Hydroclimatic Divergence and Climate-Driven Runoff Uncertainty in the Semi-Arid Fen River Basin

Climate change can alter runoff in semi-arid basins even when annual precipitation changes little. We combined historical meteorological observations, daily WEP-L simulations, and bias-adjusted CMIP6 projections for the Fen River Basin. Historical precipitation and runoff covered 1956–2025, and temperature covered 1956–2018. Future simulations covered 2026–2050 under SSP2-4.5 and SSP5-8.5. Land surface conditions were fixed at 2020, and withdrawals, irrigation, and reservoir operation were excluded. Historical precipitation showed no significant trend, whereas temperature increased by 0.307 °C decade⁻¹. Simulated annual runoff declined at five reporting locations, with trends of −12.1 to −187.2 × 10⁶ m³ decade⁻¹. Future temperatures averaged 10.57 and 11.00 °C under the two scenarios, with trends of 0.392 and 0.652 °C decade⁻¹, respectively. Across seven reporting locations, SSP5-8.5 ensemble-mean runoff was numerically 3.4–6.8% lower. Paired differences at the six locations with complete ten-model coverage were not significant and varied in sign among models. These tests are interpreted descriptively because some GCMs belong to related families. Hejin results are supplementary because model coverage is incomplete. Annual precipitation and runoff anomalies were positively correlated. Downstream inter-model runoff IQRs were comparable to ensemble means. These projections provide a climate response baseline with substantial model uncertainty, rather than forecasts of regulated river flow.