Comparison of sediment yields from forested and agriculturally-developed montane humid-tropical watersheds, Puerto Rico

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

Abstract

Global-scale conversion of forested tropical landscapes to agricultural use has been rapid since the middle of the 20th century. Interests favoring or opposing forest conversion debate issues of sustainability, decreasing biodiversity, soil degradation, and erosion, among others. The effect of forest conversion on erosion can be quantified, but detailed suspended-sediment sampling of tropical watersheds before and after deforestation is rare. However, the concept of space-for-time substitution allows for comparison among landscapes under varying land use if other factors are relatively constant. The quantification of sediment yields in two pairs of humid-tropical watersheds in Puerto Rico (18°) latitude) provides an approximate measure of the effect of forest conversion. These watershed pairs have comparable lithology, soil, climate, and topography.

Estimated annual suspended-sediment yield from two relatively undisturbed forested watersheds located in the Luquillo Experimental Forest, Puerto Rico, is on the order of 200 to 400 tonnes/km²/year. I n one stream draining Tertiary intrusive bedrock, drainage area, mean annual runoff, mean annual precipitation, slope, and bed type in the watershed are 3.3 km², 4.0 m, 4.2 m, 0.0005, and sand (D₅₀ = 0.6 mm), respectively. In the other stream, draining Cretaceous volcaniclastic bedrock, these same variables are 17.8 km², 2.9 m, 3.5 m, 0.007, and boulder (D₅₀ ~290 mm).

Two nearby agriculturally-developed watersheds are used for cattle grazing and small-scale plantain and root-crop farming. Estimated annual suspended-sediment yield is on the order of 1,000 to 3,000 tonnes/km2/year. In a stream draining Tertiary intrusive bedrock, drainage area, mean annual runoff, mean annual precipitation, and bed type in the watershed are 26.4 km², 1.5 m, 2.0 m, 0.0004, and sand (D ₅₀ = 0.5 mm), respectively. In the other stream, which drains Cretaceous volcaniclastic bedrock, these same variables are 25.5 km², 1.0 m, 2.0 m, 0.0063 and cobble (D₅₀ ~200 mm).

The larger area and lower runoff of the developed watersheds should mitigate against greater erosional yields. Nonetheless, markedly higher erosion rates are observed in the agriculturally-developed watersheds and are significant in regard to reservoir storage. Public water supply for the city of San Juan (population 1.6 million) is largely derived from surface water storage. The principal reservoir for the city has lost approximately 60 percent of its storage capacity since impoundment in 1954. The reduced storage capacity resulted in severe water rationing for San Juan in 1994 when precipitation averaged 60 percent of normal.

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