MANILA BAY WATERSHED SCORECARD: A GIS-BASED QUANTITATIVE WATERSHED HEALTH ASSESSMENT

Abstract. Watershed health refers to the maintenance of the normal status of a watershed. With the use of Geographic Information Systems (GIS), a watershed health assessment framework was developed and applied to the Manila Bay Watershed (MBW) based on the sub-indices used by the US EPA and the Minnesota DNR. These three sub-indices were used and modified: geomorphology, connectivity, and hydrology. Geomorphology sub-index accounted for the effects of soil erosion using the Unit Stream Power-based Erosion/Deposition model (USPED). Connectivity sub-index considered the connection between habitats within three environments: terrestrial, aquatic, and riparian zone. Hydrology sub-index accounted for the effects of impervious cover and urbanization on the movement of water using three factors, namely, natural cover, tree cover, and loss of hydrologic storage. Land cover map was the most used dataset in this scorecard, where the maintained natural land covers generally received high scores and built-up areas received the lowest. The overall watershed health score of MBW is 75.713 from the mean of the three sub-indices. Pampanga River Basin, which is the largest river basin within the MBW, got the highest score of 79.462 since it consists of huge portions of maintained natural land cover. Manila River Basin, known to have dense built-up areas, got the lowest average of 60.773. On the provincial level, the province of Nueva Ecija got the highest score, and the National Capital Region (NCR) got the lowest. The developed framework successfully quantified a relative health score which can be used to rank and prioritize subwatersheds, and to measure in totality the improvement or degradation of subwatershed/s over time.