Upper Akagera (NAKU)

The Upper Akagera catchment commences at the confluence of the Lower Nyabarongo and the Akanyaru Rivers and belongs to the Nile basin. It is a downstream catchment. The first half of the catchment is located within Rwanda but after Lake Rweru, the Akagera River forms the boundary between Rwanda and Burundi.

The Akagera continues the characteristic meandering path of a river in a wide and extremely flat floodplain that is flooded annually over its entire width. The Akagera connects with a large number of lakes and the flow direction between the river and the lakes may change several times each year depending on the flooding in the river (which is determined by the rainfall on the Nile Congo water divide) and the rainfall in the catchment area itself.

The principle tributaries cum lakes are Mugesera Lake, Rweru Lake which drains mostly from Burundi, the Nyabugongwe River, and finally the Ruvubu River which takes its water exclusively from Burundi and Tanzania and enters the Akagera just upstream of Rusumo falls.

The downstream limit of the catchment is at the Rusumo Falls where its waters fall into the lower Akagera River.

With a clearly defined upstream inflow location, a proper transboundary catchment area and an outflow location, the Upper Akagera catchment is properly demarcated. A subdivision of the catchment or a downstream extension would compromise the monitoring of the catchment outflow and thus one of the prerequisites for proper water management.

The surface areas of the different parts of the catchment are as follows:

  • Land area of the catchment within Rwanda : 2,941 km²
  • Surface area of the series of lakes (in Rwanda) : 112 km²
  • Surface area of Lake Rweru : 123 km² (of which 34 km² in Rwanda)
  • Total area of the NAKU catchment within Rwanda : 3,053 km²
  • Upstream area from Akanyaru catchment (Rwanda & Burundi) : 5,328 km²
  • Upstream area from Nyabarongo catchment : 8,601 km²
  • Catchment area in Burundi and Tanzania (Lake Rweru and Ruvubu river) : 13,714 km²
  • Grand total of the Upper Akagera at Rusumo Falls : 30,696 km².

The Rwandan catchment area is essentially covered by the following seven districts (Bugesera, Ngoma, Kirehe, Rwamagana, Kicukiro, Kayonza and Gasabo). A small area of the catchment is located in the district of Nyarugenge.

A further subdivision of the elongated level 1 catchment of 3,053 km² has been suggested as follows for the creation of two level 2 sub-catchments :

  • Akagera Mugesera (NAKU_1). The Akagera upstream of the Lake Rweru confluence where the catchment area is exclusively Rwandan (area of 1,888 km² in Rwanda).
  • Akagera Rweru (NAKU_2). The Akagera downstream of Lake Rweru where the river follows the boundary between Rwanda and Burundi and the catchment area is shared between these neighbors (area of 1,165 km² in Rwanda).

The principal water resources in the catchment are as follows:

  • the average annual rainfall is about 925 mm/annum which equates to 2,824 hm³/annum from the total land surface area of 3,053 km².
  • the combined average annual surface flow generated from the catchment area in Rwanda stands at 16 m³/s or some 504 hm³/annum.
  • the groundwater annual recharge / safe yield is about 351 hm³/annum. With a total groundwater storage of 4,580 hm³/annum the mean residence time is about 13 years.

 

The surface runoff of the catchments own resources and the excess flow from the river is stored in the series of lakes and the wide permanently inundated floodplain. The main effect of this is a significant increase of evaporation losses to a total of 2,320 hm³/annum.

Land characteristics in the catchment are summarily presented as follows :

  • the lithology of the catchment is varied with granite and pegmatite in the east, shale in the center, and quartzite and quartz rich schist in the east. The many waterways in the in the center and western part of the catchment contain alluvial material and the Akagera floodplain contains extensive areas of peat. The picture is completed with some volcanic and basalt material near the outlet of the catchment. In terms of groundwater productivity, shale and schist aquifers tent to have low permeability and are therefore not very interesting for groundwater exploitation. Granite and pegmatite can be exploited for low yields as they are generally deeply weathered to provide adequate storage capacity. Quartzite and alluvial material are generally interesting for groundwater exploitation but require proper investigation.
  • the dominant soil classes are the ferralsol with a complement of the 'nitosol - acricol -alisol - lixisol' class. The cambisol, clay soils with low infiltration rate and mineral soils conditioned by flat topography classes make up most of the remaining area. Especially the lower reach of the Akagera floodplain presents histosols. There are some very small areas with andosols. Except for clay and mineral sols conditioned by flat topography infiltration rates of these soils are generally high.
  • the land cover of the catchment is dominated by rainfed agriculture with, especially in the east significant areas of natural open land, forest plantation and built up area in small sections throughout the catchment and a larger domain for the city of Kigali. The western part of the Akagera floodplain is used for irrigated / agricultural wetland with the central and eastern part, due to humidity preserved as natural wetland.

     

As of 2012, the total population in the catchment was assessed (2012 population census) at 1,318 million dispersed throughout the catchment with highest densities in and around the urban centers of Kicukiro, Gasabo, Nyarugenge (all three for Kigali city), Rwamagana, Kabarondo, and Kibungo. Catchment wide population data are given in the table below. The provisional 2012 census data per sector have been converted to population per catchment. The 2012 census result for the NAKU catchment stands at 1,318,379 population which is 11.03 % above the high growth scenario projection. Over the period 2002 - 2012 this equates to an annual difference of 10.5 ‰ (underestimation of the projection data). It is possible that this difference is caused by immigration from the rural areas to the city of Kigali but this phenomenon is not observed for the lower Nyabarongo catchment (which houses the largest part of the city of Kigali).

As regards the development of land resources, and as already indicated above, the bulk of the land is used for rainfed agriculture, natural open land and wetland for irrigation and agriculture production in the eastern Akagera River and numerous valley bottoms of its tributaries. In as far as the open waters are stagnant (lakes) they have a significant potential for fish production provided that the halieutic resource is adequately managed (protected from overexploitation and exploited with suitable gear).

Bench - and radical terraces are implemented throughout the catchment but the surface area thus protected from erosion, remains restricted. Cheaper erosion protection works like trenches, contour plantations, etc. are widespread. Borders of lakes and water courses are protected by law from exploitation for agricultural use.

Urban areas are significant in the Kigali districts of Kicukiro, Gasabo and Nyarugenge but elsewhere they are limited in size and not very crowded.