Spatially distributed tracer-aided modelling to explore water and isotope transport, storage and mixing in a pristine, humid tropical catchment

Rapidly transforming headwater catchments in the humid tropics provide important resources for drinking water, irrigation, hydropower, and ecosystem connectivity. However, such resources for downstream use remain unstudied. To improve understanding of the behaviour and influence of pristine rainforests on water and tracer fluxes, we adapted the relatively parsimonious, spatially distributed tracer-aided rainfall-runoff (STARR) model using event-based stable isotope data for the 3.2-km(2) San Lorencito catchment in Costa Rica. STARR was used to simulate rainforest interception of water and stable isotopes, which showed a significant isotopic enrichment in throughfall compared with gross rainfall. Acceptable concurrent simulations of discharge (Kling-Gupta efficiency [KGE] 0.8) and stable isotopes in stream water (KGE 0.6) at high spatial (10m) and temporal (hourly) resolution indicated a rapidly responding system. Around 90% of average annual streamflow (2,099mm) was composed of quick, near-surface runoff components, whereas only 10% originated from groundwater in deeper layers. Simulated actual evapotranspiration (ET) from interception and soil storage were low (420mm/year) due to high relative humidity (average 96%) and cloud cover limiting radiation inputs. Modelling suggested a highly variable groundwater storage (10 to 500mm) in this steep, fractured volcanic catchment that sustains dry season baseflows. This groundwater is concentrated in riparian areas as an alluvial-colluvial aquifer connected to the stream. This was supported by rainfall-runoff isotope simulations, showing a flashy stream response to rainfall with only a moderate damping effect and a constant isotope signature from deeper groundwater (400-mm additional mixing volume) during baseflow. The work serves as a first attempt to apply a spatially distributed tracer-aided model to a tropical rainforest environment exploring the hydrological functioning of a steep, fractured-volcanic catchment. We also highlight limitations and propose a roadmap for future data collection and spatially distributed tracer-aided model development in tropical headwater catchments.