Suitability of SWAT Model for Sediment Yields Modelling in the Eastern Africa - Advances in Data, Methods, Models and Their Applications in Geoscience - page 263

River begins in the Central Ethiopian Highlands at an altitude of 3000 m to the west of
Addis Ababa. After flowing through Koka Reservoir, it flows northeastwards along the rift
valley until eventually discharging into Lake Abbe. The Koka Reservoir is situated about 90
km southeast of Addis Ababa at a longitude of 39˚ 10' E and latitude of 8˚ 25' N. The erosion
rates in the KRC and in the Awash Basin as a whole are high with values generally
exceeding 6,000 t/km 2 /yr and occasionally as high as 20,000 t/km 2 /yr.
The climate of the Awash Basin is characterized by the Inter-Tropical Convergence Zone
(ITCZ). The Mean annual rainfall (MAR) and temperature vary spatially across the
catchment between 170 and 1978 mm/annum and 12.8 and 31.5 o C, respectively. The area is
dominated by a bimodal rainfall pattern. According to the National Meteorological Services
Agency, the study area is characterized by a quasi-double maxima rainfall pattern with a
small peak in April and maximum peak in August. The rainfall in the highlands shows a
strong correlation with altitude (Lemma, 1996). The mean annual wind speed in the KRC is
1.9 m/s.
Two major relief features are found in the Awash Basin: the highlands of the Ethiopian Plateau
and the lowlands of the Rift Valley. The bedrock and soil in the area determine the amount
and composition of transported sediments in the river. The geology of the basin is dominated
by sedimentary rocks such as limestone and sandstone. The alluvium deposits consist of clay,
sand and tuff. The long rains occur between June and September.
Case study 2, the Nyumba Ya Mungu (NYM) Reservoir subcatchment, has runoff that is
highly regulated by the man-made Nyumba Ya Mungu (NYM) Reservoir of some 140 km 2 .
The NYM Reservoir subcatchment is located in the upstream part of Pangani River
catchment (PRC) (Figure 1 & Table 1). The PRC is located between coordinates 36 o 20' E,
02 o 55' S and 39 o 02' E, 05 o 40' S in the northeastern part of Tanzania and covers an area of
about 42,200 km 2 , with approximately 5% in Kenya (Figure 1). The Pangani River has two
main tributaries, the Kikuletwa (1DD1) and the Ruvu (1DC1) (Figure 1), which join at the
Nyumba Ya Mungu (NYM) reservoir.
The catchment of NYM occupies a total land and water area of about 12,000 km 2 (Ndomba,
2007). It is located between coordinates 36 o 20'00'' E, 3 o 00'00'' S and 38 o 00'00'' E, 4 o 3'50'' S. This
area has a Mean Annual Rainfall (MAR) of about 1000 mm/annum. The rainfall pattern is
bimodal with two distinct rainy seasons, long rains from March to June and short rains from
November to December. Rohr and Killingtveit (2003) indicated that the maximum
precipitation on the southern hillside of Mt. Kilimanjaro takes place at about 2,200 m.a.s.l.,
which is 400 - 500 m higher than previously assumed. The mean annual wind speed is 1.87
m/s. The altitude in the study area ranges from 700 and 5,825 m.a.s.l. with the peak of Mt.
Kilimanjaro as the highest ground. Based on the Soil Atlas of Tanzania, the main soil type in
the study area is clay with good drainage (Hathout, 1983). The landcover of the catchment is
dominated by actively-induced vegetation, forest, bushland and thickets with some alpine
desert. The majority of the population in the basin directly or indirectly depends on
irrigated agriculture. Agriculture is concentrated in the highlands with area coverage less
than 20% (Ndomba, 2007). Lowlands are better suited for pastoralism. The main runoff-
sediment generating subcatchments in the study area upstream of NYM reservoir are
Weruweru, Kikafu, Sanya, Upper Kikuletwa and Mt. Meru. The basin is also important for
hydropower generation, which is connected to the national grid. Hydropower plants, which
are downstream of NYM Reservoir are NYM (8 MW), Hale (21 MW), and New Pangani falls
(66 MW).