Heatwaves only partially offset increased water consumption from earlier greening: the case of the Kashi Basin in Central Asia

Excessive water consumption resulting from an earlier green-up date (GUD) driven by climate warming may be mitigated by reduced evapotranspiration (ET) during summer heatwaves. However, their relative contributions to water consumption remain unclear. Here, we integrate satellite-derived phenology, reanalysis-based heatwave indices and a physically based eco-hydrological model to disentangle the effects of earlier GUD and heatwaves on ET and streamflow in the Kashi Basin (KSB), an inland basin on the western slope of the Tianshan Mountains in Central Asia. From 2001 to 2020, GUD in the basin advanced by 0.38 days yr⁻¹(∼12 days earlier relative to 2001), while nighttime and daytime heatwaves (represented by TN90p and TX90p) increased by 0.53 and 0.39 days yr⁻¹, respectively. Model estimates indicate that earlier GUD increases annual basin-mean ET by 11.24 mm (5.72% of baseline ET; 95% CI: 8.47–13.99) and reduces annual streamflow by 0.56 × 10⁸m³(1.59% of baseline streamflow; 95% CI: −0.70–0.42 × 10⁸m³). By contrast, increased heatwaves are associated with a small annual ET reduction of 1.15 mm (0.58% of baseline ET; 95% CI: −1.51–0.75) and a modest annual streamflow increase of 0.06 × 10⁸m³(0.17% of baseline streamflow; 95% CI: 0.04–0.08 × 10⁸m³). The combined GUD × heatwave scenario yields a slightly larger ET increase and streamflow reduction than the linear sum of individual effects. Thus, in this cold and semi-arid basin, heatwave-related ET savings compensate for only about 10% of the additional ET linked to earlier green-up (95% CI: 7%–12.6%). A Tianshan-wide analysis further shows that 59.88% of alpine vegetation experienced both advancing GUD and late-summer browning, consistent with basin-scale evidence that enhanced spring growth can erode, rather than enhance, the carryover benefits of spring greening by depleting water resources needed to sustain summer vegetation activity.