Tracking rainfall contribution in a conceptual runoff model

• A method to track rainfall contribution in a conceptual rainfall-runoff model. • Wetter catchments respond more to short-term rainfall than drier catchments. • Modelled runoff increasingly depends on multi-annual rainfall during extended dry periods. • The method provides insight into the flow generation mechanism. This paper presents a novel method to analyse how conceptual rainfall-runoff models represent the relationship between rainfall and streamflow. Although the effectiveness of conceptual rainfall-runoff models can be limited by their inherently simplistic representation of catchment characteristics, they remain instrumental in surface water management for their ease of use and fast computation capabilities. The method introduced in this study tracks rainfall contributions through the conceptual model to determine their proportional contribution to generated streamflow outputs. By tagging input rainfall volumes, this technique effectively identifies the conceptual-modelled response of the catchment, offering insight into the temporal distribution of runoff sources. For example, the method provides a useful diagnostic for evaluating model response to rainfall inputs, identifying differences in the partitioning of rainfall contributions across timescales that may not be apparent from traditional performance metrics. The method is applied to the GR4J hydrological model, for 227 catchments with streamflow records ranging from 33 to 70 years. Case studies are used to illustrate how contributions from different rainfall scales vary across flow regimes and climatic conditions. Notably, during a prolonged drought period, the modelled streamflow increasingly reflected contributions from rainfall that occurred more than a year prior, and this longer-memory behaviour persisted even after drought conditions eased.