Reservoir Regulation and Meteorological–Streamflow Coupling: A Multi-Region Matched-Basin Analysis with Drought-Event Diagnostics

Abstract

Reservoirs can alter river responses to climatic water deficits, but distinguishing regulation-associated differences from hydroclimatic variability remains difficult. We use a large-sample matched-basin design to compare full-record meteorological–soil-moisture and meteorological–streamflow coupling between highly regulated basins and hydroclimatically similar near-natural references, with event-based drought propagation examined separately. GRDC-Caravan hydroclimatic records were combined with Global Dam Watch reservoir information, with reservoir exposure represented by a static, basin-level storage-to-discharge proxy. The main analysis retained 394 matched basin pairs for 2002–2018 (17 calendar years) with at least 80% streamflow completeness. The primary metric was zero-lag Pearson coupling computed from pairwise-complete monthly standardized anomaly series across the full analysis period; decoupling was defined as 1 − r0. Event-based metrics were evaluated separately in eligible pairs. Streamflow showed a modest regulation-associated shift: the median regulated-minus-reference decoupling difference was 0.023 (95% bootstrap CI [0.008, 0.037]), with 57.4% of pairs positive. Soil moisture showed an opposite-sign shift (median ΔDc = −0.017), indicating a compartment contrast rather than a common positive decoupling response. The streamflow signal remained positive under stricter completeness filtering, an extended analysis period, exclusion of the poorest matches, and without-replacement matching, with a complementary sign-flip sensitivity check. Country- and GRDC-subregion cluster bootstraps retained positive intervals, whereas coarse WMO-region clustering and the separate 1991–2020 eligible-subset sensitivity had intervals including zero. Storage-to-discharge intensity was positively but modestly associated with streamflow decoupling (Spearman ρ = 0.241). These results indicate that reservoir regulation is associated with weaker contemporaneous meteorological–streamflow coupling in the sampled basins and support reservoir-aware interpretation of drought signals, while residual geographic and other human water-use confounding, together with unobserved reservoir operation, preclude strict causal attribution.