Temporal and Spatial Variation Characteristics of Water Chemistry and Evaluation of Irrigation Suitability in the Qingling River Basin in the Upper Reaches of the Yangtze River

• • Dry-season pH ranged 7.36–9.18 and TDS 119–2684 mg·L−1; wet-season pH 7.12–8.92 and TDS 125–2342 mg·L−1, indicating substantial seasonal and spatial variability in water chemistry, critical for irrigation scheduling and treatment. • • SO4^2− concentrations significantly exceeded the Yangtze River Basin average, with dominant anions being SO4^2− and HCO3^−; this sulfate enrichment implies acid mine drainage or evaporite dissolution, necessitating targeted management to prevent soil salinization. • • APCS-MLR identified five controlling factors: sulfuric acid-dominated water-rock interactions (47.90% dry, 28.23% wet), carbonic acid-dominated water-rock interactions (24.80% dry, 28.94% wet), domestic sewage (16.98% dry, 27.48% wet), agricultural non-point sources (2.40% dry, 2.02% wet), and unknown sources (7.92% dry, 13.34% wet); water-rock interactions dominate, but sewage contribution rises in wet season, indicating pollution control priorities. • • Irrigation suitability evaluation showed most samples are suitable, but a few have high salinity (TDS up to 2684 mg·L−1), advising cautious use to avoid soil degradation; this directly informs water resource allocation in the Qingling River large irrigation district.