiation, confirmed in previous studies as evaporation paradox.