Effect of urbanization on the long-term change in pan evaporation: A case study of the Nanpan River Basin in China

Evaporation is a major component of both the surface energy balance and terrestrial water balance. Rapid urban development can change the climate of urban areas, which may have an impact on pan evaporation. In this study, we analyzed the differences in pan evaporation between urban and rural stations, using the Nanpan River Basin of China as the study area. Site pair analysis was conducted using evaporation and socioeconomic data from 25 meteorological and 14 hydrological stations from 1980 to 2016. An urbanization effects indicator, namely EUE, was created to quantify the urbanization effects. Subsequently, a gray relational analysis (GRA) between the urbanization effects indicator and socioeconomic indicators was conducted to identify the effect of socio-economic indicators on urbanization effects. The main results were as follows. (1) During the period 1980–2016, the pan evaporation in the Nanpan River Basin showed a decreasing trend, which was similar in both urban and rural stations, with slopes of −2.87 and −2.89 mm/yr, respectively. (2) After the sudden change in pan evaporation in the Nanpan River Basin in 2000, the slope difference between urban and rural stations increased significantly. The slope of pan evaporation at urban stations was −4.55 mm/yr, whereas that at rural stations was 1.01 mm/yr. The slope difference between urban and rural stations was −5.56 mm/yr, larger than that in 1980–2000 (−0.43 mm/yr). (3) Urbanization influenced the trend and slope of the long-term change in pan evaporation. The rising slope decreased after 2000 in two of the seven cities, and the rising trend changed to a decreasing one in five cities. The difference between the slopes in 2001–2016 and 1980–2000 △E was used as the urbanization effects indicator EUE, and the EUE of all seven cities was<0. (4) The GRA based on the established urbanization effects indicator and urbanization indicators revealed that land use indicators were the factors with the greatest influence on the urbanization effects EUE, and the remaining indicators were social, population, transportation, and national economic indicators. This study is critical important for assessing the long-term changes of pan evaporation and can provide a basis and reference for water resources management in the basin.