4. 2. Study area
The Lake Tana basin is entirely located in the Amhara regional state in the north-western
highlands of Ethiopia (Figure 1). It is one of the sub-basins of the Blue Nile River in which the
largest fresh water lake in the country is found. The total catchment area of the Lake Tana basin
is 15,321 km2. It is located at 12°00ʹ Northing and 37°15ʹ Easting (Figure 1). Altitude of the
basin varies from 1785 m to 4094 m with a mean elevation of 2418 m. Most of the basin consists
of cropland on the moderate sloping hillsides and grass in the valley bottoms. Soils vary rom
Haplic Luvisols and Alisols in the Gilgel Abay, Luvisols in the Gumara, and Eutric Leptosols
and Chromic Luvisols in the Rib and Megech . The climate in the Lake Tana basin is affected by
the movement of the inter-tropical convergence zone which results in a single rainy season
between June and September and a dry period the rest of the year. The average dry season
(November– April) rainfall is 117 mm and potential evaporation is 710 mm. The wet season
(May–October) rainfall is 1400 mm and potential evaporation is 645 mm for data used in the
calibration and validation periods. More than 90% of the annual rainfall occurs in the wet
monsoon phase. The mean annual temperature in the basin is 23°C in the relatively lower lying
areas such as Bahir Dar and ranges between 15–20°C in the middle and high altitudes (29).
Figure 1. Location map of lake Tana sub basin (source 29)
1. Study area
The Ribb and Gumara Rivers are the two major perennial rivers that pass through the Fogera
Plain and cause flooding due to their reduced local channel capacity (35). The rivers originate in
the Guna Mountains, flow westward receiving additional from tributaries, and finally enter Lake
Tana (Figure 1). Near the Lake Tana shore, the average monthly maximum temperature varies
from 17°C to 37°C and from 19°C to 39°C for the Gumara and Ribb watersheds based on data of
Woreta and Addis Zemen meteorological stations (1995– 2015), respectively. In December, the
temperature in the headwaters of the Gumara and Ribb watersheds falls below 0°C (Debre Tabor
meteorological station). The Ribb and Gumara watersheds receive an average yearly rainfall of
1300 and 1320 mm, respectively, with higher precipitation in their upper mountainous areas and
less near the lake shore. The rainfall is unimodal and 80% falls in the rainy season (June to
September) (35).
1.1. The Ribb River watershed
The Ribb River has two gauging stations: the Lower and the Upper, which encompass a
watershed area of 1501 and 844 km2, respectively (Figure 1). The watershed elevation varies
from 4081m a.s.l. near the source to 1786m a.s.l. near Lake Tana. Terrain analysis shows that
18.3%, 42.0% and 49.7% of the watershed area have a slope of less than 10%, between 10% and
20% and greater than 20%, respectively. The river slope is relatively steep (0.3%) near the
source and becomes gentle (∼0.037%) in the Fogera Plain.
2.2. The Gumara River watershed
The Gumara River has one gauging station near the Gumara Bridge, on the road connecting
Bahir Dar and Gondar, with a watershed area of 1412 km2 (Figure 1). The watershed is
characterized by undulating topography with an elevation that varies from 3704m a.s.l. at the
source to 1786 m a.s.l. near Lake Tana. 25.7% of the watershed area has a slope less than 10%,
mainly in the Fogera Plain, while 33.2% of the area has a slope between 10% and 20% and the
remaining 41.1% is characterized by steep slopes greater than 20%.
2 Study Area
Gumara watershed is found in north western Ethiopia, located between latitudes 11034′-120 N
and longitudes 37033′-380 11′ E (Fig. 1). The elevation ranges from 1784 to 3408 m above mean
sea level (amsl). It has 1470.20 km2 and forms a part of Lake Tana basin forming the source of
Blue Nile, North western Ethiopia. A mountainous and hilly dissected terrain with steep slopes
characterises the upper stream whereas an undulating topography and gentle slopes form the
downstream parts of the study watershed. The area is drained by numerous smaller streams
including Guamra (the largest stream in the watershed) that flow to Lake Tana. The soils
covering much of the watershed can be classified into clay loam, sandy loam and clayey to silt
clayey (53).
Figure 1. Location map of Gumara watershed (source 53)
Figure 1. Location map of Gumara and Ribb watersheds (source 35)