Runoff Production in the Upper Río Chagres Watershed, Panama

Abstract

Runoff production in tropical watersheds is governed by a wide variety of potential sources and there have been few rigorous studies to date. The 414 km2 upper Río Chagres basin offers a unique opportunity to better understand the runoff production mechanisms in tropical watersheds through data analysis and modeling with rainfall and runoff data. Flow data and tipping bucket rain gage data are available at both the basin outlet (Chico gage) and for an 80.6 km2 internal basin location (Piedras gage). Modeling is performed using the Sacramento Soil Moisture Accounting Model (SAC-SMA), calibrated using data from 2000 and verified using data from 2001. The flood event of 28–31 December 2000 was examined in detail. Data analysis and modeling reveal critical threshold storages in the catchment, and anomalously high runoff production at the start of the wet season. This conclusion is supported by field studies that reveal evidence of high storage capacity and dry season water repellency. Observation of discrete quasi-stable baseflows in the upper Río Chagres is not seen in the internal Rió Piedras drainage, which is shown to exhibit ephemeral behavior year-round. New data collection and monitoring is proposed for the upper Río Chagres catchment, including measurements of rainfall above canopy, cloud stripping, stemflow, throughfall, soil moisture, interflow, and overland runoff measurements.