From Summer Peak to Winter Release: Nine Decades of Flow Regime Alteration by the Lower Naryn Hydropower Cascade, Central Asia

How far can reservoir operation displace a river’s calendar? We address the question with 92 years of monthly discharge on the Naryn River, the transboundary artery of the Syr Darya basin, together with an unregulated headwater gauge, two mountain weather stations, 46 years of release data and two years of daily reservoir balances. Annual flow shows no trend, yet the centre of timing of flow moved from late June to mid-January, a displacement of 164 days (bootstrap 95% interval 158 to 173) that inverts the hydrograph. Both weather stations warmed significantly over the same decades, but the headwater gauge kept its summer regime and advanced its timing by only five days, and a paired-gauge counterfactual assigns two to five days of the downstream displacement to changing inflow and 160 to 162 days to operation. Change points near 1986, 1990 and 1994, confirmed by independent segmentation methods, coincide with the documented transition from irrigation-oriented to energy-oriented operation. Roughly 4.8 km3 per year (4.0 to 5.6) now arrives outside the growing season. Reservoir operation is the dominant driver of the inversion, and the displacement volume quantifies the seasonal redistribution at the centre of a thirty year allocation dispute.