Assessing Impact Factors on Soil Erosionand Sediment Loss in Puerto Rico Watersheds

A major environmental concern in the Commonwealth of Puerto Rico is increased soil erosion and sediment load to reservoirs, estuaries, and coral reef areas along the coast. Sediment deposition has significantly reduced the storage capacity of reservoirs and, with their associated contaminants and nutrients, can stress corals and negatively impact reef health. To prevent and manage soil erosion and sediment loss, it is important to understand local soil erosion and sediment transport processes. The main objectives of this study were to: 1) identify sediment sources; 2) determine the influence of landscape characteristics on sediment loss; and 3) seek alternative ways to reduce soil erosion and sediment loss to the reservoirs, bay areas and coastal zone. To determine the influence of landscape characteristics on sediment loss, we analyzed available precipitation, runoff and sediment data collected in Puerto Rico over the past three decades, along with information on land use, soil properties and topography. The partial least squares analysis did not definitively identify a set of major factors associated with sediment loss due to the complexity of the island geography, soil type and land use; however, the analysis showed that topography and rainfall factors do not play a leading role. Sediment loss from the ridge watersheds in Puerto Rico was mainly caused by interactions of development, heavy rainfall events (especially hurricanes; Table 1), and steep mountainous slopes associated with the ridges (Yuan et al., 2015). Additionally, we looked at the impact of increased agricultural production since the 1950s, especially the increase in sun-grown coffee plantations typically located in ridge watersheds, on soil erosion and sediment loss. To accomplish this, we conceived and developed an integrated approach using hydrological modeling with different land use and land management scenarios to assess the impact of land use and land management on sediment yields, and multiple regression analyses to determine the relative importance of other erosion related factors such as slope steepness, slope length and soil erodibility. We conducted hydrological modeling for two watersheds (Yahuecas and Adjundas; Figure 1) in Guánica Bay, using the Soil and Water Assessment Tool (SWAT). From our SWAT simulations and multiple regression results, we were able to conclude that coffee production generates the largest amount of sediment, followed by range-grass, and forest produces the least (Yuan et al., 2016). If sun-grown coffee trees were all replaced by shade-grown coffee trees, the annual sediment yield would be reduced. For the same land use and management practices, soil erodibility and slope steepness are the two factors influencing sediment yield the most. These results improve our understanding of soil erosion and sediment loss resulting from changes in land use/cover in Puerto Rico and allow stakeholders to make more informed decisions about land use planning and water resource management.