GATE Study Material Deforestation (Biotechnology Engineering)

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the content of water in the soil and groundwater as well as atmospheric moisture. 42 Deforestation reduces soil cohesion, so that erosion, flooding and landslides ensue. 43 44 Forests enhance the recharge of aquifers in some locales, however, forests are a major source of aquifer depletion on most locales. 45 Shrinking forest cover lessens the landscape's capacity to intercept, retain and transpire precipitation. Instead of trapping precipitation, which then percolates to groundwater systems, deforested areas become sources of surface water runoff, which moves much faster than subsurface flows. That quicker transport of surface water can translate into flash flooding and more localized floods than would occur with the forest cover. Deforestation also contributes to decreased evapotranspiration, which lessens atmospheric moisture which in some cases affects precipitation levels downwind from the deforested area, as water is not recycled to downwind forests, but is lost in runoff and returns directly to the oceans. According to one study, in deforested north and northwest China, the average annual precipitation decreased by one third between the 1950s and the 1980s. citation needed Trees, and plants in general, affect the water cycle significantly
their canopies intercept a proportion of precipitation, which is then evaporated back to the atmosphere canopy interception their litter, stems and trunks slow down surface runoff their roots create macropores - large conduits - in the soil that increase infiltration of water they contribute to terrestrial evaporation and reduce soil moisture via transpiration their litter and other organic residue change soil properties that affect the capacity of soil to store water. their leaves control the humidity of the atmosphere by transpiring. 99 of the water absorbed by the roots moves up to the leaves and is transpired. 46
As a result, the presence or absence of trees can change the quantity of water on the surface, in the soil or groundwater, or in the atmosphere. This in turn changes erosion rates and the availability of water for either ecosystem functions or human services. The forest may have little impact on flooding in the case of large rainfall events, which overwhelm the storage capacity of forest soil if the soils are at or close to saturation. Tropical rainforests produce about 30 of our planet's fresh water. 38
Soil Undisturbed forests have a very low rate of soil loss, approximately 2 metric tons per square kilometer 6 short tons per square mile . citation needed Deforestation generally increases rates of soil erosion, by increasing the amount of runoff and reducing the protection of the soil from tree litter. This can be an advantage in excessively leached tropical rain forest soils. Forestry operations themselves also increase erosion through the development of roads and the use of mechanized equipment. China's Loess Plateau was cleared of forest millennia ago. Since then it has been eroding, creating dramatic incised valleys, and providing the sediment that gives the Yellow River