| Forests, Climate, and Hydrology: Regional Impacts (1988) |
|3. Effects of tropical forest on water yield|
Methods of detecting the effects of forests on water yield
In order to detect the effects of forest on streamflow characteristics three operations are usually necessary: calibration of the watershed against the control, treatment of the watershed, and analysis of the resulting data to detect any changes the treatment may have on the streamflow or yield. Much of our knowledge of forest watershed hydrology is derived from experiments.
Calibration is the determination of the normal (pre-treatment) relationship between a streamflow characteristic of interest and some other variable or variables. Several calibration methods have been used to characterize hydrological relationships. A limited number of studies have been conducted using only single watersheds. In this case a watershed is calibrated on climatic data. During the calibration period the flow characteristic under study is related to climatic variables. According to Golding (1980) this method is more informative than the paired (control) basin method because it relates streamflow to factors that influence it. It also costs less than instrumenting two basins and avoids the problem of searching for two or more similar basins. Clearly, no advantage can be taken of possible contrasts between different types of cover on otherwise similar watersheds, which may facilitate analysis of the results (Ward 971, 49).
There is also the paired watershed approach, in which two comparable watersheds are selected and the hydrological variables of interest measured and uncontrolled in fluences indexed during the calibration period, which is usually a minimum of five years. One of the watersheds is chosen as a control while the other is treated. The paired watershed approach is most common and was at first assumed to be the most suitable for solving the watershed problems investigated in South Africa (Wicht 1967). Some of the best known paired watershed experiments in humid tropical regions are those of East Africa, of which two are in Kenya and one in Tanzania. Many years may be necessary to attain the level of precision required, and the length of this calibration period is difficult to determine (Wilm 1949; Kovner and Evans 1954; Singh 1974) in spite of the several prescriptions made for its determination. The period should not only be long enough to yield sufficient values for computing the significant statistics, it should also be representative of the regional climate. To permit accurate isolation of treatment effects the climatic conditions during the treatment period should be similar to those prevailing during the calibration period. It has been found in some cases that such stability cannot be guaranteed. This has led to a search for rapid calibration methods. Bethlahmy (1963) advocates the calibration of watersheds by comparing their responses to individual storm events. It is suggested that in humid regions the annual number of adequate storms will exceed 100, so that a maximum of three years will be required for calibration.
The use of multiple watershed experiments partially eliminates the deficiencies of the paired watershed approach. In this approach streamflow, for instance, from a number of treated watersheds may be compared to that from a number of control watersheds. This may eliminate the need for a pre-treatment calibration period. Here again there is need to determine the number of watersheds required to detect a specified increase in water yield at a given probability level (Swanson and Hillman 1977). There are examples of this approach in the humid tropics; some of the earlier ones include those reported by Rodier (1959) and Wicht (1961, 1967) for the tropical and subtropical regions in Africa. The multiple watershed approach requires a greater input of financial and human resources over the short period, but the problem of unstable control treatment due to natural or man-made changes in the physical environment is much reduced.
Types of Treatment
Several types of treatment have been, and are being, applied to experimental watersheds. Clear-cutting of a large part, or even the entire experimental watershed, is one of the common treatment methods. There are also cases of strip cutting in which clear-cut strips alternate with uncut strips of similar dimension (Ward 1971). Examples of clearcutting experiments in tropical rain forest include Lien-Hua-Chi Watershed in Central Taiwan; Mbeya Watershed in Tanzania; IITA experiments in Ibadan, Nigeria; the Doon Valley experiments in India; and the multiple watershed experiments in French Guiana.
Selective logging or thinning, based on either age, species, or quality, and sometimes selective patch or block cutting are also adopted as the method of treatment of an experimental watershed. The Man and the Biosphere experiment in East Kalimantan
Province, Indonesia, is a good example of treatment by selective logging. In some more recent experiments the role of chemical treatments and burning in forest clearance has also been investigated.
Substitution or changing forest cover is a rather common type of treatment in humid tropical regions, where the choice is often between tea, pine, and bamboo species. Experimental watersheds in which the natural forest cover is changed to tea or pines are found in Kericho and Kimakia, both in Kenya. There is also the Dehra Dun experiment in the Doon Valley, India, where the forest was clear-cut and reforested with Eucalyptus grandis and E. camaldulensis (Mathur et al. 1976; Bahadur, Chadra, and Gupta 1980).
The removal of forest litter is another method of treatment that may help to separate the effects of tree canopy from those of the forest floor, particularly pertinent to tropical forests. Unfortunately no example of this in the humid tropics is known to the writer. However, Tsukamoto (1975) has demonstrated the effect of forest litter in the 1.42 ha North Watershed in Aichi Forest of the University of Tokyo.
Finally, afforestation has been applied as the main treatment in a number of experimental watersheds in the tropical regions. Effects of afforestation on watersheds require longer periods of observation since tree growth is relatively slow. In South Africa watersheds under sclerophyllous scrub were afforested with pines. The treatment is similar to that of substituting one forest cover for another in the East African watersheds, except that forest development may be faster in the latter.
Examples of properly designed watershed experiments, which have as their main objective the evaluation of the effect of forests on streamflow characteristics, are few in the humid tropical regions, even though they abound in the temperate zones. There are, nevertheless, a number of watershed investigations that use forested watersheds that are being progressively deforested to examine the effects of such forest removal. These studies include those by Raghunath, Das, and Thomas (1970), who analysed data from 17 watersheds whose forest cover ranged from 2.47% to 69.33% in the Nilgiris, India. Others include the Kali Mondo Basin, Java, in Indonesia (Bruijnzeel 1983), 10 watershed investigations in French Guiana of the Amazon region (Roche 1981), and the montane cloud forests area of Costa Rica (Zadroga 1981).
According to Reinhart (1967) an ideal treatment would take full effect immediately upon application and remain in effect, at the same level, indefinitely. This situation is approximated by a clear-cutting of forests followed by frequent re-cuts. If the treatment departs from this ideal situation, analysis becomes more difficult and failure of the experiment is possible. Afforestation and reforestation treatments are exceptions to Reinhart's maxim, but time-trend analyses can be used to evaluate the changes, if any, in the streamflow characteristics.