Climate change dominates blue-green water shifts in China’s Arid Northwest: Evidence from the Heihe River Basin

As Northwest China undergoes a distinct warm-wet climate shift, the underlying processes regulating blue-green water conversion remain insufficiently characterized. Here, we establish a Soil and Water Assessment Tool (SWAT) model for the Heihe River Basin. Following rigorous calibration and overall evaluation using reliable ground observations (1997–2015), where monthly runoff simulation achieved R² = 0.51 and NSE = 0.50, and high volume accuracy (PBIAS = − 2.9%), the model reliably captured the basin’s hydrological processes. Results reveal that the green water coefficient peaked during drought years, whereas both blue and green water reached maxima in wet years. Spatially, blue and green water diminished from south to north, contrasting with the green water coefficient, which exhibited higher values in the north. Notably, climate change alone drove a significant increase in total blue and green water volumes from 1997 to 2004 to 2013–2020 (extended simulated period), accompanied by a declining green water coefficient (GWC) due to the non-linear increase in runoff. In contrast, land use change caused only a slight decrease in green water (-2.37 mm) and a marginal decrease in blue water (-0.11 mm), exerting a minor counteracting effect on water resources. Quantitative attribution confirmed climate change as the dominant driver, explaining 101.94% of blue water variation and 105.37% of green water variation, whereas land use change contributed − 1.07% to blue water and − 4.12% to green water variations. These results highlight that climate change is the primary control factor, with land use changes slightly offsetting its impact. This study provides critical insights for optimizing irrigation, land allocation, and sustainable water management in similar arid regions under changing climatic conditions.