| Forests, Climate, and Hydrology: Regional Impacts (1988) |
|3. Effects of tropical forest on water yield|
Effects of tropical forests on water yield
Hibbert (1967) summarized 39 studies of clearing forest (all the results available by that time). He shows that the upper limit of yield increase is 4.5 mm y-1 for each percentage of reduction in forest cover. As may be expected most treatments produced less than 2.5 mm increase per year. First-year responses to complete deforestation varied from 34 mm to 457 mm of increased runoff and most produced less than 300 mm. The plot of the first-year increases for thirty treatments does not, according to Hibbert (1967), reveal any strong relation between increased water yield and percentage of reduction of forest. It is, however, interesting that the highest first-year increase in water yield reported was for a 100% clear-cutting of a watershed at Kimakia (Kenya) under tropical forest cover. Hibbert drew three generalizations:
A total of 94 forest clearing or planting experiments, including those already reviewed by Hibbert (1967), were studied by Bosch and Hewlett (1982). They also made generalizations and criticisms that may lead to more careful design of catchment experiments. Incidentally, two treatments reported for Maimai, New Zealand, produced the highest first-year yield increases, of 650 and 540 mm. Bosch and Hewlett (1982) dispute Hibbert's conclusion that water yield responses to afforestation and deforestation is unpredictable and conclude that coniferous forest, deciduous hardwood, brush and grass cover have, in that order, a decreasing influence on water yield of the source areas compared to bare ground.
The following specific inferences are also made, though the authors cannot set error limits on the coefficients whose sign and magnitude they consider to be correct.
Results from Forested Tropical Catchment Experiments
Table 5 presents brief descriptions of a number of paired catchment experiments under tropical and subtropical forest conditions. The table gives details of treatments and effects on water yield during the years following treatment. Experiments 21 and 22 are not strictly controlled, while number 12 is a promising multiple catchment experiment yet to yield results. The following brief description of the selected experiments provides additional information on the treatment effects.
East Africa. Two of the experiments are in Kenya. One deals with the replacement of indigenous rain forest by plantation tea at Kericho and the other by exotic conifers, patula pine, at Kimakia. A third experiment is at Mbeya in southern Tanzania, where a pair of otherwise similar catchments, one under indigenous forest cover and the other cultivated without soil conservation, are being compared for streamflow and sediment yield. In all three experiments the water balance equation was used to evaluate, over a series of water years, the unknown components, particularly the actual water use of the paired catchments. Other conceptual models were then used to simulate streamflow responses.
TABLE 5a. Tropical and subtropical watershed experiments: Basin characteristics
|Location name & no.||Latitude||Elevation (m)||Aspect||Slope (%)||Area (ha)||Climatic classa||Vegetation and soil||References|
