Abstract:
The magnitude of interrill and rill erosion was determined on the northern slopes of the Uluguru Mountains,
Tanzania which is representative for larger areas of East African Arch Mountains, where population pressure is
high and land degradation is severe. The aim of the study was to develop a database to support soil
conservation in the area. The study was done on two distinct geomorphic units with respect to altitude and
hence rainfall distribution pattern: mountain ridges with an altitude ranging from 1000 to 1500 masl and mean
annual rainfall of 2300 mm and mountain foothills whose altitude and mean annual rainfall are 550 to 900 masl
and 900 mm, respectively. Total soil loss was measured on 36 individual bounded plots measuring 1.2 m × 20 m
using Gerlarch troughs on each day with rain from July 2000 to June 2001. The plots were located on six
different geopedologic units, nine on mountain ridges and the rest on the mountain foothills. The slope
gradient on the terrain ranged from 30% to 70%. The plots were put under maize cultivation as the main crop.
Soil loss through rill erosion was estimated by volumetric measurements of rills on each soil erosion plot. The
soil loss due to interrill erosion was obtained by subtracting soil loss through rill erosion from the total soil loss
measured in the Gerlarch troughs. The results indicate that soil loss due to both interrill and rill erosion was
very high with mean soil loss of 69 and 163 t/ha/year, respectively. Rill erosion accounted for about 58% of the
total soil loss while interrill erosion contributed to the remaining 42%. Both interrill and rill erosion were higher
in the mountain ridges with mean soil loss of 88 t/ha/year and 210 t/ha/year compared to 49 and 116 t/ha/year
in the mountain foothills, respectively. Rill erosion was significantly higher (P ≤ 0.001) in all geopedologic units
with slope gradient above 40% (mean soil loss ranged between 91 and 258 t/ha/year) compared to interrill
erosion with mean soil loss varying from 41 to 115 t/ha/year. In geopedologic units with slope gradient above
60% both interrill and rill erosion were highly active while in geopedologic units with slope gradient below 40%
the two processes were less active. The results demonstrate that rill erosion is more important than interrill
erosion in the study area particularly where the slope gradient exceeds 40%. The results further show that the
major part of the studied area has moderate interrill erosion (10–50 t/ha/year) and severe to very severe
(N 100 t/ha/year) rill erosion. This study clarifies the magnitude of interrill and rill erosion which is important
for designing soil conservation on agricultural fields.
