Global water availability is of critical significance to water resources for human livelihoods and socio-economic development. Over the past two decades, global water availability has undergone substantial changes due to the complex interactions between climate change, changes in vegetation cover, and human activities. 

Using a combination of satellite data and large-scale basin observations they defined water availability as the net difference between precipitation supply and evapotranspiration demand, researchers led by Prof. ZHANG Yongqiang from the Institute of Geographic Sciences and Natural Resources Research of the Chinese Academy of Sciences quantified the annual trends in global land water availability from 2001 to 2020.  

The researchers found that global water availability has decreased significantly, mainly due to a significant decrease in water availability in the Southern Hemisphere, especially in regions such as South America, Southwest Africa, and Northwest Australia.  

Conversely, the Northern Hemisphere has experienced  mixed trends, with some areas experiencing increased water availability and other regions, such as Southeast Asia, Eastern Europe, and Siberia, experiencing declined water availability.  

These changes are primarily driven by climate patterns, particularly those associated with the El Ni？o-Southern Oscillation. Other factors, including vegetation greening and direct human activities, play a secondary role.  

"This study highlights the importance of understanding and managing water resources in the face of climate change, which may exacerbate regional water resource challenges, especially in the Southern Hemisphere," said Prof. ZHANG.  

"It also sheds light on the limited global impact of vegetation changes on water resources, emphasizing that the relationship between vegetation and water availability is complex and requires further regional investigation," said ZHANG.   

"The findings for the past two decades suggest future water management challenges in the Southern Hemisphere," said Prof. Günter Bl？sch, a world-leading hydrologist, in an accompanying perspective. "There are water management challenges ahead due to reduced water resources in the Southern Hemisphere. Reduced water availability requires a shift from crisis response to long-term, proactive water management, as advocated in the Prague Statement of the International Association of Hydrological Sciences." he added. 

The study was published in Science on November 2.