Effects of changing land use in the Pagsanjan–Lumban catchment on suspended sediment loads to Laguna de Bay, Philippines

Suspended sediment transport into Laguna Lake (Philippines) due to soil erosion from agricultural activities in the Pagsanjan–Lumban catchment is of great concern because of its impacts on water quality. The Pagsanjan–Lumban catchment contributes about 18–20% of the total freshwater inflows to the lake but represents only 10% of the catchment area. In this study the suspended sediment transport model, SedNet, was used to assess the relative impacts of changes in land use within the catchment on the quantities of suspended sediment discharged to the lake. The SedNet model predicted the total suspended sediment supply from the Pagsanjan–Lumban catchment (280km2) to be 231.2kt/y of which almost 70% was predicted to come from Balanac River, 23% from Bombongan River and the remaining 7% from the catchment downstream of these two river systems. Overall, the suspended sediment supply was predicted to decrease by 8% for every 20% of intercropped coconut plantations converted to forest. A greater reduction in the total suspended sediment yield discharging to the lake was predicted when reforestation was targeted at the steep slopes (<6.6°) than when reforestation was untargeted (20% reduction compared with approximately 7%). The major limitation with the application of the SedNet model for this work was the limited availability of input data such as more detailed land use classifications (i.e. differentiating plantation into various kinds of crops, as well as different types of grass covers) and information about gullies throughout the catchment. This improved data input would provide greater accuracy and confidence in the modelling. However, the model was very useful for giving a preliminary assessment of the sources of suspended sediment transported from the catchment, and for targeting reforestation for reducing suspended sediment accessions to the lake.