Soil erosion studies at Mlingano on the Eastern Usambara uplands

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University of Dar es Salaam
The Usambara mountains and their surrounding uplands 'have been reported to be in an erosion susceptible zone. A field study was thus conducted to monitor erosion rates and factors influencing it in the Usambara uplands. Twenty four runoff plots were established at Mlingano on 3 sites with natural slopes of 10, 19 and 22%. The treatments consisted of plows (a) which were ploughed, harrowed, raked and kept bare (b) with similar tillage as (a) but straw mulched @ 6 ton/ha(c) kept in rough ploughed condition, and (d) left under natural grass fellows. Runoff water, soil loss and nutrients in runoff water were monitored for each erosive rainstorm. Rainstorm characteristics were computed from daily recording rain gauge charts. In addition to soil from different horizons, composite surface soil (0-30 cm) from each site was also analysed for physical properties that influence soil erodibility. Periodic measurements were made for soil water content of surface soil, water retention characteristics, infiltration and bulk density. Soil erodibility factors computed from the Universal Soil Loss Equation were compared with those estimated from Wischmeier et al., (1971) erodibility nomograph. Data was collected for two rainy seasons whose total amount of 793.2 mm was only 70% of the long term annual average. Annual soil loss from bare plots was 37.8, 92.8 and 88.2 'ton/ha from the 10, 19 and 22% slopes respectively. Soil loss rom the other treatments ranged from 13.6 to 37.9, 0.12 to 0.18 and 0.08 to 0.14 ton/ha/year for ploughed, mulched and natural grass fallows respectively. The long (Masika) rainstorms which had higher rainfall intensity and kinetic energy caused more soil and runoff loss than those from the short (Vuli) rains. The correlations of rain -fall amount and kinetic energy per storm with E130 and AI max erosivity indices were highly significant. Erosion increased with increase in slope gradient. Field determined erodibility factor (K) was, higher than that estimated from the nomograph. Field K ranged from 0.121 to 0,176 while nomograph K ranged between 0.085 and 0.110 ton/ha/year. Field erodibility was found to be influenced by % water stable aggregates and gravel content of various quantities and sizes. Bare and ploughed treatments lost sediments of coarser texture than those from straw mulched and natural grass fallow. Natural grass and straw mulched treatments had higher infiltration rates and water retention characteristics at the end than at beginning of study. Significant differences in soil, water among treatments occurred during periods of frequent rains. Runoff and soil loss were higher during periods of high antecedent soil water content. Concentration of Calcium and Magnesium in runoff water decreased with increase in runoff volume but to a lesser extent than sodium, potassium, phosphorus and No3-N. Annual nutrients lost from bare treatments ranged from 4.58 to 8.86 kg/ha for calcium, 4 .02 to 7.00 kg/ha for magnesium, 2 .97 to 4.20 kg/ha for potassium, 3.37 to 5.08 kg/ha for sodium, 0.46 to 0.85 kg/ha for No3-N and 0.66 to 1.09 kg/ha for phosphorus. Nutrient losses in runoff water from other treatments were low.
Available in print form, East Africana Collection, Dr. Wilbert Chagula Library, Class mark (THS EAF S625.T2N44)
Soil research, Soil erosion, Economic geography, Tanzania, Tanga municipality
Ngatunga, E. L. N (1981) Soil erosion studies at Mlingano on the Eastern Usambara uplandsMaster dissertation, University of Dar es Salaam. Dar es Salaam.