Effect Of Urbanization On Pan Evaporation In Nanpan River Basin,yunnan Province

The rapid development of cities will change the regional climate of cities.In the context of global warming,the impact of urbanization on climate change has attracted widespread attention.As an important link in the hydrological cycle,evaporation may also be affected by urbanization.Therefore,it is of great significance to study the change characteristics of evaporation and the influence of urbanization on evaporation.In this paper,monthly pan evaporation data from 1980 to 2016 and social and economic data in 2016 are used to analyze temporal and spatial changes of pan evaporation in Nanpan River Basin.Based on the analysis of temporal and spatial changes of pan evaporation,the differences of urban and rural evaporation in the basin are analyzed by using the station pairing analysis method,and a concise index EUE of pan evaporation urbanization effect is created to measure the urbanization effect.Furthermore,grey relational analysis method is adopted to analyze the grey relational between EUE and socio-economic indicators,and to identify the influence of socioeconomic indicators on urbanization effect.The main research results are as follows:(1)Pan evaporation in the Nanpan River Basin during the study period(1980-2016)showed a downward trend on the whole,except in autumn,spring,summer,winter,wet and dry seasons and annual evaporation showed a downward trend.Spatially,pan evaporation in each season and year is low in the northeast and high in the southwest.Pan evaporation had a sudden change around 2000,and was Hurst greater than 0.50 in both year and season,showing a strong positive persistence in the trend of change.(2)Among different types of stations(urban stations,mixed stations and rural stations),pan evaporation of the three types of stations decreased in the rest seasons and the whole year during 1980-2016,except for urban stations and mixed stations in autumn.The trend of annual and seasonal pan evaporation during 1980-2000 was consistent with that during 1980-2016.During 2001-2016,the annual change was consistent with that of spring and summer.The evaporation of urban and mixed stations decreased,while that of rural stations showed an upward trend.The three types of stations in autumn and winter and dry season all showed a downward trend,while that of urban stations only showed a downward trend in wet season.The change trend of pan evaporation in rural stations after 2000 shows a deviation from that before 2000.The results showed that urbanization caused the decreasing trend of pan evaporation.(3)After one-to-one pairing between urban and rural stations.At the basin scale,the urbanization effect(EUE)of the whole year and each season is less than 0.At the station scale,EUE of 5 of the 7 groups of station is less than 0,accounting for 71.43%of the selected station.In each season,the proportion of EUE less than 0 is higher.It shows that urbanization is one of the factors leading to the decrease of pan evaporation.(4)The correlation analysis between EUE and various urbanization indicators shows that the indexes of science,education,culture and health,land use indicators and population indicators are the urbanization indicators with higher correlation with urbanization effect.The correlation coefficient of Luxi Station,Xuanwei Station,Yuxi Station and Mengzi Station is larger in the seven groups,and the correlation degree is all greater than 0.5.Among the four groups of stations,EUE value of 75% stations is less than 0 in the whole year,and that of 54.17% stations is less than 0 in each season.It is indicated that the indexes of science,education,culture and health,land use indicators and population indicators are the main factors affecting the decrease of pan evaporation in urbanization development.