Transient storage volume and transit time vectors to infer geometrical constraints on the hydrogeological functioning of karst aquifers

Evaluation of transient storage in karst aquifers provides critical insights into factors influencing its hydrogeological behavior. This study assesses the applicability of a new approach that combines two key metrics to characterize internal flow constraints within an aquifer located southern Spain: a quantitative index of the hydrodynamic state (TSV) and hydrogeochemical parameters indicative of groundwater transit time (Total Organic Carbon –TOC-, Mg²⁺, δ¹³C_TDIC). Transient storage volume –TSV- is here defined as the difference between the cumulative functions derived respectively from daily global aquifer recharge and discharge. A common breakpoint of 3.2 hm³ in the relationships between TSV and discharge from two springs (Algarrobal and Garciago) suggests the presence of a geometric constraint (threshold) affecting groundwater flows, which drastically impact on the hydrodynamic functioning of the aquifer. The existence of this threshold is also deduced from hydrogeochemical transit time indicators. Bellow the TSV breakpoint, only Cornicabra spring exhibits natural responses induced proportionally to the magnitude of the recharge. For TSV values above 3.2 hm³ fast flows are activated within the aquifer with the subsequent effects on the natural responses of all springs, due to the mix of recently infiltrated water with previously stored in the saturated zone. Sensitivity analysis of the equations for estimating potential evapotranspiration (one of the key parameters driving recharge) could further enhance the calculations representativeness. At any case, improvements in the conceptualization through such approaches can be integrated into numerical modeling algorithms, thereby enhancing the model plausibility and alignment with the hydrogeological processes.