SWAT-based sediment yield simulation with mining land-use change scenario in Lanuza Bay, Philippines

The Philippines is known for its rich marine biodiversity and is deemed as the apex of the world’s coral triangle. However, sediment yield studies and river discharge measurements in the country are sparse if not non-existent. High sediment rates have detrimental effects on water quality and consequently to coral reef health and marine biodiversity. Thus, modeling of runoff and sediment yield at a watershed level is important in assessing coral community environments.

In this study, a Soil and Water Assessment Tool (SWAT) based sediment yield simulation was done for Lanuza Bay-- a site with high productivity but increasing mining activity. Two simulations were conducted. The first utilized a land-use map before January 2011 and was made to run from January 1998 to August 2018 in order to simulate a scenario in which mining operations did not occur in the area. The second simulation utilized an updated land-use map that incorporated mining sites from January 2011 to August 2018.

SWAT model results indicate that slope class was the primary determinant of erosion rates (slope band > 20%). The study suggests that consecutive precipitation occurrences affected soil erodability and induced a time lag between precipitation and sediment yield peaks. The highest contributors to sediment yield at a sub-basin level were identified to be areas adjacent to or coinciding in mining or excavation sites. Comparing the scenarios with and without mining, mining contributed to 4% of the increase in the watershed’s total annual sediment yield.

Qualitative and historical validation shows reasonable agreement between simulated values and satellite images. The output of this study can be used as a science-based reference in crafting laws and policies for land-use management and Marine Protected Area (MPA) planning.