Effects of land use on upland erosion, sediment transport, and reservoir sedimentation, Lago Loiza basin, Puerto Rico

Abstract

Lago Loiza, the major water-supply reservoir for San Juan, Puerto Rico, has lost 47 percent of its storage capacity to sedimentation since impoundment in 1953. This report describes results of a study on the effects of land use on upland erosion rates, sediment yield, and reservoir sedimentation rates. Based on Cesium-137 and bathymetric survey data, sedimentation rates documented for the period 1964-90 were slightly less than the rates documented for the period 1953-63. The major land use in the early history of the reservoir was cropland, which comprised 48 percent of basin area in 1950. Following economic shifts on the island in the 1960's, agricultural land was abandoned and replaced by forest, which increased from 7.6 percent in 1950 to 20.6 percent of the basin in 1987. The boundary of the Lago Loiza basin is 22 kilometers south-southeast of San Juan. Population in the basin increased 77 percent from 1950 to 1990 and housing units increased 194 percent. Even with the increase of forest cover in the basin, reservoir sedimentation rates did not decrease until 1979-90. This lag time may reflect the transport of sediment that was stored in the fluvial system during the period of deforestation and mobilized during the period of afforestation. In the last 20 years, the substantial increase of population and housing in the basin caused more land area to be disturbed and may have caused sedimentation rates to remain high.