The South West Monsoon rains are once again ravaging the Philippines at this time of year. For the past several years, these monsoon rains have been accompanied by destructive winds and heavy rainfall resulting to extreme floods. These, on occasion, lead to the loss of lives and livelihoods, and the destruction of homes, properties and infrastructure especially in Metro Manila.
Despite the fact that the monsoons regularly and predictably occur at this time of year, it seems as though prevention and preparation efforts for these rains show no foresight—they remain stagnant and have shown little progress if at all. To raise awareness, we aim to illustrate why floods continue to surge in Metro Manila.
A way to explain flood occurrences is by understanding the surface water channels and outflow points that serve the area. This is where the importance of watersheds come into place.
A watershed is an area where streams and rainfall are drained into a common outlet. It may be pictured as having three divisions – collection, transport, and depositional zone. The process starts when rain waters are collected at the top of a hill, a mountain, or a ridge – which are then transported through the tributaries such as rivers – and deposited into the catchment areas or basins to join the larger water bodies (e.g. seas and oceans).
Forests of the watersheds play a significant role in the regulation of water flow. Leafy canopy intercepts rainfall; while forest floor absorbs precipitation slowing down surface runoffs that go into the catchment basins. Such ecosystems service highlights its importance in the event of heavy rainfall, storms, and typhoons. Likewise, when watersheds are degraded, they fail to provide these services resulting to increased rate and amounts of runoffs leading to hydrological disasters, including floods, affecting low-land areas and settlements.
Among the factors that cause watershed degradation are forest cover loss and land use change as may be evident in parts of the Marikina Watershed located in Rizal, and some areas surrounding its basin, the Laguna Lake. The latter serve as depositories of waters coming from the mountains of nearby provinces – Rizal, Laguna, and Cavite (see Figure 1). Implications of the mentioned issues contribute to the occurrence of flashfloods in Metro Manila, during typhoons and heavy rainfalls.