Historical pan evaporation changes in the Qiantang River Basin, East China

Pan evaporation (E-pa) has been decreasing in many regions of the world in the past decades. Analysis of the changes in E-pa helps to understand the variation of vapour in the hydrological cycle and the impact of climate change on E-pa. This study aims to investigate the spatial and temporal trends in E-pa and analyse the causes of E-pa variation in the Qiantang River Basin, East China. A self-organizing map neural network is first applied to classify the 40 hydrological stations. Based on the clustering results, a trend-free pre-whitening Mann-Kendall test is then used to investigate the historical trends in E-pa based on more than 50 years of monthly observations. Finally, sensitivity analysis is conducted for evaluating the relationship between E-pa and five climate variables and for investigating the causes of changes in E-pa using a global, variance-based method. It is found that the hydrological stations located in this basin can be classified into six sub-areas and the E-pa trends vary substantially at six sub-areas. Generally, there is a decreasing trend of E-pa in a majority of months in all sub-areas except one and the most significant decreases occur in summer. The results of sensitivity analysis reveal that E-pa is more sensitive to solar radiation, followed by minimum air temperature, wind speed, relative humidity and maximum air temperature. The contribution of interactions between different climate variables to the variation of E-pa cannot be ignored in some specific stations. Overall, the change of E-pa is mainly due to the decrease of solar radiation. This study provides an in-depth understanding of the causes of E-pa variation and valuable information for sustainable water and crop management in this region.