Spatial Distribution And Temporal Trend Of Pan Evaporation In TGRA In Recent 60 Years

Evaporation is a key component of the global water and energy cycle.Decreases in pan evaporation over the past half century have been reported in many regions around the world associated with climate change.This trend is contrary to the expectation that global warming will be accompanied by an increase in pan evaporation,known as the “evaporation paradox”.Under climate warming conditions,spatial distribution and temporal trend of pan evaporation in the Three Gorges Reservoir Area(TGRA)and its surroundings offer valuable information about regional hydrology and ecosystem succession.Data obtained from 27 meteorological stations during 1952-2013 in the TGRA and its surroundings were used to compare and analyze.Their spatial distribution and temporal trend were examined and the causes for the variations are discussed.The results show that:(1)During 1952-2013,the annual average pan evaporation in the TGRA and its surroundings is 1208.03 mm.It shows a significant downward trend at the rate of-20.12 mm/10 a.Seasonal climate change rate in descending order is summer > spring > autumn > winter,with corresponding values of-7.56 mm/10 a,-5.51 mm/10 a,-5.03 mm/10 a,-2.87 mm/10 a.Evaporation decreased slowly in 60 s,and rose rapidly in 70 s.After 80 s,the rate of decline gradually increased.(2)The annual average evaporation has gradually increased from southwest to northeast in space.From winter to summer,the high value region extends from northeast to southwest,while the low value area shrinks to the southwest.From summer to winter,the situation was the contrary.Evaporation significantly decreased region is mainly concentrated in two areas.One is the combination area of Da Ba Mountain,Wu Mountain,and the Yangtze River Valley,and the other is urban area of Chongqing.(3)The average annual pan evaporation is mainly affected by the sunshine duration and relative humidity in the spatial distribution,with an east–west downward gradient.The climate change rate shows the distribution characteristics of descending from the East and the West to the middle,which is consistent with the distribution characteristics of the ground temperature,air temperature and air pressure,but opposite to the precipitation.During the study period,the main positive influential meteorological factors to the pan evaporation are sunshine duration and wind speed,and the main negative influential meteorological factor is precipitation.(4)After the impoundment of the TGR,the annual average pan evaporation was decreased by 5.7%.Before the impoundment,the climate change rate is-18.9 mm/10 a,which was slightly lower than that of-20.1 mm/10 a during 1952-2013.The meteorological factors that mainly affected the pan evaporation in descending order is relative humidity,ground temperature,and wind speed before the impoundment.But it turned to wind speed,air temperature and relative humidity after the impoundment.