RUNOFF PRODUCTION IN THE RIO CHAGRES CATCHMENT, PANAMA

The upper Rio Chagres catchment provides the opportunity to examine tropical hydrology in a nearly-pristine environment. The upper catchment is almost entirely free from development. Furthermore,long-term stream flow records collected by the former U.S. Panama Canal Commission and the current Panama Canal Authority are available. Discharge hydrographs from two stations within the upper catchment are analyzed over a number of years. These hydrographs clearly show the influence of seasonality at the annual time-scale. However, they also show some peculiar features. First, there are occasional intense runoff events early in the wet season, as determined by the peak discharge. Secondly, the groundwater discharge to the Rio Chagres, as identified through base flow levels, exhibits two quasi-stable states as the wet season progresses. Preliminary investigation of the watershed characteristics seems to indicate that soil-surface water-repellency may play a role in exaggerated runoff production early in the wet season. However, this hypothesis does not necessarily agree with other physical characteristics observed in the catchment. Field surveys at nearby sites reveal very significant soil crack development during the dry season. These cracks have a significant water retention capacity. Additionally, the role of rainfall interception by vegetation is unclear. Finally, the two quasi-stable base-flow states indicate a fundamental hydrogeological transition during the wet season that is poorly understood. This presentation focuses on the application of a conceptual hydrologic model that is employed to obtain estimates of possible hydrogeological storage reservoir volumes, and changes in runoff production efficiency.