|(1)Kericho, Sambret||0°21'S||2,200||NW||4.5||702||Awb||Montane forest with bamboo. Phonolitic lava; deep friable clay||Pereira 1962, 1964, 1973; Blackie 1972; Edwards & Blackie 1981|
|(2)Kimakia||0°48'S||2,438||S||-||35.2||Awb||Bamboo forest (evergreen forest islands). Miocene basalts; deep porous soils||Pereira 1962, 1964, 1973; Blackie 1972 Edwards & Blackie 1981|
|(3) Mbeya||8°50'S||2,428||-||58||20.2||Awb||Volcanic ash and gneiss; soils very porous, stable structure||Pereira 1962, 1964, 1973; Blackie 1972; Edwards & Blackie 1981|
|(4) Lien-Hua-Chi 4||24°N||-||SE||40||5.86||Caw||Mixed evergreen hardwoods. Shaley; yellow fine silt loam||Hsia & Koh 1983.|
|(5) IITA||7°N||-||-||-||44||Am||Secondary rain forest Altisol||Lawson et al. 1981; Lal 1983|
|(6) II IA 2-1||7°N||-||-||-2.6||Am Alfisol||Secondary rain forest Lal 1983||Lawson et al. 1981;|
|(7) IITA 2-2||7°N||-||-||-||3.1||Am Alfisol||Secondary rain forest, Lal 1983||Lawson et al. 1981;|
|(8) IITA 2-3||7°N||-||-||-||3.2||Am Alfisol||Secondary rain forest Lal 1983||Lawson et al. 1981;|
|(9) IITA 2-4||7°N||-||-||-||2.7||Am Alfisol||Secondary rain forest Lal 1983||Lawson et al. 1981;|
|(10) IITA 2-5||7°N||-||-||-||3.2||Am Alfisol||Secondary rain forest. Lal 1983||Lawson et al. 1981;|
|(11) IITA 2-6||7°N||-||-||-||4.0||Am Alfisol||Secondary rain forest. Lal 1983||Lawson et al. 1981;|
|(12) Amazon||3°-5°N||-||-||15-50||1-1.5||Am||Crystalline rocks; weathered and moderately permeable||Roche 1981|
|(13) N. Creek, Babinda, Queensland||17°21'S||-||W||34||18.3||Am||Mesophyll vine forest. Deep silty clay loam to clay soils||Gilmour et al. 1982|
|(14) Doon Valley, Dehra Dun, W1F||30°21'N||-||SE||5.1||1.45||Cawb||Derived scrub with sal seedlings. Very deep, mod. permeable silt loam (silty clay loam below)||Mathur et al. 1976; Bahadur et al. 1980|
|(15) Natal, Cathedral Peak II||25°-30°S||2,073||NE||-||190||Cbw||Themeda grassland. Basalt; deeply weathered saprolite||Nanni 1970; Bosch 1979, 1982|
|(16) Biesievlei, Jonkershoek||25°-30°S||398||SW||-||27||Cbw||Sclerophyllous scrub. Granitic soils||Wicht 1967; Bosch 1982c|
|(17) Bosbouloof, Jonkershoek||25°-30°S||532||SW||-||200||Cbw||Sclerophyllous scrub. Granitic soils (sandstone upslope)||Bosch 1982c|
|(18) Tierkloof, Jonkershoek||25°-30°S||682||SW||-||157||Cbw||Sclerophyllous scrub. Granitic soils (sandstone upslope)||Bosch 1982c|
|(19) Transvaal, Mokobulaan A||25°-30°S||141||E||-||26||Cbw||Grassland. Shale; deeply weathered, shallow clayey sand||Van Lill et al. 1980; Bosch 1982c|
|(20) Lambre- chtbos B. Jonkershoek MADAGASCAR||25°-30°S||600||SW||-||65||Cbw||Sclerophyllous scrub. Granitic soils||Bosch 1982d|
|(21) D||15° 21°S||-||-||-||7||Cbw||Natural forest||Bosch & Hewlett 1982d|
|(22) D5||15°-21°S||-||-||-||13||Cbw||Eucalyptus robusta||Bosch & Hewlett 1982d|
|(23) Rio Grande do Sul|
|A1||28°S||100-600||-||40||92.2||Cb||Maize and soybean. Basattic soils||Bordas et al.1980|
|B1||28°S||100 600||-||20||53.2||Cb||Maize and soybean. Basaltic soils|
aSee Appendix: bhighland;cciting Van Wyk 1977;d citing Bailly et al. 1974
TABLE 5b. Tropical and subtropical watershed experiments: Treatment and water balance
|Expt. no. (table 5a)||Treatment||Method||Mean annual rainfall (mm)||Mean annual streamflow (mm)||
Post-treatment change in water yield (mm)
|Year 1||Year 2||Year 3||Mean (years)|
