Reply to Ogden and Stallard: Phenomenological runoff models in the Panama Canal watershed

Abstract

Ogden and Stallard (1) argue that our estimates of the effect of reforestation on runoff in the Panama Canal watershed (2) are invalid because we use a particular phenomenological runoff model: the US Department of Agriculture–Natural Resources Conservation Service Curve Number (CN) model. The authors claim that the CN model, originally developed to describe runoff from single storm events in temperate watersheds, does not “work” in humid tropical watersheds and cannot be applied at other temporal or spatial scales. We disagree. The test of any phenomenological model is whether it predicts well at the scale on which data are observed. The CN model does not offer the same insights into hydrological processes as physics-based models, but it has been applied at various temporal (3) and spatial scales (4, 5) with reasonable predictive power. We calibrated our model using precipitation and hydrograph data for the Candelaria subbasin and validated against observed flows for six subbasins in the watershed. At the subbasin scale the model predicted flows within . The predicted margin of error for the whole watershed was found to be 1%. Sensitivity analysis showed that the estimated hydrological flows were relatively stable with respect to CN variations.