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Recent climate warming and rapid urban development in the Pearl River Delta (PRD) of China exerted great impacts on the reference evapotranspiration (RET), which in turn affects the management of water resources and the quality of the urban environment. The objectives of this study are to examine (i) the temporal variability of the RET in the PRD and (ii) the underlying causes responsible for the temporal variation in the RET across space inside the PRD. The results indicate the following. (1) The RET in the PRD had an overall increasing trend caused by the increase of construction land during 1960-2016. (2) The increase of surface albedo caused by land cover conversion from woodland to grassland played an important role in the noticeable decline of the RET in Guangzhou and Zengcheng. (3) The dominant factors triggering RET variation varied across space in the PRD. In detail, the decline of sunshine duration (SD) decreasing Rn and the decline of wind speed (WS) weakening energy exchange were the dominant factors in decreasing RET in Guangzhou and Zengcheng. In contrast, the daily maximum temperature, daily minimum temperature, and relative humidity (RH), which were the factors causing the increase of vapor pressure deficit (VPD), were responsible for the RET increase in Taishan, Zhongshan, and Shenzhen. Overall, our results indicated that the RET in PRD exhibited a strong spatial heterogeneity due to differences in land use change and climatic conditions. Therefore, the improvement of water resources management and urban environment in the PRD should consider the spatial variation and underlying forces of RET changes.

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