Suspended sediment transport in small upland humid watersheds undergoing afforestation following human disturbance: a comparison of tropical and temperate environments

¹U.S. Geological Survey, GSA Center, Suite 400-15, 651 Federal Dr ive, Guaynabo, Puerto Rico, 00965-5703, USA

²U.S. Geological Survey, 3039 Amwiler Rd., Atlanta, GA, 30360, USA

Abstract

Forested landscapes in the Caribbean region and southeastern USA were extensively converted to agriculture during the 18th and l9th centuries. Subsequently, due to changes in land use, net afforestation has occurred. However, on a global scale, deforestation continues in response to population growth and increasing resource consumption, particularly in the tropics. An understanding of the response of fluvial suspended-sediment transport to landscape disturbance and climate change is essential for effective water, soil, and forest resource management.

The U.S. Geological Survey WEBB (Water, Energy, and Biogeochemical Budgets) Program supports the study of small watersheds in the Luquillo Mountains, Puerto Rico, and Panola Mountain, Georgia. These watersheds have sufficiently similar lithology (granodiorite), soil (Ultisols and Inceptisols), land-use history, precipitation, and stream-bed material (median particle size of 0.5 mm) to permit comparison of their suspended sediment yields.

The Rio Cayaguás, Puerto Rico, drains a 26 km² catchment wh ere land use is currently 23% forest, 47% pasture, 26% crop, and 4% developed; however, as late as the 1940's, less than 10% of the watershed was forested. The nearby 3.3 km² Icacos basin, Puerto Rico is 100% forested. The Panola Mountain stream drains a 0.41 km2 piedmont upland. The watershed, largely cleared of forest during the l9th century for crop production, is presently93% forested (7% in bedrock outcrop). Although small, the Panola sediment yield is comparable to other, larger Piedmont basins. The 50-times greater sediment yield from the Cayaguas basin, in spite of its larger size compared to the Panola stream, appears to result primarily from continued human disturbance, and secondarily from higher runoff, storm intensity, relief, and stored sediment availability in Puerto Rico.

• WEBB Project Bibliography
• WEBB Project Chief Home Page
• Caribbean Water Science Center home page