Evapotranspiration Trends and Interactions in Light of the Anthropogenic Footprint and the Climate Crisis: A Review

Evapotranspiration (ET) is a parameter of major importance participating in both hydrological cycle and surface energy balance. Trends of ET are discussed along with the dependence of evaporation to key environmental variables. The evaporation paradox can be approached via natural phenomena aggravated by anthropogenic impact. ET appears as one of the most affected parameters by human activities. Complex hydrological processes are governed by local environmental conditions thus generalizations are difficult. However, in some settings, common hydrological interactions could be detected. Mediterranean climate regions (MCRs) appear vulnerability to the foreseen increase in ET, aggravated by precipitation shifting and air temperature warming, whereas in tropical forests its role is rather beneficial. ET determines groundwater level and quality. Groundwater level appeared to be a robust predictor of annual ET for peatlands in Southeast Asia. In semi-arid to arid areas, increases in ET have implications on water availability and soil salinization. ET-changes after a wildfire can be substantial for groundwater recharge if a canopy-loss threshold is surpassed. Those consequences are site-specific. Post-fire ET rebound seems climate and fire-severity-dependent. Overall, this qualitative structured review sets the foundations for interdisciplinary researchers and water managers to deploy ET as a means to address challenging environmental issues such as water availability.

Effects of the freeze-thaw cycle on potential evapotranspiration in the permafrost regions of the Qinghai-Tibet Plateau, China

Potential evapotranspiration (ET₀) is one of the important indicators for characterizing atmospheric evapotranspiration, but arduous observation conditions lead to a relative lack in the understanding of the ET₀ mechanism in cold, high-elevation regions. The study of the sensitivity coefficient and contribution rate of meteorological elements to ET₀ under the effects of the freeze-thaw cycle in the permafrost regions of the Qinghai-Tibet Plateau (QTP) showed that air temperature (T) and ET₀ had similar change trends, which reached a peak during the summer thawing period (ST). Furthermore, analysis of meteorological elements in different freeze-thaw stages of the active layer soil revealed that the contribution rate of each meteorological element to ET₀ showed a seasonal distinction. The sensitivity coefficient and contribution rate of the net radiation (R_n) and vapor pressure deficit (VPD) to ET₀ were high, and their mean values were 0.52 and 0.44, respectively. In contrast, the sensitivity coefficient and contribution rate of wind speed (u₂) and T to ET₀ were very small, and the mean values of the sensitivity coefficient were 0.08 and 0.01, respectively. Additionally, T had the lowest contribution rate to ET₀. These results indicated that permafrost had a stable regulatory effect on T. The results are expected to be helpful in developing a process-based frozen soil hydrological model.

Periodic fluctuation of reference evapotranspiration during the past five decades: Does Evaporation Paradox really exist in China?

Evidence that the pan evaporation or reference evapotranspiration (ET₀) as the indicator of atmospheric evaporation capability have decreased along with the continuous increase in temperature over the past decades (coined as “evaporation paradox”) has been reported worldwide. Here, we provide a nationwide investigation of spatiotemporal change of ET₀ using meteorological data from 602 stations with the updated data (1961–2011). In addition, we explore the trigger mechanism by quantitative assessment on the contribution of climatic factors to ET₀ change based on a differential equation method. In despite of different shift points regionally, our results suggest that the ET₀ generally present decadal variations rather than monotonic response to climate change reported in previous studies. The significant decrease in net radiation dominate the decrease in ET₀ before early 1990s in southern regions, while observed near-surface wind speed is the primary contributor to the variations of ET₀ for the rest regions during the same periods. The enhancements of atmospheric evaporation capability after early 1990s are driven primarily by recent relative humidity limitation in China. From a continental scale view, as highly correlating with to Pacific Decadal Oscillation, the shift behaviors of ET₀ is likely an episodic phenomenon of the ocean-atmosphere interaction in earth.