ABSTRACT:

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. In this study, using monthly monitoring in the data scarce (2370 km2) San Carlos catchment in northeastern Costa Rica, we determined runoff-area relationships and related those 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 Bayesian mixing models to assess spatial water source contributions to the outlet. Generally, the chemical and isotopic imprint at the outlet is dominated by the adjacent lowland catchments (68 %) with much less influence from the headwaters. In contrast, flow volumes seemed to be linearly scaled by area, which was only true in terms of tracers for the physically more homogeneous headwater catchments. 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. Stable isotopes indicated mean recharge elevations above the mean catchment altitude, which provide further support 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.