Projected shifts and dynamics in blue and green water resources availability

A deep understanding of the dynamics of green and blue water resources is crucial for accurately estimating future water availability. Although projections of precipitation trends are robust in many regions, changes in precipitation partitioning into green and blue water fluxes present a significant source of uncertainty for water management. To quantify water partitioning dynamics, we introduce the Blue-Green Water Share (BGWS) metric. This metric utilizes monthly precipitation data, while monthly runoff and transpiration data are used as proxies of blue and green water fluxes. We investigate the output of fourteen CMIP6 models for the historical period and three Shared Socioeconomic Pathways to assess the scenario dependency of the BGWS changes. Most importantly, we examine how and why the BGWS varies across different regions. Additionally, primary drivers of the BGWS changes are identified by applying a multivariable regression analysis and computing the permutation importance of selected ecohydrological variables.

The results illustrate a strong regional dependency and interplay of the driving variables. However, clustering the variable importance demonstrates that BGWS changes in higher latitudes tend to be more dependent on temperature, while precipitation patterns dominate partitioning changes in the tropics. Several regions, including the Mediterranean, Northern South America, and Eastern Australia, show a substantial influence of vegetation alterations on the BGWS change, shifting the partitioning imbalance towards more green water flux. We discuss the varying variable importance based on the counteracting mechanisms of increased CO₂ concentrations, altered growing seasons, and changed precipitation patterns. Our results highlight the importance of comprehensively understanding green and blue water dynamics in the context of water resources availability under a changing climate.