Reply on RC1

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> The role of topography on water transit times and pathways through catchments is unclear, especially in mountainous environments &ndash; 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 (&lt; 3 km²) across the transition from the Andes mountains to Amazon floodplain in southern Peru. We use the stable isotope composition of water (&delta;¹⁸O_H2O) to calculate the fraction of streamflow comprised of recent precipitation (&ldquo;young water fraction&rdquo;) for each of the seven small catchments. Mean unweighted young water fractions (F_yw) are 3&ndash;10 % in the Andes, 15&ndash;23 % at mid-elevation and 3&ndash;4 % in the foreland floodplain. Weighting the F_yw calculation by volume of streamflow and precipitation yield F_yw of 7&ndash;47 %. Across these catchments, topography does not exert a clear control on water transit; instead stream F_yw is controlled by a combination of hydroclimate and bedrock permeability. Mid-elevation sites are posited to have the highest F_yw 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 F_yw &ndash; particularly highlighting the role of bedrock permeability and hydroclimate in determining water transit times in a tropical mountain setting.