Choosing a reference catchment for the flow that matters

A neighbouring catchment is not automatically a credible hydrological reference. This evidence review synthesises paired-catchment research, reference-by-reference diagnostics, and evidence on catchment-dependent water-balance performance. It argues that selection should begin with the response variable and timescale, distinguishing annual yield, summer low flow, and snowmelt timing. A fictional calibration shows how a reference-gauge change can inflate an apparent target effect even when target observations remain unchanged. The review separates calibration representativeness from later stability and considers multiple references, meaningful temporal validation, extrapolation, stewardship, and monitoring history. It performs no site selection and claims no disturbance effect. Its practical implication is to state exactly which behaviour a reference predicts and how that relationship will be tested, rather than treating proximity or a high overall correlation as sufficient evidence.