Separating the effects of climate change and human activity on water use efficiency over the Beijing-Tianjin Sand Source Region of China.

Water use efficiency (WUE) is a central parameter for linking carbon and water exchange processes in terrestrial ecosystems. The Beijing-Tianjin Sand Source Region (BTSSR) in China has undergone tremendous vegetation restoration and climate change. Understanding the WUE responses to climate change and human activity and their relative contributions to the trends and inter-annual variations (IAVs) in WUE is necessary to improve water use efficiency and strengthen water resource management. The evapotranspiration (ET) dataset based on the model tree ensemble (MTE) algorithm which was a machine learning approach using flux-tower ET measurements and the GLASS GPP dataset, as well as the variance decomposition method, were used to analyze the spatiotemporal changes in water use efficiency and inherent water use efficiency (IWUE) and the impacts of climate change and human activities. The results showed that the annual WUE and IWUE exhibited significantly increase in most regions of the BTSSR. The trend of human activity played the most important role in the increases of WUE and IWUE, with relative contributions of 88.2% and 85.9%, respectively, followed by the IAV of human activity for WUE (6.1%) and the trend of climate change (8.7%) for IWUE. The contribution of IAV to climate change was relatively small. Moreover, WUE and IWUE were all positively correlated with precipitation and temperature in most regions. Our results indicated that ecological restoration projects had significantly improved water use efficiency in BTSSR and may decrease the water burden in the BTSSR.

Multi-scale assessment of eco-hydrological resilience to drought in China over the last three decades.

Understanding to what extent can terrestrial ecosystems maintain their structure and functions and recover after being hit by drought is critical for sustainable ecosystem management and drought mitigation practices. This study assesses multi-scale (i.e., at grid, climate type, land use, basin, province scales) eco-hydrological resilience to drought over China during the period of 1982-2015, based on Standard Evapotranspiration Deficit Index as well as satellite-retrieved evapotranspiration (ET) and net primary production (NPP). Over the last three decades, the ecosystem water use efficiency (eWUE) increased in most regions of China (especially in Northeast China and North China Plain) in the context of climate change. The western China showed a significant wetting trend with ascending ET, GPP and eWUE. The SEDI and ET showed significantly increasing tendencies but the ecosystem GPP and eWUE did not exhibit obvious responses to regional wetting in the middle and lower reaches of Yangtze River basin. Most terrestrial ecosystems in China were found overall resilient to drought except for mild temperature-fully humid-warm summer at the climate type scale, water type at the land use scale, Northwest Rivers at the basin scale as well as Ningxia, Qinghai, Shanghai and Hong Kong at the province scale. These findings would facilitate the identification of most drought-vulnerable regions for ecosystem management and taking reasonable adaptation measures (e.g., use of alternative water resources or reuse water, water conservation and smart irrigation) to ensure the sustainability of terrestrial ecosystems during the drought periods over China.