Comparative Study of the Morpho-Hydrological Variables of the Djibonker and Agnack Watersheds (Lower Southern Casamance, Senegal)

ABSTRACT: The hydrological functioning of watersheds is closely linked to their physical characteristics. In this context, GIS tools are essential for the morpho-hydrological study of watersheds shaped by climatic dynamics and land-use practices. This study aims to determine the morpho-hydrological variables of the Djibonker and Agnack watersheds, to compare them descriptively, and to relate them to hydrological functioning. Because the comparison involves only two watersheds, a formal multivariate statistical treatment, such as Principal Component Analysis, was not applicable; the 20 morphometric variables were instead compared directly. The results show that the two watersheds share broadly similar dimensional characteristics (area, perimeter, altitude) but differ markedly in shape, drainage density, and network configuration. Both watersheds have a complex hydrographic network dominated by intermittent channels. Concentration times obtained from Giandotti’s (1934) empirical formula are 5.12 h (Djibonker) and 5.26 h (Agnack), and flow velocities are 0.46 m/s (Djibonker) and 0.80 m/s (Agnack), consistent with the low overall slope indices and with the more elongated shape of the Djibonker basin. These morphometric contrasts help explain the marked difference in salinity dynamics between the two basins. The approach developed in this study can be useful for a better understanding of the functioning of small, ungauged rural watersheds.