No abstract

The study of the rainfall-runoff relation on annual and monthly scales is based on the hydrological data measured in various stations of the Kouilou-Niari basin. The parameters of the rainfall-runoff modeling were obtained with satisfaction by today's standards of quality. The objective of this study is to evaluate the impacts of the climate changes on the water resources of the catchment area of Kouilou-Niari using hydrological modeling. With this intention, a sample of twelvestations was selected on a hydrological cycle of more than thirty years (1952-1982). The results obtained show that the variations of the flows follow those of precipitation: reduction in N'douo zone and Niarizone and increase in Kouilou zone. These stations present a bimodal mode, with a double maximum during the rainyseasons (MAM and OND). Season (MAM) is themost rainy and season (JJAS) is the least rainy in all the stations. The relation between rainfalls and runoffs is satisfactory, because the hydrological mode models itselfonprecipitation one. The flows of the basin are rising, with anaveragestreamflow of 445 m 3 /sincluded between a minimum of 86.2 m 3 /s and a maximum of 930.4 m 3 /s for respective specific modules of 16.91 and 23.73 l.s-1 /km 2. The return periods of the floods are estimated of 5, 10 and 50 years for respective streamflows of 2470, 2620 and 2682 m 3 /s. This particular behavior of the basin finds its reasons not only in the pluviometric mode but also in the physical characteristics (relief, geologyand pedology). The Kecoefficient of flow is 42.9% and the seasonal variation K3 is 13.3 atSounda station.

The hydrologic simulation of a catchment area, described as the transformation of rainfall into runoff, generally uses hydrologic model. This work opts for the global conceptual hydrologic model GR2M, a monthly time step model, to study the Kouilou-Niari basin, the second most important ones of the Republic of Congo. This includes two parameters to model the hydrologic behavior of a catchment area. The choice of the conceptual model GR2M is justified by the reduced number of parameters and the monthly time scale. The objective of this study is to determine the characteristic parameters of the GR2M model, by a calibrating and a validating procedure. The use of these parameters enables to follow the evolution of the water resources from the climatic variables. It has been first carried out a characterization of some physical, geological and climatic factors governing the flow, by dealing with the main climatic variables which constitute the inputs of the hydrologic model. Then, a hydrologic rainfall-runoff modeling allows to calibrate and validate the model at monthly time scale. Taking into account the number of parameters involved in hydrologic processes and the complexity of the cathment area, this model gives acceptable results throughout the Kouilou-Niari basin. The values of the Nash-Sutcliffe criterion and those of the correlation coefficient obtained are greater than 80% in validation, which explains the performance and robustness of the GR2M model on this basin.