Paired-Watershed Study of Water and Solute Budgets in a Tropical Rainforest

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9903243Genereux The proposed project is a paired-watershed study of the hydrogeology of lowland tropical rainforest in Costa Rica. The proposed project will use hydrologic and geochemical measurements:1 to determine annual and monthly water and solute budgets for the Arboleda and Taconazo watersheds, and2 to evaluate our conceptual model for streamflow generation at the study site: that high solute concentrations found in the Arboleda are due to interbasin transfer of geothermally-modified groundwater, and streamflow represents a mixture of this groundwater and low-solute local water from hillslpes adjacent to the study streams.Water and solute budgets will be constructed for the two adjacent watersheds by measuring inputs (from the atmosphere, and local/regional groundwater systems) and outputs (via streams and the groundwater systems). The solutes to be studied are CL, SO4,Na,K,Mg,Ca, and total phosphorus. The proposed 30-month project will give 2 full years (24 months) of water and chemical budget data.The proposed study will utilize and build on unique, newly-constructed hydrologic infrastructure at La Selva Biological Station in Costa Rica (perhaps the best at any rainforest research site), enabling the study to avoid the large uncertainty (found in many previous studies) caused by inadequate measurement of the hydrologic cycle. Results will be among the first high-quality water and chemical budgets for lowland rainforest watersheds. These budgets will explicitly include the significant Subsurface interbasin transfer (groundwater flow beneath surface topographic divides) that occurs at La Selva; the paired watershed approach proposed is well suited for this. While interbasin transfer is often assumed not to exist or is considered a "problem" to be avoided, experience at many field sites shows that it is probably the rule rather than the exception and therefore should be directly addressed. The proposed project will serve as an example of how this can be done at other study sites. The proposed study will also provide critical hydrologic and chemical background information needed for larger, interdisciplinary projects with ecologists on the rates of carbon cycling in the rainforest and the effects on rainforest stream ecology from groundwater discharge to the stream.