Abstract
Contrary to the common expectation that the reference evapotranspiration (ETo), which is an indicator of the atmospheric evaporation capability, increases in warming climate, the decline of the ETo has been reported worldwide, and this contradiction between the expected increasing ETo and the observed decreasing one is now termed the “evaporation paradox”. Based on the updated meteorological data (1960–2019), we separately detected the spatiotemporal characteristics and the causes of the “evaporation paradox” in three subregions, namely Huaibei, Jianghuai, and Sunan, and throughout the entire province of Jiangsu in southeastern China. Different from the reported continuous unidirectional variations in the ETo, in the province of Jiangsu, it generally showed a decreasing trend before 1990 but followed an increasing trend from 1990 to 2019, which led to the different characteristics of the “evaporation paradox” in the periods from 1960 to 1989, from 1990 to 2019, and from 1960 to 2019. In the first 30 years, the reduction of the wind speed (WS) was the main reason for the decreased ETo, which consequently gave rise to the “evaporation paradox” in spring and winter in the Huaibei region and only in winter in the two other subregions and throughout the entire province. We noticed that the “evaporation paradox” in spring in the Sunan region was expressed by the decreased daily mean air temperature (Tmean) and the increased ETo which was chiefly induced by the decreased relative humidity (RH) and the increased vapor pressure deficit (VPD). After 1990, the decreased WS also dominated the decreased ETo and resulted in the “evaporation paradox” in winter in the Jianghuai region. Furthermore, the decreased sunshine hour (SH) was the main factor influencing the decreased ETo, thereby inducing the “evaporation paradox” in summer and autumn in the Jianghuai region and only in autumn in the Huaibei region and throughout the whole province from 1990 to 2019. In the whole study period from 1960 to 2019, the decreased SH was also found to be responsible for the decreased ETo and for the “evaporation paradox” in summer in all the subregions and throughout the whole province. However, regarding the “evaporation paradox” in autumn, in winter, and in the entire year in the Huaibei region and throughout the whole province, the observed decreased ETo was largely due to the reduced WS from 1960 to 2019. In summary, in addition to the air temperature, the ETo has shifted due to the other meteorological variables (especially the WS, the SH, and the VPD) and shaped the unique spatiotemporal characteristics of the “evaporation paradox” in the province of Jiangsu in southeastern China. Moreover, future studies and simulations addressing the regional climate change and hydrological cycles should take account of the changeable key meteorological variables in different subregions and seasons of the province of Jiangsu.
Highlights
It is widely accepted that the global air temperature has risen by 0.85 ̊C in the past five decades and will continue to rise by 0.3–4.8 ̊C by the end of this century [1]
We divided the entire study period into two 30-year periods; the World Meteorological Organization (WMO) recommends that a 30-year period should be used as a reference to describe the climate state
During 1960–1989, we found that the Sunan region had the highest annual air temperature variables, that is, a Tmean, Tmax, and Tmin of 15.43, 19.86, and 11.85 ̊C, while the Huaibei region had the lowest Tmean, Tmax, and Tmin of 13.84, 19.15, and 9.3 ̊C, respectively (Table 1)
Summary
It is widely accepted that the global air temperature has risen by 0.85 ̊C in the past five decades and will continue to rise by 0.3–4.8 ̊C by the end of this century [1]. Along with the warming trend, there has been a long-term expectation that the pan evaporation (ETpan) or the reference evapotranspiration (ETo), which indicates the atmospheric evaporative demand, will increase [2]. These variables have been found to decrease in many regions worldwide [3,4,5,6,7,8] since the first report in 1995 [9]. Roderick et al [10] defined the inconsistency between the expected increasing ETpan or ETo and the observed decreasing ones as the “evaporation paradox”. In recent years, based on the updated meteorological data, some studies have presented a view that the ETo has had different change points regionally [25, 26], which may impact the spatiotemporal characteristics of the “evaporation paradox” in different regions of China
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