Research On Pan Evaporation Changes In Hydro-meteorological Environment Of Guangdong

The observation data of 20cm caliber evaporation pan and related meteorological elements were collected from 86 weather stations of Guangdong Province for 1961-2003. Statistical analysis of the pan evaporation and Penman evaporation and actual evaporation were done. With the Penman equation, the characters of the energy balance term and the aerodynamic term were analyzed and their attributions to pan evaporation were investigated. The trends of pan evaporation caused by percentage of sunshine duration, wind speed, vapor pressure deficit, and daily temperature deficit were discussed.(1)The annual pan evaporation in the northern area was less than in the southern area. In the last 43 years, the pan evaporation in Guangdong had continuously decreased at a rate of 46.8mm/10a, its abrupt change happened in 1981. Except 13 stations had increasing trend,73 of 86 sites had descending trend, in Guangdong province. The regions with a significant drop in pan evaporation were the Pearl River delta, south-west part and the eastern coastal region of Guangdong. Only a small part of regions with a rise in pan evaporation concentrated in the central region and the eastern region of Guangdong. Through analysis on variations of the dynamic term and energy term in Penman equation, separately, the dynamic term had a major role. It was found that the drop of pan evaporation was mainly attributed to decreases in the energy term and the dynamic term in south-west part and the eastern coastal region of Guangdong, was mainly attributed to decreases in the energy term in the Pearl River delta, the rise of pan evaporation was mainly attributed to increases in the dynamic term was greater than the energy term decreases. The correlation analysis between annual pan evaporation and the impacting factors showed that the sunshine duration and wind peed were key influencing factors.(2)The pan evaporation in the northern area was less than in the southern area in spring, the first rain season and autumn, the latter rain season was the opposite in Guangdong. Seasonal pan evaporation had a significant decrease trend in the last 43 years, it was more significant in spring, the first rain season and the latter rain season. Spatially, the pan evaporation decrease in spring was the best, and the decrease area was the largest, secondly in the first rain season, the decrease was smaller in the latter rain season and autumn, the increase area had a gradually increasing trend from spring to autumn; anomalous centers of the seasonal pan evaporation were mainly located in the Pearl River delta, the south-west and eastern coastal region of Guangdong, and the anomalous centers were different in different seasons. The energy term fell in every season, in the Pearl River delta, and the south-west coast and Meizhou it declined significantly, the dynamic term declined in the south-west and eastern coastal and the northern region of Guangdong, the dynamic term rising area was concentrated in Meizhou and the Pearl River delta, the increase area had a gradually increasing trend from spring to autumn. The decrease in pan evaporation was mostly resulted from reduction of sunshine duration and wind speed and daily temperature range over Guangdong.(3)The Penman evaporation in the northern area was less than in the southern area in spring, the first rain season and autumn, it evenly distributed in the latter rain season.The penman evaporation had a decrease trend in spring, the first rain season and the latter rain season, but less than the pan evaporation, it had a slightly rising trend in autumn. For pan is more sensitive to the environment than the large area of the natural water body. The annual Penman evaporation was usually 60 percents of the pan evaporation, and the coefficient of evaporation had a slightly decreasing trend from the coastal region to the northern part of Guangdong, in spring and the first rain season and autumn it had a slightly decreasing trend too, evenly distributed in the latter rain season.(4)The annual and autumn actual evaporation of the eastern area were less than the western areas', in spring and the first rain season inland actual evaporation were greater than the coastal areas', in the latter rain season it was opposite. The actual evaporation had an increase trend in spring, the first rain season and the latter rain season, it had a slightly decrease trend in autumn, and less than the pan evaporation. The actual evaporation was usually 50 percents of the pan evaporation, and percentage had a slightly increasing trend from the coastal region to the center and eastern part of Guangdong.