A unified physically based recession model reveals contrasting functioning of monsoon and Mediterranean karst aquifers

Abstract. Karst aquifers supply drinking water to hundreds of millions of people but remain among the least understood freshwater systems because of their strongly heterogeneous conduit–matrix structure. Spring recession analysis is widely used to infer karst storage dynamics, yet most existing models either treat different parts of the hydrograph separately or rely on empirical formulations with limited physical interpretability. Here we derive a unified analytical model for karst spring recession by combining turbulent conduit flow, represented by a Forchheimer-type relationship, with linear drainage from a porous matrix. The resulting governing equation, dQ/dt = -αQⁿ + γe^-λt, simultaneously describes nonlinear conduit depletion and delayed conduit–matrix exchange. The parameters α, n, γ and λ can be related to aquifer properties, reflecting conduit drainage efficiency, flow nonlinearity, and the magnitude and timescale of matrix drainage.We apply the model to hourly discharge records (2013–2023) from climatically and geologically contrasting karst systems in monsoon-influenced southwest China and Mediterranean central Italy. Event-based calibration shows that the unified recession model reproduces complete recession limbs and consistently outperforms a classical dual-reservoir benchmark. Beyond goodness of fit, the inferred parameter patterns reveal systematic regional contrasts: Chinese spring exhibits higher nonlinear exponents (n = 2) and larger α, indicating strongly turbulent, conduit-dominated drainage with short memory, whereas Italian spring is characterised by n = 1 and negligible γ, consistent with large, slowly draining matrix storage. Using regional-average parameter sets without further calibration, the unified recession model also reproduces independent multi-week drought recessions in both regions, demonstrating that these parameter contrasts are robust and transferable within each hydrogeological setting. Mapping events in the (n, S~) parameter space delineates distinct functional regimes of karst aquifers that can be related to differences in drought resilience and baseflow support. The unified recession model thus provides a diagnostic tool to infer karst aquifer functioning from discharge data alone, supporting water-resource assessment and climate-impact studies in data-scarce regions.