| Sumario: |
Headwaters are generally assumed to contribute the majority of water to downstream users, but
how much water, of what quality and where it is generated are rarely known in the humid tropics.
Here, using monthly monitoring in the data scarce (2370 km2) San Carlos catchment in
northeastern Costa Rica, we determined runoff-area relationships linked to geochemical and
isotope tracers. We established 46 monitoring sites covering the full range of climatic, land use
and geological gradients in the catchment. Regression and cluster analysis revealed unique
spatial patterns and hydrologically functional landscape units. These units were used for seasonal
and annual Bayesian tracer mixing models to assess spatial water source contributions to the
outlet. Generally, the Bayesian mixing analysis showed that the chemical and isotopic imprint at
the outlet is throughout the year dominated by the adjacent lowland catchments (68 %) with
much less tracer influence from the headwaters. However, the headwater catchments
contributed the bulk of water and tracers to the outlet during the dry season (>50 %) despite
covering less than half of the total catchment area. Additionally, flow volumes seemed to be
linearly scaled by area maintaining a link between the headwaters and the outlet particularly
during high flows of the rainy season. Stable isotopes indicated mean recharge elevations above
the mean catchment altitude, which further supports that headwaters were the primary source
of downstream water. Our spatially detailed “snap-shot” sampling enabled a viable alternative source of large-scale hydrological process knowledge in the humid tropics with limited data
availability.
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