Impacts of land-use and land-cover change on blue–green water partitioning

Abstract. Land-use and land-cover change (LULCC) is a major driver of terrestrial water cycle changes, yet its effects on how precipitation is partitioned into blue (runoff) and green water (transpiration) flows remain unclear. Here we address this knowledge gap using Earth system model simulations from the Land Use Model Intercomparison Project (LUMIP) under contrasting socioeconomic pathways (SSP1-2.6 and SSP3-7.0). We find that future sustainable LULCC (i.e., predominantly avoided deforestation and preservation of natural non-forest ecosystems) significantly impacts blue-green water partitioning, with regions showing positive leaf area index (LAI) and gross primary productivity (GPP) responses generally corresponding to larger green water shares. These effects are strongest in the tropics and particularly during dry seasons, where LAI and GPP responses are largest. Regions with the strongest green water gains show the highest sensitivity of blue-green water partitioning to vegetation responses, with the largest partitioning shifts per unit change in LAI or GPP. Precipitation responses to LULCC further modulate the strength of blue–green water partitioning shifts. In some regions, higher transpiration is partly offset by increased rainfall, limiting reductions in blue water availability. While we find consistent ecohydrological responses to LULCC across the multi-model ensemble mean, substantial regional inter-model disagreement arises due to differences in model-specific plant functional types and their parametrisations. Our results underscore that the water cost or benefit of land management depends jointly on vegetation function, precipitation feedbacks, and model structural uncertainty.