 | Sustaining the Future. Economic, Social, and Environmental Change in Sub-Saharan Africa (UNU, 1996, 365 p.) |
 |  | Part 2: Environmental issues and futures |
| | Tropical deforestation and its impact on soil, environment, and agricultural productivity |
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Deforestation and hydrological balance
Deforestation of TRF can drastically alter the components of the hydrological cycle:
P = I + R + DS + D + > Edt,
where P is precipitation, I is infiltration, R is surface runoff, DS is soilwater storage, D is deep drainage, E is evapotranspiration, and t is time. Deforestation decreases I and DS and increases R and D components. In general, deforestation may also increase E. The change in E, however, may also depend on the land use.
Several experiments have demonstrated the effects of clear-cutting on the increase in total water yield. The impact of deforestation on the hydrological balance of a 44 ha watershed was studied at the International Institute of Tropical Agriculture (IITA), Ibadan, Nigeria. Prior to partial deforestation in 1978 and complete defor estation in 1979, measurements of surface and subsurface flow were made under the forest cover from 1974 to 1977. Under the forest cover, the interflow was 0.4 per cent to 1.4 per cent and total flow 0.8 per cent to 2.7 per cent of the total rainfall. Partial clearing in 1978 increased interflow to 1.2 per cent and increased total flow to 6.6 per cent of the total rainfall (table 9.13).
Table 9.13 Effects of partial clearing in 1978 on total water discharge from Watershed 1
| Parameters | |
| Rainfall (mm) | 785.8 |
| Surface flow (mm) | 42.7 |
| Surface flow (% of rain) | 5.4 |
| Subsurface flow (mm) | 9.4 |
| Subsurface flow (% of rain) | 1.2 |
| Total yield (mm) | 52.1 |
| Total yield (% of rain) | 6.6 |
Note: The partial clearing was of 3.1 ha out of 44.3 ha.
Table 9.14 Hydrological components on an annual basis for Watershed 1, 1979-1986
| Year | Annual rainfall (mm) | Subsurface flowb (mm) | Surface flow (mm) |
Total water yield |
Apparent evapo transpirationa |
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| mm | % of rainfall | mm | % of rainfall | ||||
| 1979 | 1,435.5 | 28.0 | 73.4 | 101.4 | 7.1 | 1,334.1 | 92.9 |
| 1980 | 1,449.7 | 73.1 | 90.0 | 163.1 | 11.3 | 1,286.6 | 88.8 |
| 1981 | 1,074.5 | 58.9 | 28.9 | 87.8 | 8.2 | 986.7 | 91.8 |
| 1982 | 851.5 | 50.9 | 25.9 | 76.8 | 9.0 | 774.7 | 91.0 |
| 1983 | 897.6 | 45.8 | 21.3 | 67.1 | 7.5 | 830.5 | 92.5 |
| 1984 | 1,162.2 | 58.9 | 27.1 | 86.0 | 7.4 | 1,076.2 | 92.6 |
| 1985 | 1,675.7 | 18.5 | 93.2 | 111.7 | 6.7 | 1,563.9 | 93.3 |
| 1986 | 1,164.1 | 1.9 | 51.7 | 53.8 | 4.6 | 1,110.3 | 95.3 |
a. Evapotranspiration includes soil water storage and groundwater
recharge.
b. Subsurface flow is underestimated during wet years because it
is computed as a part of surface flow during the storm runoff.
The entire watershed was cleared in 1979 and cultivated to food crops. The data in table 9.14 show that the total water yield ranged from 4.6% to 11.3% of the rainfall received. Because of the bimodal distribution of the rainfall, the hydrologic balance was computed separately for each growing season. The hydrologic balance showed that total water yield ranged from 1.4% to 11.8% for the first season (table 9.15) and from 0.8% to 18.1% for the second season (table 9.16). The intermittent stream, with a trace of flow after heavy rain and no flow during the dry season, became a perennial stream that recorded a measurable flow throughout the dry season (table 9.17).
