The effects of afforestation and deforestation on water yields

The exploitation of land and water resources to sustain an ever-increasing population inevitably involves the utilisation for both urban and agricultural development of rural areas and the natural landscape. This process can result in profound changes to the how regime of river basins that are so affected, the scope and magnitude of which have been investigated by means of experimental catchment studies. The extensive data base that has resulted from such activities has provided a basis for developing a series of generalised relationships which can be used by water resources planners to anticipate the changes in water yield that can result from alterations to the predominant vegetative cover of a catchment. The application of fuzzy linear regression analysis to data from 145 experiments has shown that, for a 10% reduction in cover, the yield from conifer-type forest increased by some 20-25 mm, whereas that for eucalyptus-type forest increased by only 6 mm. Both values were somewhat lower than those previously published, as was the 5 mm decrease in yield associated with a 10% afforestation of scrub. A 10% reduction in the cover of deciduous hardwood gave a 17-19 mm increase in yield, broadly in line with earlier estimates.