Abstract:Inland valleys with wet lowlands are an important water source for farming communities in the sub-humid zone of West Africa. An inland valley and surrounding contributing watershed area located in the sub-humid zone near M'bé in central Côte d'Ivoire was instrumented to study surface runoff and base flow mechanisms. Four flumes at different distances down the main stream and more than 100 piezometers were installed. Measurements were taken during two rainfall seasons in 1998 and 1999. Under initial wet conditions, a typical single-peak hydrograph was observed. Under low antecedent moisture conditions, however, runoff was characterized by a double-peaked hydrograph. The first peak, which occurred during the storm, was caused by rain falling on the saturated valley bottom. The second peak was delayed by minutes to hours from the first peak and consisted of rain flowing via the subsurface of the hydromorphic zone that surrounds the valley bottom. The duration of the delay was a function of the water table depth in the hydromorphic zone before the storm. The volume of the second peak constituted the largest portion of the stream flow.

The GaMampa wetland, a palustrine wetland, comprises less than 1% of the catchment but is widely believed to make a significant contribution to dry-season river flow in the Mohlapitsi River, a tributary of the Olifants River, in South Africa. The contribution of the GaMampa wetland to dry-season flow in the Mohlapitsi River and the impact of increasing agriculture on its hydrological functioning were investigated. Economic analyses showed that the net financial value of the wetland was US$ 83 263 of which agriculture comprises 38%. Hydrological analyses indicated that the Mohlapitsi River contributes, on average, 16% of the dry-season flow in the Olifants River. However, the wetland contributes, at most, 12% to the increase in dry-season flow observed over the reach of the river in which the wetland is located. The remainder of the increase originates from groundwater flowing through the wetland. Furthermore, despite the conversion of 50% of the wetland to agriculture since 2001, there has been no statistically significant reduction in dry-season flow in the Mohlapitsi River. These results highlight the importance of understanding the nature of the full suite of services being provided by a wetland in order to make informed decisions for appropriate management.Key words agriculture; ecosystem services; hydrology; livelihoods; South Africa; wetland Une étude de l'hydrologie et de la fourniture de services écosystémiques des zones humides: la zone humide de GaMampa, Afrique du Sud Résumé La zone humide de GaMampa, une zone humide palustre, représente moins de 1% du bassin versant auquel elle appartient, mais il est communément admis qu'elle apporte en saison sèche une contribution significative au débit de la rivière Mohlapitsi, affluent de la rivière Olifants, en Afrique du Sud. Nous avons étudié la contribution de la zone humide de GaMampa au débit de la rivière Mohlapitsi en saison sèche et l'impact croissant de l'agriculture sur son fonctionnement hydrologique. Les analyses économiques ont montré que la valeur financière nette de la zone humide était de 83 263 US$ dont 38% correspondent à l'agriculture. Les analyses hydrologiques ont montré que la rivière Mohlapitsi contribue en moyenne pour 16% au débit de la rivière Olifants en saison séche. Cependant, la zone humide contribue à 12% au plus de l'augmentation du débit observé en saison séche sur la portion de la rivière sur laquelle la zone humide est située. Le reste de l'augmentation provient de l'eau souterraine qui coule au travers de la zone humide. Par ailleurs, malgré la conversion de 50% de la zones humide en terres agricoles depuis 2001, il n'y a pas eu de réduction statistiquement significative du débit de saison sèche de la rivière Mohlapitsi. Ces résultats soulignent combien il est important de comprendre la nature de la gamme complète de services fournis par une zone humide, afin de prendre des décisions éclairées pour une gestion appropriée.

No abstract

No abstract