|(1)||1959 1960 34% area clear-cut for clean-weeded tea garden; 1959- 1963 53% area clear-cut for tea garden||paired||2,236 (1959-1973)||789 (1958-1964)||+ 103||-||-||-|
|(2)||1956 100% cleared, pine planted; inter-cropped vegetables 3 years until canopy closed||paired||2,198 (1958-1973)||1,104 (1958-1973)||+457||+229||+ 178||-|
|(3)||1958 50% cultivated in any given season forested 16.3 ha control||paired||1,658 (1958-1968)||667 (1958-1968)||-||-||-||+ 220 (14 yr)|
|(4)||1978-1979 100% clear-cut (skyline logging) after 7 yr calibration||paired||-||1,100||+448||+204||-||-|
|(5)||1978 100% clear-cut after 4 yr calibration||forest control||1,450||35||+ 340||-||-||-|
|(6)||Traditional farming control||15 ha forest||1,450||35||+ 3||-||-||+ 2.1 (3 yr)|
|(7)||Manual clearing; no tillage control||15 ha forest||1,450||35||+ 16||-||-||+ 5.2 (3 yr)|
|(8)||Manual clearing; conventional tillage||15 ha forest control||1,450||35||+ 54||-||-||+ 27 (3 yr)|
|(9)||Shear blade clearing; no tillage||15 ha forest control||1,450||35||+ 86||-||-||+ 35 (3 yr)|
|(10)||Tree pusher + root rake: no tillage||15 ha forest control||1,450||35||+ 153||-||-||+ 57 (3 yr)|
|(11)||Tree pusher + root rake; no conventional tillage||15 ha forest control||1,450||35||+250||-||-||+ 110 (3 yr)|
|(12)||1979-1980, clearing replacing with pine, orchard, etc. after 3 yr calibration||multi-basin; 2 forest controls||3,500||500-925||-||-||-||-|
|(13)||1971 1973 67% area logged, cleared, raked, ploughed; bare 2 yrs; prior 3 yr calibration||paired (S. Creek forest control 25.7 ha)||4,239 (1970-1977)||2,873||+ 393||-||-||+ 293 (2 yr)|
|(14)||1969 100% clear-cut; E. grandis+ E. camaldulensis planted,, after 8 yr calibration||monthly data (annual inadequate)||1,677||-||-||-||-||28 % (1969-1974) monthly yield|
|(15)||1951 74% afforestation with Pinus patula||Cathedral Peak III control; no significant change in quick- flow volume||1,400||650||-||-||-||-257(>20 yr) Max -440, 1973|
|(16)||1940 98% afforestation with Pinus radiata||Lambrechtbos A as control||1,400||660||-||-||-||-323 (>20 yr) Max -400, 1955|
|(17)||1940 57% afforestation with Pinus radiata||Tierkloof as control||1,390||590||-||-||-||-270 (>20 yr) Max 325, 1963|
|(18)||1956 36% afforestation with Pinus radiata||Lambrechtbos A as control||1,809||1,100||-||-||-||- 130 (>20 yr) Max 170, 1964|
|(19)||1969 100% afforestation with Eucalyptus grandis||Mokubalaan B as control||1,150||173||-||-||-||-340 (10 yr) Max - 403, 1974|
|(20)||1961 84% afforestation with Pinus radiata||Lambrechtbos A as control||1,451||460||
No significant effect
|(21)||Natural forest||Brush control||2,100||730||-||-||-||+250|
|(22)||Eucalyptus robusta plantation||Natural forest control||1,600||700||-||-||-||-400|
|(23)||1979 cleared for agriculture||Forest control (67.8 ha)||-||-||-||-||-||-|
At Kericho (Sambret Watershed) there was overall reduction in water use (about 11%) and some increases in water yield during the initial clearing and clean-weeding. The replacement of tropical forest by tea estates has, however, not resulted in long-term reduction in water yield. Differences revealed in interception (lower from tea) and transpiration (higher from tea) might alter the seasonal distribution of streamflow, which for some water uses may be as important as changing the annual yield.
