تغییرات در تبخیر تشتکی و نسبت دادن آن به ۵۰ سال گذشته در چین Trend in pan evaporation and its attribution over the past 50 years in China

۱ Introduction Evaporation is an important component of both hydrological and energy cycles. Change of pan evaporation has been recognized by most hydrologists as an integrative indicator of hydrological response to climate change. Therefore, study of pan evaporation trends has become a pertinent issue. It has been reported that pan evaporation has recently been in significant decline globally (Peterson et al., 1995; Michael et al., 2004; 2005; 2007), especially in the Northern Hemisphere (Chattopadhyay et al., 1997; Brutsaert et al., 1998; Cohen et al., 2002; Zeng et al., 2007). This phenomenon is in apparent contradiction with the expected increasing potential evaporation from global warming, e.g. the so called pan evaporation paradox (Michael et al., 2002). This has given rise to many studies on pan evaporation trends and the attribution of their causes, as well as the relationship between pan evaporation and actual evaporation (Burn et al., 2007; Sun, 2007). China is located in the eastern part of the Eurasian Continent and has intricate topography and diverse climate types including continental monsoon climate. Therefore, the study of pan evaporation trends in China is of high importance not only regionally, but also globally as an indicator of response to climate change over diverse settings. Similar to elsewhere, pan evaporation in most of China has been shown to be trending downward. Qiu et al. (2003) found that pan evaporation was on a significant descending trend in the Yellow River Basin. Guo et al. (2005) reached a similar conclusion in the Huang-Huai-Hai Plain, and the attributing factors were thought to be wind speed and solar radiation. However, Liu et al. (2006) found close relationships between pan evaporation and diurnal temperature range (hereafter DTR) and wind speed. Zuo et al. (2005) found that pan evaporation had a good correlation with relative humidity based on the climate trend analysis for China as a whole. The goal of this study is to analyze pan evaporation trends over the past half century in China on a regional basis and as a whole, and to investigate possible relationships with other climatic variables. Trend analysis was conducted using the Mann-Kendall non-parametric test method, and a regional mean pan evaporation (hereafter Epan) changing pattern obtained by 5-point moving average. For trend attribution, we compared the complete correlations of pan evaporation with 7 climatic variables: precipitation, air temperature, relative humidity, wind speed, DTR, sunshine duration, and low clouds. ۴٫۲ Conclusions Epan has been on a decreasing trend in most parts of China over the past 50 years. In the humid region, the Middle-Lower Yangtze River Basin, South China and Yunnan/Guizhou provinces have experienced significant decrease in Epan. While in the semi-humid/semi-arid region, areas with significant decreasing trend in Epan were mainly detected in the Huang-Huai-Hai Plain and eastern Tibet, and in the arid region, provinces of Xinjiang, Qinghai and central Gansu. 5-point moving average analysis showed that Epan was on a steadily decreasing trend during the 1960s to 1990s. Among the three regions, Epan in the humid region showed the largest decreasing rate (29.7 mm/10a); the second was in the semi-humid/semi-arid region (17.6 mm/10a); and the smallest decreasing rate was in the arid region (5.5 mm/10a). Among the four seasons, Epan in summer experienced the most significant decreasing trend; the second was in spring; decreasing trends in autumn and winter were relatively moderate. Decreasing Epan in summer and spring contributed most to annual decreasing Epan. Decrease in Epan related most to the decreases in DTR, SD and wind speed in China over the past 50 years. In the arid region, water factors were also important. Decreasing trends in DTR and SD in Northwest China and areas south to the Middle-Lower Yangtze River Basin were mainly attributed to the increase in precipitation and low cloud, while in the Huang-Huai-Hai Plain, it may relate to the increase in aerosols and other pollutants. Increase in low cloud and aerosols leads to a decline in solar radiation, which in turn results in decreased Epan. Decrease in wind speed relates more to the reduction of the Asian winter and summer monsoons because of global climate warming. However, detailed understanding of the decreasing DTR, SD and wind speed requires further investigation, and studies on this question should be strengthened.