A process-based diagnosis of catchment coevolution in volcanic landscapes: synthesis of Newtonian and Darwinian approaches

Abstract. Catchment coevolution is a framework that seeks to find a rigorous connection between landscape evolution and the emergent hydrological responses, and formulate hypotheses about how such evolution affects their hydrological response. Empirical studies previously conducted by the authors in volcanic catchments in Japan have revealed that the hydrological signatures baseflow index and slope of the flow duration curve (SFDC) decline with the age of the catchment bedrock; however, the possible influence of external climate forcing on these signatures could not be eliminated, and the causality behind the empirical relationships could not be identified. To test the robustness of the relationship and attempt to understand why such simple and predictable relations have emerged across a climate gradient, we used a process-based hydrological model that was independently calibrated for eight catchments underlain by volcanic rock of different ages and conducted a numerical experiment to decouple the effects of internal and external properties of the catchments. The eight calibrated catchment models were independently forced by the eight different sets of climate properties corresponding to the prevailing climates of the eight catchments to investigate how the simulated relationship between SFDC and catchment age deviates from the empirically derived relationship on both per catchment (each catchment forced by eight climates) and per climate (each climate filtered by eight catchments) basis. We found that the mean of the residuals of the observed versus predicted SFDC from each catchment, exhibited a significant positive correlation with catchment age, providing numerical evidence of catchment coevolution to support that of the empirical study. We further investigated the causality of this relationship and found from several simulated time scales and model fluxes that younger catchments on average (1) require longer for the transmission zone storage to fill and empty, (2) take longer to release water from deep aquifers, and (3) have greater recharge to deep aquifers. These findings corroborate the hypothesis of coevolution of volcanic catchments, which is that in younger catchments more water percolates to the subsurface storage and the deeper hydrologically active systems release water at a slower rate. The analysis also revealed that the external climate characteristics interact with the catchment internal properties in forming the catchment hydrological responses. The mean throughfall rate, which is controlled by rainfall intensity and should be independent of catchment internal properties, significantly declined with catchment age in our data set. The mean transpiration rate was the only candidate for a causal link between climate and the hydrological signatures.