Intense rains and steep slopes make the Forest's soils highly erodible when vegetative cover is removed. The sandy diorite-derived soils lack cohesion, so they are particularly susceptible to erosion. Montane wetlands occur on slopes where soils are shallow and poorly drained. Rainfall is high and can be supplemented by cloud water interception. Evaporation is low, and water does not drain rapidly.
Water frequently stands on the surface, which is covered by a slimy algal growth in some places.
The extreme wetness of the soil gives rise to bog conditions, including growth of sphagnum moss, superficial plant roots, and accumulation of incompletely decomposed organic matter. Forest Rainfall and Stream Water Quality El Yunque is the wettest area in Puerto Rico, receiving an average of 120 inches of rainfall annually. Heavy rain may occur throughout the year, but on average May is the wettest month and March is the driest. Higher elevations have about 250 rainy days annually. About 1,700 showers occur annually producing about 0.