The sponge effect

The theory that forests act as a sponge and absorb all floodwaters is scientifically not viable. It is already relegated to the heap of tried and tested theories that are now debunked. However, the "sponge phenomena" that is better referred to as the "infiltration of water into the soil and on to the water table" does work for small subcatchments with forest. During the rainy season, such infiltration reduces water flooding over the surface (surface flow) and slows down the water as it gravitates through the soil to the river from all sides of a valley. Because the water moves slower, the water reaches the river in a more staggered manner and so the river storm flow is not as high as when the water flows directly over the surface of deforested land in a more concentrated form with eroded soil to reach the river.

Furthermore, a percentage of the rain that is stored in the subsurface and deeper soils finds its way down the slope during the drier months feeding the water table and keeping the river flow higher than in those rivers fed by deforested hills. This is one of the major ecological services that forests provide and its value should never be underestimated. However, such a volume of water is in no way comparable to the total volume of water in a major area storm event that cannot be contained by subsurface storage and delayed flows in the total watershed.

Localized sub-catchment events were assessed for decades as to the impact of logging on storm flows. However, simple extrapolation of small area processes - with relatively delayed delivery of water over a shorter time in forested areas - to large areas is to miss the importance of differences of scale, with respect to both land area and storm size. The differences in land area make irrelevant the relative delay in subsurface water delivery. The differences in storm size and duration necessary to affect an entire major watershed are of magnitudes greater than a small hillside valley of which the land area, even when forested, could not "detain" such a volume of water. The total area rainfall that enters a major river system and the reduction in slope of that river on entering the lowlands allows the water cut through its meandering seasonal path and spread out on the flood plains. Such an event as that which occurred in Quezon Province is one of a very different frequency and composition to that of sub-catchment infiltration during seasonal events.