River systems have been identified as major pathways and transporters of wastes, including plastics, that ultimately end up in the oceans. The Imus River Watershed (IRW) is located in the Philippine Province of Cavite, one of the provinces in the CALABARZON Region of southern Luzon. This study delineated and mapped the physical boundaries of the Imus River watershed and determined the topographic features, stream characteristics, geomorphology, political subdivisions, barangay communities, population distribution, land use and land cover, and hydro-climatic characteristics of the watershed. Both primary and secondary data sources were used in making comprehensive land use maps, population maps, and hydroclimatic data analyses.
The boundary of the Imus River Watershed was initially established through an unsupervised delineation process using a digital elevation model of Cavite with a 5-meter resolution in ArcGIS. Sangley Point Synoptic Station in Cavite City and the CvSU-PAGASA Agrometeorological Station in Indang were used to define the general hydroclimatic condition of IRW due to their close proximity to the watershed. The total drainage area of IRW is 11,259.80 hectares, covering portions of Tagaytay City, Amadeo, Silang, Dasmariñas, Imus City, Bacoor City and Kawit. Elevation within the watershed ranges from 0 to 655 meters above sea level. The lowland area covers parts of Kawit, Imus City, and Bacoor City; a central hilly area covers parts of Imus City, Bacoor City, and the majority of communities in Dasmariñas and Silang. The upland area covers parts of Silang, Amadeo, and Tagaytay City. There were 56 perennial streams identified with a total length of 186.15 km and 36 river segments. The Imus river system is a combination of headwaters and medium-sized streams. The sub-watersheds, labeled A, B, and C, have drainage densities of 1.15 km/km2, 1.95 km/km2, and 1.41 km/km2, respectively. The sub-watersheds A and C have stream frequencies of 0.20/km2 and 0.25/km2 while sub-watershed B has a stream frequency of 0.39/km2. In alphabetical order, these sub-watersheds have bifurcation ratios of 5, 3.31, and 2.5, elongation ratios of 0.33, 0.26, and 0.43, and circulatory ratios of 0.18, 0.11, and 0.26.
A total of 222 barangay communities are located within the boundaries of the watershed with a total population of 1,351,057 in 2015. 90.67% of the province is classified as alienable and disposable land, while the remaining forest land represents only 9.33%. Alienable and disposable lands are further classified as production land (55.24%) and built-up areas (44.76%). The Sangley Point Synoptic Station has a normal mean temperature of 28.53°C while the CvSU-PAGASA Agromet Station has a normal mean temperature of 26.20°C. The average total annual rainfall recorded at Sangley Point Synoptic station and CvSU Agromet Station were 2,265.69 mm and 2,483.05 mm, respectively. The average flow during wet season was 1,601.84 liters per second, while the average flow during dry season was 1,337.42 liters per second.