Assessing the impact of land-use change on surface runoff generation within the Panama Canal Watershed
MA: University of Colorado Boulder, 2017.
Land-use in the tropics has changed dramatically with increased conversion of forests to subsistence farms and cattle pastures. Land-use change alters soil properties that drive the hydrological processes of infiltration and surface runoff. We compared surface runoff generation between two steep, humid, tropical lowland catchments in Panama: a mature forest and an actively grazed cattle pasture. Soil hydrologic properties, soil moisture and surface runoff were measured along hillslopes of each land-use type. We input soil characteristics and rain event data to HYDRUS-1D to simulate surface runoff, which was then compared to that observed at the forest and pasture. Runoff ratios were generally higher at the pasture site, though we did not observe any overall trends between rainfall characteristics and runoff ratios across different land-uses. We observed significant differences in saturated hydraulic conductivity, bulk density and porosity between the forest and pasture (p < 0.05). Simulating surface runoff in HYDRUS-1D produced reasonable outputs for the pasture, but little to no surface runoff was simulated at the forest. Results from our study suggest the combination of a leaf litter layer and the activation of shallow preferential flow paths are the main drivers for surface runoff generation at the forest site, while Hortonion overland flow is the main driver for surface runoff at the pasture site. Results from this study contribute to the broader understanding of the delivery of freshwater to streams, both in terms of timing and quantity, which will become increasingly important in the tropics in light of freshwater resource scarcity.