Geomorphology and lithology as factors influencing hydrological dynamics. Comparative analysis of karst vs. non-karst watersheds in the central-western region of Portugal

The geomorphological and geological characteristics of karst regions are very specific and complex, affecting the hydrological processes significantly. Due to the enormous heterogeneity and non-linearity of karst terrains, the hydrodynamic functioning of karst watersheds considerably differs from non-karst watersheds. Quantifying and understanding the influence of these watershed characteristics in the hydrodynamics of karst and non-karst areas is crucial for managing water resources and extreme hydrological episodes, particularly in the present scenario of hydroclimatic variability and uncertainty.

The present study establishes a comparison between two watersheds: (i) a karst basin (Anços) composed of Middle Jurassic karstified limestones, dominated by a rough plateau that promotes the infiltration, and (i) a non-karst basin (Arunca) characterized by Upper Jurassic and Cretaceous marly limestones, and Cenozoic thick mudstones deposits (low permeability promoting surface runoff). The rivers' water level was logged every 20 minutes by a gauge station from 2010 to 2020, and the rainfall was also registered for the same period.

The objectives of this study are (i) to examine, from a quantitative perspective, the influence of geomorphological and geological characteristics on the runoff coefficient using statistical analysis; (ii) to quantify the rainfall-runoff lag time and the recession coefficients from the analysis of storm hydrographs simultaneously in both watersheds; (iii) to deepen the knowledge about the behavior of each watershed in flood episodes through hydrological simulation.

The results showed that geomorphological and geological watershed characteristics play an essential role in the hydrodynamic functioning of the Anços and Arunca watersheds. Furthermore, the outcomes of this study revealed a pronounced contrast between the two watersheds in the reaction to rainfall and mainly in the first stage of the recession limb. It should be highlighted that the non-karst watershed presents a faster and more extreme hydrological behavior expressed in a peak hydrograph. On the reverse, the karst watershed demonstrated a higher storage capacity, revealed by the shape of the recession curve.