Comment on hess-2022-188

Abstract

The role of topography on water transit times and pathways through catchments is unclear, especially in mountainous environments – yet these environments play central roles in global water, sediment, and biogeochemical fluxes. Moreover, the vast majority of intensively monitored catchments are located in northern latitudes. As a result, the interplay between water transit, topography and other landscape characteristics is particularly underexplored in tropical environments. Here we present the results of a multi-year hydrologic sampling campaign (twice-monthly and storm sampling) to quantify water transit in seven small catchments (< 3 km2) across the transition from the Andes mountains to Amazon floodplain in southern Peru. We use the stable isotope composition of water (δ18OH2O) to calculate the fraction of streamflow comprised of recent precipitation (“young water fraction”) for each of the seven small catchments. Mean unweighted young water fractions (Fyw) are 3–10 % in the Andes, 15–23 % at mid-elevation and 3–4 % in the foreland floodplain. Weighting the Fyw calculation by volume of streamflow and precipitation yield Fyw of 7–47 %. Across these catchments, topography does not exert a clear control on water transit; instead stream Fyw is controlled by a combination of hydroclimate and bedrock permeability. Mid-elevation sites are posited to have the highest Fyw due to less permeable bedrock, poorly developed soils and more frequent and intense rainfall. The data presented here allow us to explore relationships between topography, bedrock permeability, hydroclimate and stream baseflow Fyw – particularly highlighting the role of bedrock permeability and hydroclimate in determining water transit times in a tropical mountain setting.