Streamflow generation in a nested system of intermittent and perennial tropical streams under changing land use

Abstract. Despite the increased interest in the hydrology of intermittent hydrological systems in recent years, little attention has been given to tropical forest environments. We present a unique set of hydrological, stable isotopic, geochemical, and landscape mapping information to obtain a mechanistic understanding of streamflow generation in an intermittent system of 20 nested catchments (<1–159 km²) draining intermittent and perennial streams and rivers in the Chocó-Darien ecoregion, a tropical biodiversity hotspot, located in the Pacific lowlands of northern Ecuador that has been strongly degraded by deforestation and cultivation during the last half-century. Intermittent streams mainly located in conserved forested headwaters present a faster streamflow response to rainfall and shorter recession times than degraded perennial streams in the catchment's middle and lower parts. Isotopic information shows that rainfall during the wet period (January to May) contributes to streamflow generation in intermittent streams possessing shallow soils and a low bedrock permeability, in contrast to perennial streams in which rainfall during the wet season recharges their high bedrock permeability. Lower concentrations of major ions and electrical conductivity were observed in intermittent streams compared to higher concentrations in perennial streams. We found a strong correlation between the catchments’ geology and their geochemical signals and a weak correlation with their topography, land cover, and soil type. These findings indicate that shallow subsurface flow paths through the organic horizon of the soil dominate streamflow generation in intermittent streams due to the limited water storage capacity of their bedrock with very low permeability. On the contrary, high bedrock permeability increases the water storage capacity of perennial catchments replenished during the wet period, helping sustain streamflow generation throughout the year. These findings highlight the key role geology plays in driving hydrological intermittency, even in highly degraded tropical environments, and provide key process-based information useful for water management and hydrological modelling of intermittent hydrological systems.