Hydrogeomorphology influence on pan-tropical transit times

The tropics are one of the most diverse and highly dynamic regions on Earth. Despite their importance, our understanding of tropical hydrological processes remains a significant challenge mostly due to limited monitoring. Here, we used high-resolution daily input-output isotope data sets from seven pan-tropical catchments ranging in size from 3 to 990 km2 , located in Australia, Costa Rica, and Ecuador to estimate and compare streamflow transit times (TTs) with potential explanatory hydrogeomorphological variables. Pan-tropical catchments resulted in short TTs from 49 to 497 days using a simple lumped convolution integral model with a Gamma distribution as transfer function (best-fit Kling Gupta efficiencies up to 0.92). The gamma distribution alpha parameter was in 5 out of 7 catchments below the previously identified global pattern of around 0.5. A random forest (RF) ranking approach identified water storage capacity, the presence of sedimentary rocks (%), and the annual precipitation amount as the most important TT controls. In addition, the TT distribution as indicated by the alpha parameter was best explained by annual evapotranspiration, the soil texture, and the Andosol soil cover (%). Our findings identified the key TT and TT distribution controls in fast responding tropical catchments compared to other geomorphic and climate zones emphasizing the value of TT as a simple catchment descriptor