Spatially distributed simulations of dry and wet season sediment yields: A case study in the lower Rio Loco watershed, Puerto Rico

Identifying sources of terrestrial sediment to the ocean is important for watershed managers because of the harmful impacts of sediment on nearshore marine ecosystems. The Rio Loco watershed (RLW) in southwest Puerto Rico has been extensively modified, currently comprised of small mountainous rivers connected by reservoirs and irrigation canals to the Lajas Valley Agricultural Reserve (LVAR). The RLW has regularly supplied sediment to Guanica Bay; however, the total annual yield and subbasins most responsible have remained unknown. We estimated sediment yield for the lower RLW using the Soil and Water Assessment Tool (SWAT) in separate scenarios to assess a range of conditions; a dry season scenario that modeled flow through the irrigation system and a wet season scenario that modeled flow through the natural river channel. Precipitation was similarly correlated with sediment yield in both seasons, with 1.5x more sediment per month predicted during the wet than dry season. The distribution of sediment yield among subbasins was the same in both seasons. The foothills north of the cement lined irrigation canal and subbasin north of Lago Loco were the largest source of sediment, both combined to produce nearly 40% of the total sediment reaching Guanica Bay. The next largest source was from subbasins that comprised a combination of agricultural and suburban land uses. Relative basinscale model outputs for both the irrigation system and natural channel agree with empirical measurements from 2015. The SWAT model output provided some guidance and justification for sediment management strategies in the foothills, upstream of Lago Loco, and within the LVAR.