| Forests, Climate, and Hydrology: Regional Impacts (1988) |
|3. Effects of tropical forest on water yield|
Wide variations in experimental methods have often made pooling of results difficult.
In the present review of tropical forest experiments the methods adopted by investigators include the control watershed approach, in which the yield (RA) of the treated watershed is predicted by the yield (RC) of the control watershed by the relation
where a and b are intercept and slope constants respectively.
In the East African experiments with the paired catchments under the same land use, no initial calibration period was possible. But through the use of better methods of estimating evaporation and improved methods of measuring the major components of the hydrological cycle, it was possible to produce comparative estimates of the water use of different types of vegetation. This, however, means that any differences in seasonal streamflow distribution between the paired catchments could not be attributed, by pre- and post-treatment comparison, to a particular cause, such as land use change. Intensive and long-term methods (with calibration and treatment) should be regarded as complementary rather than as alternative methods in the tropical forest environments.
One of the major weaknesses of many traditional studies that should be avoided is the measurement of only one or two of the components of the water balance. All the components of the hydrological cycle ought to be measured or accurately estimated, if necessary, by employing watershed model techniques. Only as empirical coefficients are made more deterministic and bulked processes of the hydrological cycle further separated can hydrological models be improved. This is perhaps why Pereira (1973) announced "the era of calibrate, cut and publish, is over and a far more detailed study of the components of the hydrological cycle is necessary."
In evaluating the effects of land use on water yield, the use of the "best" hydrological year is desirable. The best hydrological year is associated with the highest coefficient of determination when runoff is related to rainfall by a simple correlation model (Sharp et al. 1960). They also found that the variable hydrological year did not prove to be as good as the "best" hydrological year of uniform length and dates.
There is certainly an urgent need to undertake properly designed catchment experiments in tropical forests in order to understand and control the impact of forest harvesting on water quantity in these active hydrological environments. The development of catchment studies should enable the developing nations that inhabit most of the region to apply the knowledge gained to land management, particularly in water conservation and erosion control. It is encouraging that a number of such studies recently established in parts of Brazil, Guyana, Argentina, India, and Malaysia are expected to produce results that will considerably increase our present knowledge of the hydrology of the enigmatic tropical forest.