At Kimakia replacement of bamboo forest by patula pine and vegetables initially decreased the water use by 19% and resulted in a large increase in streamflow. Once the pine canopy closed (1967-1973) no significant difference in water yield could be detected as water use by both bamboo and pine was 76% of the Penman evaporation from water surface (E`,).
At Mbeya, when evergreen forest gave way to shamba or small holder cultivation, a large increase in water yield resulted. Very little of this increase resulted from overland flow because of the remarkably high infiltration capacity of the very stable and porous ash-derived soils. On the other hand, the dry-season baseflow was doubled. The difference in dry-season transpiration accounted for much of the change in water yield, for water use by plants decreased from 0.92 E0, under forest to 0.65 E0 for cultivated land. Also, under forest only 28% of annual precipitation resulted in streamflow, while the percentage was 40% for the cultivated watershed during 1958-1968.
Taiwan. A small low-altitude watershed, Lien-Hua-Chi, near Sun Moon Lake in central Taiwan, originally under subtropical montane hardwood forest, was clear-cut during 1978-1979. The surface disturbance was kept to a minimum as skyline logging was used; roads were constructed around the basin periphery, away from streams, and all yarding was uphill. The yield increases for the first and second years after clear-cutting were 58% and 51% respectively of the annual flow. The effect was greater on the low flows, which increased by 108% and 293% respectively during those years, while the wet-season flow increased by 55% and 47% respectively.
Ibadan, Nigeria. The effect of clearing a 44 ha watershed at Ibadan, western Nigeria, in early 1979 was a significant increase in total water yield. Prior to the clearing streamflow was negligible. Following clearing runoff increased by some 340 mm, or 23% of the annual rainfall. Both direct runoff and baseflow components increased after deforestation (fig. 2). The baseflow increased steadily from "an unmeasurable trace" in the forested catchment in 1978 to about 0.1, 0.12, and 3.22 mm per month in 1979, 1980, and 1981 respectively. Gradual and persistent deterioration of the surface soil structure is said to account for the increase in overland flow. The reduced water use was an important factor of the increase in total water yield.
Experiments 6 to 11 compare methods of land clearing and tillage systems. These six experiments are calibrated against a 15 ha forested catchment that experienced only an insignificant amount of runoff during 1979-1981. Traditional farming is based on incomplete clearing. Among basin treatments involving complete clearing (i.e. 2-2 to 2-6), manually cleared plots are associated with an average of 16 mm per year increase of yield in 1979-1981. The corresponding figure for mechanically cleared plots is 67 mm per year.
The results of these experiments indicate the need to investigate further the effect of clearing and tillage methods, as much of the differences in experimental results might be explained by the techniques of deforestation employed.
Dehra Dun, India. A pair of small forested (brush) watersheds were calibrated for 8 years (1961-1968). W1F was then clear-cut and re-afforested with Eucalyptus species. The post-calibration relationship indicates that reforestation resulted in 28% reduction in streamflow. The resulting plantation was fully stocked, with dense undergrowth. Information was not available on the effect of clearing on water yield during the first year after clearing (1969) due to failure of the flow-recording gauge.
North Creek, Babinda, Queensland, Australia. Two catchments were selected in the tropical rain forest in Babinda, North Queensland, in 1969 to study the effects of logging and land clearing. The experimental catchment, North Creek, was calibrated against the South Creek control watershed from 1969 to 1971. It was then logged in June 1971, and in 1973 67% of the experimental catchment was completely cleared, stick-raked, and partially ploughed. The treated areas of the catchment remained completely bare for two years.
Logging produced little detectable hydrological change, but the two years following clearing produced an average increase in annual water yield of 10.2%, or 293 mm. Furthermore, weekly minimum discharge in the treated catchment increased by 40 to 60% for flows less than 5 litres s-1 after clearing. Soil moisture levels in the experimental basin remained higher because of reduced transpirational demand, and soil moisture deficits were drastically reduced. For instance, at the end of the first dry season after clearing in 1973, the treated North Creek catchment required only 94 mm of rain to bring the soil moisture in the surface 3 m to field capacity, whereas the control catchment required 291 mm. Equations relating soil moisture in the two catchments before and after clearing indicate that the moisture level rose by 33% in North Creek after clearing.
