Streamflow and satellite evapotranspiration are asymmetric calibration targets in water-limited catchments

Abstract. Reliable hydrological modelling in data-scarce regions is constrained by the scarcity of streamflow observations, which has driven increasing adoption of satellite-derived actual evapotranspiration (AET) as a supplementary calibration target. Whether AET provides parameter-constraining information complementary to streamflow, or merely trades discharge skill for evapotranspiration reproduction, remains contested. This study compared three calibration schemes: streamflow-only (Q-only), AET-only, and joint streamflow-AET calibration (Q+AET), applied to three conceptual rainfall-runoff models (GR6J, HBV, IHACRES) across 17 water-limited watersheds spanning the hydroclimatic gradient of the West African Sahel during 1991–2020. Parameter identifiability was assessed via variance reduction and distributional shift from prior to behavioural samples. Long-term water-balance estimates were evaluated against the Budyko-Fu framework, with the reference evaporative index anchored both on GLEAM AET and independently on water-balance closure. Results show that streamflow provided substantially more parameter-constraining information than AET. AET-only calibration reproduced evapotranspiration dynamics but left the production, soil-moisture and loss-module parameters governing runoff generation unconstrained. Q-only calibration constrained water partitioning sufficiently to recover AET dynamics in HBV and IHACRES, while AET in GR6J remained poorly constrained under streamflow-only conditioning. Joint calibration preserved streamflow skill of Q-only while matching or exceeding AET-only on AET simulation. It emerged as the only scheme to yield a simultaneously coherent long-term runoff ratio and evaporative index across all three model structures, which held under both Budyko reference anchors. The cost of multivariable calibration was governed by model structure: the direction of parameter displacement imposed by each variable (opposing in GR6J, weakly coupled in HBV and coincident in IHACRES) predicted whether joint calibration was structurally costly or essentially free. These results suggest that streamflow should remain the primary calibration target wherever observations are available, however, satellite-derived AET estimates remain valuable as an independent check on long-term water-balance plausibility rather than as a substitute calibration signal.