Assessing plot-scale impacts of land-use on overland flow generation processes in central Panama

Land-use in Panama has changed dramatically with ongoing conversion of forests to subsistence farms and cattle pastures, potentially altering soil properties that drive the hydrological processes of infiltration and overland flow. We compared overland flow generation between hillslopes in forested and actively cattle grazed watersheds in central Panama. Soil physical and hydraulic properties, soil moisture, and overland flow data were measured along hillslopes of each land-use type. Soil characteristics and rain-event data were input into a simply representative model, HYDRUS-1D, to simulate overland flow that we use to make inferences about overland flow response at forest and pasture sites. Runoff ratios (overland flow/rainfall) were generally higher at the pasture site, though we did not observe any overall trends between rainfall characteristics and runoff ratios across the two land-uses at the plot scale. Saturated hydraulic conductivity (Ks), bulk density and porosity had strong evidence for differences between the forest and pasture sites (p < 10-4). Simulating overland flow in HYDRUS-1D produced outputs similar to the overland flow recorded at the pasture site, but little to no overland flow could be simulated at the forest site. Results from our study indicate that, at the plot scale, Hortonian overland flow is the main driver for overland flow generation at the pasture site, whereas the combination of a leaf litter layer and the activation of shallow preferential flow paths are likely the main drivers for overland flow generation at the forest site. Results from this study contribute to the broader understanding of the delivery of freshwater to streams, which will become increasingly important in the tropics considering freshwater resource scarcity and changing storm intensities.