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 streams in recent years, little attention has been given to these systems in tropical forest environments. We present a unique set of hydrometric, stable isotopic, geochemical, and landscape mapping information to obtain a mechanistic understanding of streamflow generation in 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 agricultural encroachment during the last half-century. Hydrological intermittency is mainly controlled by antecedent wetness due to the strong seasonality of precipitation. Nevertheless, the streambed of catchments draining intermittent streams remains humid throughout the year, even when surface water stops flowing, since evapotranspiration is reduced due to continued cloudy and foggy conditions during the dry season. Intermittent streams mainly located in conserved forested headwaters with shallow soils and a low permeability bedrock have a faster streamflow response to rainfall and shorter recession times than the perennial streams with high permeability bedrock in the catchment's degraded middle and lower parts. Isotopic information shows that rainfall during the wet period (January to May) contributes to streamflow generation in the intermittent streams, whereas rainfall during the wet season recharges the subsurface water storage of the perennial streams. Concentrations of major ions and electrical conductivity were lower in intermittent streams compared to perennial streams. We found a strong correlation between the catchments' geology and geochemical signals and a weak correlation with the 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 and is replenished during the wet period, helping sustain streamflow generation throughout the year for the perennial streams. These findings suggest that geology may play an important role in driving hydrological intermittency, even in highly degraded tropical forest catchments, and provide key process-based information useful for water management and hydrological modelling of intermittent hydrological systems.