Sensitive analysis of potential evapotranspiration to key climatic factors in Taohe River Basin

Abstract: Sensitive analysis of potential evapotranspiration (ET0) to main climatic factors is an important content of the land surface vapor flux research, which has remarkable theoretical and practical significance for understanding regional hydrological cycle and for effective use of agricultural water resources in the context of climate change. Over the past few decades, many parts of the world experienced a decline of ET0 under a warming-up background. Relationship between ET0 and the climatic factors has once again become people's attention. The Taohe River Basin (TRB) is located in the transition zone of the two major geomorphic units of China, the Qinghai-Tibet Plateau and the Loess Plateau. Area of the upstream of the TRB (Gannan Plateau) features an altitudinal cold humid and semi-humid climate, while the downstream of that (Loess Plateau) presents a temperate and semi-arid climate. Natural pattern of the regional climate differentiation provides a good platform for studies on sensitive analysis of ET0 to climatic factors. In this study, the Penman-Monteith (P-M) equation and the Beven sensitivity method were used to estimate the ET0 and sensitivity coefficient related to the main climatic factors. The Sen's slope and Mann-Kendall approaches were used to quantitatively test the amplitude and significance of the sensitivity from year 1981 to 2010. Based on above studies, we comprehensively analyzed the sensitivity of ET0 to climatic factors and discussed the reasons of the ET0 variability under a changeable regional climate in the TRB over the past 30 years. The P-M module outputs indicated a spatially gradual increase of ET0 from the upper reaches to the lower reaches of the TRB. The multi-yearly averaged annual ET0 across the basin was in a range of 633-789 mm. During the period from 1981 to 2010, there was an up-growing tendency of ET0 at a rate of 3.43 mm per year. In the upper reach area of Gannan Plateau, the increase of ET0 was less obvious than that in the lower reach area of Loess Plateau, with an increase rate of 3.39 mm per year and 3.54 mm per year, respectively. Application of Beven method resulted in an obvious differences of ET0 sensitivity to climatic factors. According to the sensitivity coefficients, the sensitive significance was in an order of net solar radiation > relative humidity > maximum air temperature > minimum air temperature > wind speed, while there were relatively high correlation between ET0 and the sensitivity coefficient of the net solar radiation, the maximum air temperature and the wind speed, respectively. Especially, higher values of sensitivity and correlation coefficients were found with the first two factors. The results showed that given the same increasing amplitude (+1%) of net solar radiation and maximum air temperature, the relative increment of ET0 due to the net solar radiation was 0.73%, while that due to the maximum temperature was 0.31%. Over the past 30 years, change of net solar radiation (1%) was less than that of the maximum air temperature (5%), thus, the sensitivity of ET0 to the regional climate was dominated by a weakness to net solar radiation and an enhancement of maximum air temperature. Therefore, the increase of ET0 during the studied period was mainly due to increment of the air temperature in the area.