Memoire Online - Hydrological modeling of the Congo River basin: Asoil-water balance approach

The Congo has a regular flow, which is fed by rains throughout the year. As recorded at Kinshasa, the flow has for years remained between the high level of 65411.92 m³/s, recorded during the flood of 1908, and the low level of 21407.54 m³/s, recorded in 1905. During the unusual flood of 1962, however, by far the highest for a century, the flow probably exceeded 73623.8 m³/s (Encyclopedia Britannica, 2007). At Kinshasa, the river's regime is characterized by a main maximum at the end of the year and a secondary maximum in May, as well as by a major low level during July and a secondary low level during March and April (Figure 4, Table 1). In reality, the downstream regime of the Congo represents climatic influence extending over 20° of latitude on both sides of the equator, a distance of some 2253km.

The Congo River's flow and water levels are affected by the rains all year round. It is the effects of rainfall throughout the regions whose rivers and tributaries contribute to the Congo River that influence the fluctuations in the flow of the river. However, because the Congo basin has an immense area, the weather pattern in one particular region will not have much effect on the river's overall levels. For example, heavy rainfall in the northern areas that contribute to the

Table 1 River discharge at KINSHASA gauge (after Vorosmarty et al, 1998) Station: Kinshasa, Latitude: 4.3o S/ Longitude: 15.3o E, Elevation: River: Zaire,
Country: Congo D.R., Area: 3475000 km2

-	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec	Ann
m ³ /s	47494	37649	34713	37172	39150	36717	31703	31087	36366	43172	51708	56082	40251
mm	36.6	26.4	26.8	27.7	30.2	27.4	24.4	24	27.1	33.3	38.6	43.2	366
km ³	127	91.9	93	96.3	105	95.2	84.9	83.3	94.3	116	134	150	1270
l/s/km ²	13.7	10.8	9.99	10.7	11.3	10.6	9.12	8.95	10.5	12.4	14.9	16.1	11.6
%	9.83	7.79	7.19	7.7	8.11	7.6	6.56	6.44	7.53	8.94	10.7	11.6	100

Figure 3 Mean Discharge Regime of the Congo River Basin at the Kinshasa gauge

Figure 4 Monthly discharge of the Congo River (Kinshasa gauge). Mean 1960-1990

Patterns have been established in the past and the river can be expected to have higher levels
in December and May due to the rainy season. The levels are expected to be low in March and

April and even lower in July in response to the dry season. If some of the weather patterns change drastically, resulting in floodwaters arriving at the same or different times, then the anticipated water levels are affected accordingly.

2.4 Climate

The Congo basin is located in the equatorial belt. This location ensures that different parts of the Congo basin receive substantial rainfall throughout the year; with a decreasing trend of rainfall with latitude. The northern and central portions of the basin have two major rainfall seasons which begin in March and October each year (Kazadi, 1996). The northernmost points of the basin, situated in the Central African Republic, receive 8 to 406.4 mm during the course of a year, which is less than the average near the equator; the dry season, however, lasts for four or five months, and there is only one annual rainfall maximum, which occurs in summer.

In the south, the two rainfall seasons gradually merge into a single season beginning in December and lasting for six months each year. In the far southern part of the basin -- at a latitude of 12° S, in the Katanga region -- the climate becomes definitely Sudanic in character, with marked dry and wet seasons of approximately equal length and with mean rainfall of about 1245 mm a year.

The rainfall peaks are associated with the passage of the Inter-Tropical Convergence Zone (ITCZ), which is a large zone of low pressure caused by excessive heating from an overhead sun. During the northern summer, the midday sun is directly overhead in the tropical regions of the Northern Hemisphere. This results in higher temperatures and consequently, lower air pressures at the surface. Moist air flows from the oceans towards these low pressure areas. The moisture is released as rainfall on the land surface when the air is forced to rise on entering the convergence zone or by orographic effects. The ITCZ causes heavy rainfall in the areas it passes over as it moves north and south between the tropics during the respect northern and southern summers. The Congo basin is thus representative of a large river basin in which the spatial distribution of input varies significantly with time.

Figures 6 bellow shows the monthly distribution of rainfall, Evapotranspiration and temperature for 8 stations over the Conog Basin. Figure 7 and Table 2 give the evective rainfall for three virtual stations located in the southern hemisphere, northern and the center of the basin.

Long	Lat	Dec	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov
12	-14	32	30.1	40.4	75.6	44.6	1.8	-0.5	-1.2	-2.8	-5.1	0.7	32
23.58	-2	131.3	113.6	104.2	129.9	126.3	106	57.1	61.7	99.4	137.6	139.2	131.3
35	10	5.8	11.9	12.7	43.5	92.7	114.8	84.7	115.5	129.8	121.1	98.7	5.8

Figure 6 Long-term monthly average of Effective Rainfall (1961-1990) at grid cell.

Figure 7 Effective rainfall distribution for three selected grids in the Area

Hydrological modeling of the Congo River basin: Asoil-water balance approach

2.3 Hydrology

2.4 Climate