Table 9.15 Hydrological components for the first growing season (March-July), 1979-1987
| Year | Annual rainfall (mm) | Subsurface flowb (mm) | Surface flow (mm) |
Total water yield |
Apparent evapo transpirationa |
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| mm | % of rainfall | mm | % of rainfall | ||||
| 1979 | 846.1 | 7.0 | 89.8 | 96.8 | 11.4 | 749.3 | 88.6 |
| 1980 | 604.3 | 1.2 | 7.0 | 8.2 | 1.4 | 596.1 | 98.6 |
| 1981 | 636.8 | 20.2 | 17.3 | 37.5 | 5.9 | 599.3 | 94.1 |
| 1982 | 615.2 | 28.4 | 17.6 | 46.0 | 7.5 | 569.2 | 92.5 |
| 1983 | 580.9 | 22.3 | 15.2 | 37.5 | 6.5 | 543.4 | 93.5 |
| 1984 | 681.6 | 23.6 | 13.9 | 37.5 | 5.5 | 644.1 | 94.5 |
| 1985 | 935.7 | 10.8 | 52.1 | 62.9 | 6.7 | 872.8 | 93.3 |
| 1986 | 714.2 | 1.8 | 36.9 | 38.7 | 5.4 | 677.3 | 94.8 |
| 1987 | 723.5 | 36.4 | 49.2 | 85.6 | 11.8 | 637.9 | 88.2 |
a. See notes to table 9.14.
b. See notes to table 9.14.
Table 9.16 Hydrological components for the second growing season (AugustNovember), 1979-1986
| Year | Annual rainfall (mm) | Subsurface flowb (mm) | Surface flow (mm) |
Total water yield |
Apparent evapo transpirationa |
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| mm | % of rainfall | mm | % of rainfall | ||||
| 1979 | 585.8 | 0.03 | 4.6 | 4.6 | 0.8 | 581.2 | 99.2 |
| 1980 | 845.4 | 71.90 | 81.1 | 153.0 | 18.1 | 692.4 | 81.9 |
| 1981 | 432.4 | 33.50 | 11.6 | 45.1 | 10.4 | 387.3 | 89.6 |
| 1982 | 223.6 | 19.20 | 8.1 | 27.3 | 12.2 | 196.3 | 87.8 |
| 1983 | 230.6 | 19.20 | 6.1 | 25.3 | 11.0 | 205.3 | 89.0 |
| 1984 | 480.6 | 30.50 | 13.2 | 43.7 | 9.1 | 436.9 | 90.9 |
| 1985 | 735.5 | 6.90 | 41.1 | 48.0 | 6.5 | 687.5 | 93.5 |
| 1986 | 379.2 | 0.10 | 13.9 | 14.0 | 3.7 | 365.2 | 96.3 |
a. See notes to table 9.14.
b. See notes to table 9.14.
Table 9.17 Hydrological components for the dry season (December-February) for Watershed 1, 1979-1987
| Year | Seasonal rainfall (mm) | Subsurface flow (mm) | Surface flow (mm) | Total water yield (mm) |
| 1979 | 3.6 | 0.0 | 0.0 | 0.0 |
| 1980 | 23.6 | 0.0 | 0.0 | 0.0 |
| 1981 | 5.3 | 2.0 | 0.08 | 2.1 |
| 1982 | 12.7 | 5.0 | 0.10 | 5.1 |
| 1983 | 0.0 | 3.6 | 0.03 | 3.6 |
| 1984 | 86.1 | 4.1 | 1.1 | 5.2 |
| 1985 | 0.0 | 3.6 | 0.0 | 3.6 |
| 1986 | 7.6 | 0.0 | 0.0 | 0.0 |
| 1987 | 18.8 | 3.9 | 0.0 | 3.9 |
Note: The data for December are taken from the previous year.
An increase in the magnitude of interflow and its continuous discharge throughout the dry season may be attributed to the replacement of deep-rooted perennials with high water requirements with shallow-rooted annuals with relatively fewer water requirements. Further, annuals were not grown during the dry season.
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