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

Abstract

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‐km2 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 (10 m) and temporal (hourly) resolution indicated a rapidly responding system. Around 90% of average annual streamflow (2,099 mm) 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 (~420 mm/year) due to high relative humidity (average 96%) and cloud cover limiting radiation inputs. Modelling suggested a highly variable groundwater storage (~10 to 500 mm) 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.

Tracing Water Sources and Fluxes in a Dynamic Tropical Environment: From Observations to Modeling

“…T r flux is of particular interest, since it accounts between 60-90% of global terrestrial ET (Jasechko et al, 2013;Wei et al, 2017), with the Leaf Area Index (LAI) and the growing vegetation stage collectively explaining 43% of the T r /ET variance (Wang et al, 2014) so that for tropical regions with dense vegetation, like most of Costa Rica, the T r /ET ratio is highly relevant and may require in situ water vapor measurements above and under the canopy cover. In Costa Rica, a recent isotope ecohydrological study in a pristine tropical rainforest, using the spatially distributed tracer-aided rainfallrunoff model for the Tropics (STARRtropics; Dehaspe et al, 2018), resulted in a water partioning of 65% of actual ET being driven by vegetation with greater transpiration rates over the dry season (Correa et al, 2019). Overall, this system was dominated by young water, ranging from 1-hr-transpiration to 3.3 years-old groundwater (Correa et al, 2019).…”

“…Two different methods were used to extract soil water: centrifugation (extracts the most available moisture in soils) and cryogenic distillation (extracts the complete moisture hold in soils) (see Orlowski et al, 2016 for a detailed description on methods). Soil water from a rainforest at central Costa Rica was extracted by centrifugation (topsoil sampling during wet season; red triangles Figure 10A; Sánchez- Murillo et al, 2019b), while soil water coming from a rainforest at northern Costa Rica was extracted through cryogenic distillation (sampled between 5 and 110 cm during dry season; yellow dots Figure 10A; Dehaspe et al, 2018). Indistinctively of soil depth, extraction method, season or region, water extracted from soil samples exhibited a clear meteoric origin in line with rainfall isotope seasonality and with minimal enrichment.…”

“…(ii) Spring water, surface water and groundwater (both timeseries and seasonal spatial snapshots), hydrothermal fluids, and soil water (Sánchez-Murillo et al, 2014, Sánchez-Murillo et al, 2017a, Sánchez-Murillo et al,2019bCorrales-Salazar et al, 2016;Ramírez-Leiva, et al, 2017;Salas-Navarro et al, 2019). (iii) Useful isotope-informed applications and estimations, such as stratiform and convective rainfall separation, high resolution isoscapes, recharge mechanisms and elevations, and contribution of magmatic fluids; and tracer-aided rainfall-runoff modeling to estimate flow pathways, runoff generation processes and water ages Birkel and Soulsby, 2016;Dehaspe et al, 2018;Sánchez-Murillo et al, 2020).…”

Iron oxides control sorption and mobilisation of iodine in a tropical rainforest catchment

“…The dry season with lower rainfall probability lasts from January until April followed by the rainy season between May and December. See Dehaspe et al (2018) and Solano-Rivera et al (2019) for a more complete description of the study catchment. 15 https://doi.org /10.5194/soil-2020-20 Preprint.…”

“…if water logged) releasing the adsorbed DOC-I complexes. However, Fe and iodine mobilization in ReBAMB soils caused by reducing conditions is unlikely to occur because of the high infiltration rates (> 100 mm h -1 ) and steep slopes that quickly and continuously transfer water towards the 5 next stream (Dehaspe et al, 2018;Solano-Rivera et al, 2019). Moreover, the low mobility of iodine as DOC-I-Fe-oxidecomplex was caused by the fact that Fe-oxides protect OM against degradation.…”