3.5.1 Soil erosion hazard assessment
Emphasis was put on the role of water erosion which is governed by the following land characteristics: climate, land surface, soil, land cover and management. The erosion risk is assessed by:
erodibility + erosivity + slope steepness
3.5.1.1 Soil erodibility
The resistance of the soil to disintegration of soil aggregates and the dispersion and detachment of the soil particles was assessed using clay and silt percentage, organic matter content. Organic matter and silt/clay ratio ratings were derived from the soil survey data (topsoil 0 – 30 cm) at the National Agricultural Research Laboratories, Kawanda (See soil map above). The ratings (Siderius, 1992) are indicated in the tables below:
Table 3: Soil organic matter and carbon rating
| Rating | % Organic matter | % Carbon |
| 1 | > | > 3.0 |
| 2 | 2 – 5 | 1.2 – 3.0 |
| 3 | < 2 | < 1.2 |
Table 4: Rating of the silt/clay ratio
| Rating | Ratio |
| 1 | < 0.2 |
| 2 | 0.2 – 0.6 |
| 3 | 0.6 – 1.0 |
| 4 | > 1.0 |
Soil erodibility factor rating
The final soil erodibility rating was obtained by adding the soil organic matter and silt/clay ratio sub ratings. The final soil erodibility rating is shown in table
Table 5: Final soil erosion erodility factor rating
| Rating | Description | Sum |
| 1 | High resistance to erosion | < 3 |
| 2 | Medium resistance to erosion | 3 – 5 |
| 3 | Low resistance to erosion | > 5 |
| Siderius 1992. Soil derived land qualities, ITC Enschede, The Netherlands | ||
The slope factor
Slope gradient was derived from the SRTM digital elevation model using the IntegratedLand and Water Information System (ILWIS). The slope was categorized into 5 classes namely:
Table 6: Erosional slope rating and categories
| Rating | Erosional slope (%) | Categories |
| -2 | 0 – 3 | Nearly level |
| -1 to 0 | 3 – 8 | Undulating to gently sloping |
| +1 to + 2 | 8 – 16 | Rolling to steep |
| +3 | 16 – 30 | Hilly to moderately steep |
| +4 | > 30 | Steep to very steep |
< Slope erosion map here>
The rainfall factor (Erosivity)
The energy generated by the falling raindrops that disintegrates soil aggregates and dispersion was derived from mean annual rainfall values using the following equation
developed by Moore (1979) for East African conditions:
R = 0.029 × (3.96× P + 3122)− 26 (Moore, 1979) (7.4)
where:
R= Rain erosivity (J mm /m2/h)
P= Annual rainfall (mm/year)
Table 7: The rainfall erosivity factor rating and categories
| Rating | Erosivity (J mm /m2/h) | Categories |
| 1 | 0 – 144 | Very low |
| 2 | 144 – 172 | Low |
| 3 | 172 – 199 | Medium |
| 4 | 199 – 227 | High |
| 5 | 227 – 254 | Very high |
< Erosivity map here>
The final ratings depicting soil erosion risk was obtained by crossing of the soil erodibility, erosivity and slope steepness maps (See erosion risk map). The erosion risk map shows areas that require soil conservation measures to minimize surface water runoff.
Reference
Chenery, E.M., 1960. An Introduction to the Soils of the Uganda Protectorate – Memoir 1, Department of Agriculture; Kawanda Agricultural Research Station, Kampala, 1960.
Siderius 1992. Soil derived land qualities, ITC Enschede, The Netherlands
Moore, T. R., 1979. Rainfall erosivity in East Africa: Kenya, Tanzania, Uganda. Geografiska Annaler. Series A. Physical geography 61, 147-156.