Watershed Sensitivity to Land use and Lithology: a Comparison of Fluvial Sediment Yield in Four Small Montane Tropical Catchments, Puerto Rico

U.S. Geological Survey, GSA Center, Suite 400-15, 651 Federal Drive, Guaynabo, Puerto Rico, 00965-5703, USA

Abstract

The low-latitude regions of the Earth are presently undergoing profound, rapid landscape conversion. An appreciation of the impacts of these land-use changes demands careful analysis of watershed-scale hydrologic and geomorphic processes in an attempt to manage what is largely an economically-driven, often disorganized process. Four small, montane, humid-tropical watersheds in the Luquillo Experimental Forest and nearby Río Grande de Loíza watershed, Puerto Rico, were paired to compare and contrast the effects of land use and bedrock geology on fluvial sediment yield. During the period 1991 to 1995 sediment concentration (yield normalized to runoff) was about 3.5 times greater in the two basins in secondary forest and pasture compared to sediment concentration in the basins in primary forest. However, the influence of lithology was almost as great: sediment concentration in the intrusive-bedrock watersheds was about 3.2 times higher than sediment concentration in the volcaniclastic-bedrock watersheds. These contrasts highlight the well-known impact of land use on sediment yield but stress the strong control of bedrock geology as well. Most importantly, the high sediment yields in the watersheds converted to pasture and secondary forest, even after 60 years of forest expansion, provide a glimpse into the future of tropical catchments elsewhere that are now undergoing deforestation.

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