USING THE SOIL AND WATER ASSESSMENT TOOL (SWAT) TO MODEL DRY AND WET SEASON SEDIMENT YIELDS TO CORAL REEFS IN THE LOWER RIO LOCO WATERSHED

The transport of sediment from land to coral reefs is of primary concern, as sediment is one of the most harmful pollutants to nearshore coral reefs. Located in Southwest Puerto Rico, the Rio Loco flows through a highly manipulated watershed comprised of a series of small mountainous rivers (SMRs) connected via multiple reservoirs. They supply water for an extensive irrigation system for the Lajas Valley agricultural reserve. Drainage from the agricultural fields is collected, carried to Guánica Bay, and is ultimately transported to the Caribbean Sea. Coincidently, the nearshore coral reefs of the adjacent Guánica Bay have been degraded over time due to the influx of terrestrial sediment. This study seeks to determine whether the upland SMRs or the lowland Lajas Valley agricultural reserve is the predominant source of sediment to the nearshore reefs. To model the influence of dry/wet seasonality on sediment yield, two scenarios were constructed using the Soil and Water Assessment Tool (SWAT) to mimic observed changes in the hydrologic flow regime in each season. The dry season scenario modeled the flow of water through the irrigation system and found that ~50% more sediment was produced by the SMRs over the agricultural reserve. The wet season scenario modeled the flow of water through the natural river channel that bypasses the irrigation reserve. Results showed the largest sources of sediment were the SMRs and cement-lined irrigation canal, both of which produced ~45% more sediment than the agricultural reserve. Overall, preliminary results indicated that the largest annual source of sediment in the watershed was the headwater SMR systems, and not the lowland agricultural reserve, thus stressing the importance of land conservation measures in the headwater regions. Furthermore, these estimates are conservative as the Rio Loco watershed is connected to four additional upstream mountainous reservoirs that may contribute to sediment loading and not modeled here. Using SWAT to model the predictions of sediment yield in this watershed can lead to informed land management decisions that improve water quality and help preserve the health of nearshore coral reefs.