Assessment of reference evapotranspiration across an arid urban environment having poor data monitoring system

Estimation of reference evapotranspiration (ET0) in urban areas is challenging but essential in arid urban climates. To evaluate ET(0)in an urban environment and non-urban areas, air temperature and relative humidity were measured at five different sites across the arid city of Isfahan, Iran, over 4 years. Wind speed and sunshine hours were obtained from an urban surrounding weather station over the same period and used to estimate ET0. Calculated ET(0)was compared with satellite-based ET(0)retrieved from the MOD16A2 PET product. Although MODIS PET was strongly correlated with the Valiantzas equation, it overestimated ET(0)and showed average accuracy (r= 0.93-0.94,RMSE= 1.18-1.28 mm/day,MBE= 0.73-0.84 mm/day). The highest ET(0)differences between an urban green space and a non-urban area were 1.1 and 0.87 mm/day, which were estimated by ground measurements and MODIS PET, respectively. The sensitivity of ET(0)to wind speed and sunshine hours indicated a significant effect on cumulative ET(0)at urban sites compared to the non-urban site, which has a considerable impact on the amount of irrigation required in those areas. Although MODIS PET requires improvement to accurately reflect field level microclimate conditions affecting ET0, it is beneficial to hydrological applications and water resource managers especially in areas where data is limited. In addition, our results indicated that using limited data methods or meteorological data from regional weather stations, leads to incorrect estimation of ET(0)in urban areas. Therefore, decision-makers and urban planners should consider the importance of precisely estimating ET(0)to optimize management of urban green space irrigation, especially in arid and semi-arid climates such as the city of Isfahan.