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Comparison of Surface Runoff and Soil Loss from Runoff Plots in Forest and Small-Scale Agriculture in the Usambara Mts., Tanzania

Lill Lundgren
Geografiska Annaler. Series A, Physical Geography
Vol. 62, No. 3/4 (1980), pp. 113-148
Published by: Wiley on behalf of the Swedish Society for Anthropology and Geography
DOI: 10.2307/520673
Stable URL: http://www.jstor.org/stable/520673
Page Count: 36
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Comparison of Surface Runoff and Soil Loss from Runoff Plots in Forest and Small-Scale Agriculture in the Usambara Mts., Tanzania
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Abstract

The Usambara Mts., like other Tanzanian mountain areas, have a high potential for crop production, thanks to a favourable climate and fertile soils. This has resulted in a high population density and, in consequence, heavy pressure on the land resources. Deforestation and soil erosion are increasing threats to the long-term potential of the mountains to sustain a growing population. The aim of the present report has been to compare surface runoff and soil loss, and soil characteristics influencing these, in natural montane forest and under small-scale agricultural farming in the Usambara Mts., Tanzania. Two sites of undisturbed natural forest, one in Intermediate Evergreen Forest and one in Highland Evergreen Forest, were selected, together with one very well-kept agricultural site. On each site, four runoff plots of 12 sq.m each were installed--two on "gentle" slopes (10-15°) and two on "steep" slopes (20-25°). On these twelve plots (3 sites × 4) surface runoff and soil loss were recorded together with rainfall/throughfall in 1974 and 1975 (24 months). Soil samples down to a depth of 50 cm were collected and analysed for pH, organic carbon content, bulk density, texture, water-stable aggregates, porosity, and water-retention properties. In addition, infiltration capacity was measured, with a double-ring infiltrometer, at each site plus at a badly eroded area nearby. Non-stable soil parameters of importance to erosion are normally slightly less favourable in the agricultural topsoil (0-10 cm) than in the forest topsoils. Thus, organic matter content was appr. 25% in the Highland forest topsoil and appr. 10% in the Intermediate forest, compared with appr. 8% in the agricultural topsoil. Bulk density was appr. 0.4 g/cm3 in the Highland forest topsoil, appr. 0.8 g/cm3 in the Intermediate forest and appr. 0.9 g/cm3 in the agricultural topsoil. The forest topsoils showed high porosity values, the Highland forest had appr. 80% pores of the soil volume, the Intermediate forest appr. 70%, and the agricultural land appr. 60%. In addition, the percentage of water-stable aggregates was lower in the agricultural soil than in the two forest soils. Infiltration capacities of the two forest sites were 850 mm/h and 830 mm/h respectively, of the agricultural land 635 mm/h and of the badly eroded agricultural land only 270 mm/h, all after 90-minutes measurements. Water-retention properties differed greatly between the three sites. At field capacity, for example, the plant-available moisture in the top 50 cm of the soil was 129 mm in the Highland forest soil, 101 mm in the Intermediate forest soil, but only 52 mm in the agricultural soil. Certain soil properties, such as pH and clay content, indicate that there might be inherent soil differences between the agricultural site and the forest sites. Average annual rainfall at the Intermediate forest site was 1115 mm, at the Highland forest site 820 mm, and in the agricultural area 635 mm for the two years of study. This is lower than normal. Average surface runoff as a percentage of rainfall was 0.4% on the gentle-slope plots and 1.0% on the steep-slope plots in the Intermediate forest. The corresponding figures for the Highland forest were 0.8% and 1.3%, and for the agricultural area 0.1% and 0.1%. Average soil loss in g/m2/year was 4.2 from the gentle-slope plots and 10.1 from the steep-slope plots in the Intermediate forest. The corresponding figures for the Highland forest were 3.7 and 7.5, and for the agricultural area 1.0 and 0.9. The very low values of surface runoff and soil loss from the small-scale agricultural land are discussed in the report. Good land management, including, for example, no burning and leaving weed trash as mulch, is the most probable explanation. Large seasonal variations of runoff as a percentage of rainfall/throughfall, with high percentages (up to 14%) at the onset of the rainy seasons, were observed. This was especially pronounced on the Highland forest plots. Retarded rainfall acceptance of the strongly humic topsoils when they are dry is a probable explanation. The study has shown that with good land management it is possible to minimize surface runoff and soil loss from agricultural land on moderate slopes, at least during years of below average rainfall. Since no extreme rainfall events occurred during the two years of study it is impossible to say how the agricultural soil would have reacted to this. It is wellknown, however, that rare and extreme rainfall intensities cause most of the erosion that occurs on badly managed land, a fact that has been observed also in the Usambara mountains.

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