Linking tracers, water age and conceptual models to understand ecohydrological processes in humid tropical catchments

Abstract:

Assessing catchment runoff response remains a key research frontier due to limitations in current observational techniques to fully characterize water source areas and transit times in diverse geographical environments with implications for ecohydrological processes. Here, we report from experiments that combine empirical data with modelling to identify dominant runoff processes in sparsely monitored humid tropical catchments in Costa Rica. The analysis is based on the integration of isotope tracers into conceptual rainfall-runoff models of varying complexity that are able to simulate discharge and tracer concentrations and to track the evolving age of stream water and evaporative fluxes exiting the catchment, as well as the age of stored waters. Different model structures can be seen as competing hypotheses of catchment functioning and effects on ecohydrological water partitioning.

We show that a variety of tested model structures reproduce water and tracer dynamics in stream, but the simpler models fail to adequately reproduce both. The resulting water age distributions of the tested models varies significantly with little similarity between the stream water age and stored water age distributions. Sensitivity analysis reveals that only some of the more complex models can be more adequately constrained to infer more plausible water age distributions and catchment storage estimates. These models indicate that the age of water stored in the catchment is generally older compared to the age of water fluxes with evapotranspiration age being younger compared to streamflow. However, the water age distributions follow a similar temporal behaviour dominated by climatic seasonality. Stream water ages increase during the dry season (> 1year) and decrease with increased streamflow (a few weeks old in stream) during the wet season. We further show that the ratios of the streamwater age to stored water age distribution and the water age distribution of actual evapotranspiration to the stored water age distribution from constrained models can potentially serve as useful indicators of ecohydrological processes.