HYDROGEOCHEMICAL STUDY OF RUNOFF GENERATION IN THE SEASONAL TROPICS OF CENTRAL PANAMA

We have established an integrated international research program that combines hydrological, geochemical, and geophysical measurements in the seasonal tropics of central Panama. Our research includes in situ data collection and analysis enhanced by natural and introduced tracers and modeling. The overall objective of this collaborative research is to understand important runoff generating processes in the seasonal tropics and the roles of the seasonal transitions and land-use changes on runoff mechanism thresholds. The existing 12 ha instrumented Cerro Pelado hillslope old-regrowth micro-catchment enables detailed field studies including imaging of shallow subsurface flow using electrical resistivity tomography, and studies of soil surface hydrophobicity using alternating Cl and Br applied tracers. Multi-scale hydrologic, geochemical and light- and heavy-stable isotopic data collected using a paired-basin methodology compare the virtually pristine 414 km² old-growth Upper Rio Chagres and adjacent 330 km² largely deforested Rio Pacora watersheds to examine land-use change effects at a range of scales. Different lithologies underlay these basins and this knowledge is used to assist in identifying flow paths and groundwater residence times. This presentation discusses our research design and preliminary results from over six-years of hydrogeochemical sampling and measurements.