DETAILED FIRST-ORDER CATCHMENT INVESTIGATIONS OF RUNOFF PRODUCTION MECHANISMS IN THE SEASONAL TROPICS OF PANAMA

Inaccessible and inhospitable working conditions have resulted in a lack of detailed hydrological field investigations in the tropics. We have established a new field observatory located near the town of Gamboa, Panama, near the mid-point of the Panama Canal, to provide detailed measurements of the hydrologic cycle at the first-order catchment scale. Instrumentation includes measurements of evapotranspiration, precipitation, stemflow, throughfall, soil moisture, shallow groundwater levels, and stream flow. Simple models were applied at a workable scale in an attempt to close the water budget. Our investigations reveal that competing runoff production mechanisms are responsible for the observed streamflow. Both of these mechanisms are strongly dependent upon rainfall rate and secondarily upon cumulative rainfall volume. We have identified a small but hydrologically significant riparian zone that is decoupled from the hillslope, and in contrast to the remainder of the catchment shows signs of frequent overland flow. Upslope regions occasionally produce overland flow during high-intensity rainfall. Micrometeorological measurements of evapotranspiration (ET) using an eddy covariance system, coupled with a mass balance approach highlight the careful consideration required in utilizing traditional estimation techniques or the simplifying assumption of declaring a catchment “watertight.” Implications of seasonal changes in soil moisture storages for an improved conceptual modeling framework will also be presented.