South Africa. The replication in time over a set of six watersheds shown in table 5 for South Africa has brought the streamflow comparison method to its highest degree of development (Pereira 1973). The experiments were begun at Jonkershoek Research Station in 1940 and at the Cathedral Peak Research Station in 1951 to study management policies for the steep grass or scrub covered watersheds. Plantings were done at 8-year intervals on a 40-year forest rotation cycle. This made possible comparisons of the effects of pine and Eucalyptus at different stages of tree growth.
A simple model derived from the Cathedral Peak results by Nänni (1970) is used to predict the effect of afforestation on average annual water yield. The model consists of a set of curves relating mean annual streamflow reductions, due to afforestation, to mean annual runoff before afforestation (fig. 3). The model has limited use as it neither provides information on seasonal patterns of water yield nor on changes in the components of streamflow. Maximum flow reductions in the South African experiments following afforestation conform to expected first-year increases after clear felling (table 5).
First-Year or Maximum Yield Changes
Hibbert (1967) appeared narrowly to resist the temptation to pronounce 450 mm as the upper limit of the first-year increase following clear-cutting. He noted, however, that "exceptional climatic conditions must prevail if larger increases are to be obtained." The maximum value of 457 mm was produced in one of the two tropical forest experiments included in his review. However, the results in table 5 and the plot of 12 of the results (fig. 4) indicate that the first-year maximum yield changes following clearcutting or afforestation may be up to 6 mm for each percentage of change in forest cover. There is obviously some relationship between streamflow response to afforestation or deforestation and the regional mean annual rainfall in tropical forest environments. Unfortunately, the availability of results presently for the region is inadequate to depict any such relationship. Nevertheless, if any region is to provide maximal streamflow responses, that region appears to be the forested areas of the humid tropics, where high rainfalls coupled with large radiation energy surpluses react virtually continuously throughout the year with deep-rooted evergreen trees on deeply weathered soils.
It would appear, however, that the effect of clear-cutting in high rainfall areas is shorter lived than in low rainfall areas due to rapid regrowth of vegetation in the former. The maximum yield reductions following afforestation are comparable to the first-year increases after clear-cutting. There is no evidence that such reductions are lower; they may indeed be higher than the observed increases following clear-cutting in the tropical forest (fig. 4).
Average Annual Changes in Total Yield
Figure 5 shows the plot of average annual changes in yield against the change in forest cover. A "best-fit" line (not shown) to the plot indicates that there is about 5 mm change in annual water yield for each percentage of change in forest cover. Average values obtained for afforestation over longer periods are more reliable in this respect, provided the effect of climatic trends has been removed. The plot also suggests that any change in forest cover of less than 15% may result in no change in streamflow.
Seasonal Distribution of Yield Changes
It is important to consider the behaviour of the stream during different seasons of the year. It is possible for the annual total yield to remain unchanged while the stream regime itself is so altered that the use of the streamflow is greatly impaired or enhanced. The role of forests in this case is similar to that of engineering structures on streamflow.
At Mbeya, East Africa, and North Creek (Babinda), Australia, there was no detectible change in overland flow, whereas the dry-season baseflow increased significantly. Where there is evidence of change in overland flow, as in Ibadan (Nigeria) and Taiwan, the increase in baseflow is much higher, even though the actual volume flow is relatively small.
The drastically reduced transpirational demand, especially during the dry season, from clear-felled areas is primarily responsible for the large increase in low flow of streams. Maintenance of a good soil structure and high infiltration after treatment further enhances the contribution of baseflow to the total yield.