Spatio-temporal variability and trend of potential evapotranspiration in Xinjiang from 1961 to 2013

Abstract: Evapotranspiration is one of the important parameters in hydrological and meteorological studies. Under the background of global warming, it is essential to accurately estimate the potential evapotranspiration in arid and semiarid areas where the water resources are limited and excessively explored, which would be greatly helpful for planning the irrigation water supply and understanding the effects of climate change on hydrological processes. In this study, observed daily meteorological data during 1961-2013 from 55 meteorological stations, including temperature, relative humidity, wind speed and sunshine hours, are used to calculate the potential evapotranspiration based on Penmen-Monteith equation, the methods of Mann-Kendall test, Cramer's test, correlation analysis and the contributions analysis, for analyzing the spatial distribution and temporal changes in the potential evapotranspiration in XinJiang during 1961-2013.Major results were as follows: from the 1960s to the 1970s, potential evapotranspiration was dominated by positive anomaly, then changed into negative anomaly from 1980s to 1990s, and the annual and seasonal potential evaporation anomaly reached a low point in the 1990's. Potential evapotranspiration anomaly descent in spring and summer is the key contributors to the annual potential evapotranspiration anomaly descent.There was a change point for ET0 series around the year 1994 based on the Cramer's test. For the average in the region, ET0 decreased from 1961 to 1994 by 3.21 mm/a, while ET0 began to increase since 1995 at a rate of 3.51 mm/a.Spatial distribution of ET0 showed that during 1961-1994, annual ET0 in most sites (about 75%) had decreasing trends; on the contrary, during 1994-2010, about 76% sites had increasing trends.Based on the latitude, altitude and terrain, the study area was further divided into five climatic regions: the Altay Mountains, the south slope of Tianshan Mountains, the north slope of Tianshan Mountains, the Tianshan Mountain and the north foot of Kunlun Shan. The Cramer's test for detecting a change point in annual ET0 of five climatic regions found that a change point for ET0 series was identified around the year 1994. Annual ET0 of five climatic regions decreased from 1961 to1994 and increased significantly from 1995 to 2013.The results indicated that wind speed, sunshine duration and relative humidity showed reverse trends during the period of 1995-2013 compared to the period of 1961-1994, and the temperature increasing was more significant since 1994.During 1961-1994 and 1995-2013, among all tested meteorological factors, wind speed had the highest complete correlation coefficient with ET0 in XinJiang. There does exist difference of the highest complete correlation coefficients among five climatic regions.During 1961-2013, wind speed became the biggest contributor in XinJiang and this was also apparent in the south slope of Tianshan Mountains, the north slope of Tianshan Mountains and the north foot of Kunlun Shan; while relative humidity was the biggest contributor in the Altay Mountains. From 1961 to 1994, hours of solar shine was the biggest contributor, whereas during 1994-2013, temperature became the biggest contributor in the Altay Mountains.