Variation characteristics and cause analysis of potential evapotranspiration in Poyang Lake Basin from 1981 to 2019

Potential evapotranspiration (ET0) is an essential indicator to determine the level of climate dryness and water shortage, particularly for crop water demand and irrigation. Recent studies have found that the ET0 in most areas of China changed from a downward trend to an upward trend in the early 1990s, closely relating to the tendency of meteorological factors. Seasonal water deficit has been the main limiting factor for agricultural production in summer and autumn, especially in Poyang Lake Basin (PLB), one of the most important commodity grain bases in China. However, the ET0 trend and the seasonal differences still remain unclear. In this study, a Penman-monteith formula was utilized to calculate the ET0 in PLB during 1981-2019 using the daily observed data from 74 meteorological stations. Mann-Kendall trend test and the sensitivity-contribution method were applied to reveal the trend of ET0 on an annual and seasonal scale, and thereby to compare the contribution rates of climate factors, including the wind speed, solar radiation, vapor pressure, as well as the minimum and maximum temperature. Results showed that: 1) The annual ET0 in the PLB first declined and then rose during the study period, where a downward trend was found before 2000s followed by a weak downward trend after 2000 s, and an upward trend in the second half of 2000s. The ET0 showed an increasing trend in other seasons except for summer and the annual scale in 1981-2019. The annual trend of ET0 was 1.50 mm/a (P<0.05), with the highest increasing rate in spring (0.81 mm/a, P<0.05) followed by autumn (0.64 mm/a, P<0.05). While the summer ET0 experienced a decreasing trend (-0.23 mm/a, P>0.05). 2) The ET0 reduced by 3.97% in 1981-2019, with the decrease in wind speed and solar radiation, while the increase in the water vapor pressure. Nevertheless, the cumulative contribution rate of the minimum and the maximum temperature was 9.79%, which ultimately increased ET0 by 5.84%. Therefore, the warming temperature was the main reason for the annual increase of ET0 in PLB, where the highest temperature had the largest contribution rate (5.90%), indicating the dominant factor. Seasonally, spring and autumn ET0 rising was also mainly caused by warming the minimum and maximum temperature. The contribution rate was 14.40% and 11.79% for spring and autumn, respectively. The maximum temperature was the dominant factor for these two seasons, with the contribution rate of 10.04% and 6.98% for the spring and autumn, respectively. In addition, the dominant factor for winter ET0 rising was the warming minimum temperature, of which the contribution rate was 8.77% (correspondingly 5.58% for the maximum temperature). Although the high temperature increased ET0 by 3.89% in summer, the contribution rate of the decreasing solar radiation was -5.34%. Thus, the decreasing of solar radiation was the dominant factor for summer ET0 declining. 3) The positive contribution of warming temperature exceeded the negative contribution of wind speed and solar radiation in the decadal change of contribution rate for various climate factors. That was the main reason for the turning point of the annual ET0 from falling to rising during the 2000s. The finding can provide a sound reference to regional water management and seasonal drought monitoring in the Poyang Lake Basin of southern China.