An accurate quantification of the ratio of transpiration to total terrestrial evapotranspiration (T/ET) is indispensable in the estimation of land water flux, which in turn can provide a deeper insight into the global interactions between the atmosphere and the terrestrial ecosystems. Quantifying the impact of changing environmental on T/ET is an important topic of research. A wide range of approaches has been used to estimate T/ET, while none of these studies has quantified the relative contributions of multiple factors to the current terrestrial T/ET trend. 

Prof. HE Honglin's team at Institute of Geographic Sciences and Natural Resources Research of Chinese Academy of Sciences and National Ecosystem Science Data Center applied a model-data fusion method that integrates the Priestly-Taylor Jet Propulsion Laboratory (PT-JPL) model with multivariate observational datasets (transpiration and evapotranspiration) to quantify the relative contributions of multiple factors to the T/ET trend for the terrestrial ecosystem of China from 1982 to 2015. This work was published in Agricultural and Forest Meteorology. 

Their study revealed T/ET increased remarkably at a rate of 0.0019 yr^-1 (P < 0.01) during the study period. Leaf area index increased significantly over the period, by 0.0031 m² m^-2 yr^-1. It appears that greening and climate change were the most likely causes of the increasing T/ET in China, directly explaining 57.89% and 36.84% of the T/ET trend, respectively. Particularly, in the subtropical-tropical monsoonal region, greening directly contributed 24.43% to the T/ET trend whereas climate change contributed 60.95%.  

The study result also shows that the influences of greening and climate change on T/ET trends are mutually reinforcing. 

Additionally, researchers analyzed the partial correlation between the climate-driven T/ET and the climate variables which indicated that warming (0.04 °C yr^-1, P < 0.01) was the major driving force of the climate-induced interannual variability of T/ET across the whole study area (R = 0.84), especially in the subtropical-tropical monsoonal region (R = 0.89). 

“Our results may help elucidate the interactions between terrestrial ecosystems and the atmosphere within the context of long-term global climate changes,” Prof. HE said. 

The study was done in collaboration with Lanzhou University, Institute of Tibetan Plateau Research Chinese Academy of Sciences, Tianjin University and Southwest University. The work was supported by the National Key Research and Development Program of China and the Strategic Priority Research Program of the Chinese Academy of Sciences.

Figure: Relative importance of environmental factors on the T/ET trend in China from 1982 to 2015. ‘LAI only’, ‘CLIM only’ and ‘RAD only’ represent the direct influence of LAI, climate, and energy, respectively. ‘LAI+’ and ‘CLIM+’ represent both the direct and interactive influences of LAI and climate. (Image by Dr. NIU Zhong-en)

Reference:

Niu Z, He H^*, Zhu G, Ren X^*, Zhang L, et al. An increasing trend in the ratio of transpiration to total terrestrial evapotranspiration in China from 1982 to 2015 caused by greening and warming[J]. Agricultural and Forest Meteorology, 2019, 279: 107701.