Forests are important regulators of ecosystems. They exert significant effects on the water budget and the hydrological cycle.
In areas of heavy rainfall, a large fraction of the rain is intercepted by the tree crowns and other forest plants. This water evaporates quickly and returns to the hydrological cycle. Some of the water reaching the forest floor penetrates into the soil through the litter and the loose soil surface, and there is little surface run-off. It is only after some period of time that the seeped water reaches the streams and rivers.
This time lag is an important device to regulate the water discharge into rivers, and in this way, flooding is prevented or minimized. On the other hand, in dry periods also the forest soil continues to supply water slowly to the streams and rivers.
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Destruction of forest changes the above situation immediately. The hydrological cycle is disrupted and the water level of the rivers cannot be properly regulated. This causes flooding. In dry periods, the rivers tend to dry up, affecting irrigation and power generation.
In deforested areas, erosion of soil occurs fairly briskly, especially on steep slopes. This removes the fertile top soil and also loads the rivers with much suspended matter. Deforestation thus greatly increases the quantity of detritus in many tropical rivers. This detritus causes premature filling up of water reservoirs, etc.
Large-scale forest destruction often produces grave climatic consequences, especially desertification and aridity. These result from reduction of evaporation as the tree canopies (cut and removed) no longer intercept rain water, and also because rapid run-off of precipitation occurs in the absence of the forest cover.