Historical pan evaporation changes in Qiantang River Basin, China

Abstract:

Pan evaporation has been decreasing in many regions of the world in the past decades. Analysis of the changes in pan evaporation helps to understand the variation of vapor in the hydrological cycle and the impact of climate change. This study aims to investigate the spatial and temporal trends in pan evaporation and analyze the causes of its variation in 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 used to investigate the historical trends based on more than 50 years of observations. Finally, global sensitivity analysis is conducted for evaluating the relationship between evaporation and five climate variables and for investigating the causes of changes. It is found that the hydrological stations can be classified into six sub-areas and the evaporation trends vary substantially at different sub-areas. Generally, there is a decreasing trend in a majority of months and the most significant decreases occur in summer. The results of sensitivity analysis reveal that pan evaporation is more sensitive to solar radiation, followed by minimum air temperature, wind speed, relative humidity and maximum air temperature. The contribution of interaction effects between different climate variables to the variation of pan evaporation cannot be ignored in some specific stations. Overall, the change is mainly due to the decrease of solar radiation. This study provides an in-depth understanding of the causes of pan evaporation variation and valuable information for sustainable water and crop management in this region.