Informations et ressources scientifiques
sur le développement des zones arides et semi-arides

Titre : Understanding Hydrological Processes in an Ungauged Catchment in sub-Saharan Africa

Auteur : Mul, M.L.

Grade : Degree of Doctor 2009

Université de soutenance : UNESCO-IHE, Institute for Water Education

Résumé partiel
Ungauged catchments can be found in many parts of the world, but particularly in subSaharan Africa. Information collected in a gauged catchment and its regionalisation to ungauged areas is crucial for water resources assessment. Especially farmers in semi-arid areas are in need of such information. Inter and intra-seasonal rainfall variability is large in these areas, and farmers depend more and more on additional surface and groundwater resources for their crop production. As a result, understanding of the key hydrological processes, and determination of the frequencies and magnitudes of stream flows, is very important for local food production. This is particularly true for the ungauged Makanya catchment in Tanzania, which is the subject of this study.

In the absence of long-term hydrological data, hydrological processes have been studied through a multi-method approach. Regular rainfall and runoff measurement devices were installed in a nested catchment approach. High spatial and temporal resolution data have been collected over a period of 2 years to capture all the hydrological processes. Spring samples have been taken to identify groundwater flow systems. Hydrograph separation with hydrochemical data has been performed to identify and quantify the origins and flow pathways of the water during flood flows. Electrical resistivity tomography (ERT) has been used to map the subsurface structure at selected sites. Finally, a conceptual model has been developed to test the hypothesised conceptualisation of the flow paths.

Agricultural practices by farmers in the catchment vary as a function of location as they are influenced by the local climate, water resources availability and soil type. Three zones with distinct features have been identified within the study area. In the highlands, it is cooler, rainfall is more abundant and there are perennial springs. Here irrigation is practiced as supplementary irrigation in the wet season and as full irrigation during the dry season. In the midlands only supplementary irrigation is practiced using the remainder of the perennial streams coming from the highlands. Here the high probability of occurrence of dry spells requires supplementary irrigation. In the lowlands, base flow has dried up and spate-irrigation is practiced during the rainy season, whereby flash floods are diverted from the main river onto the farm land.

In the highlands, the occurrence of perennial springs is defined by the geology. These springs discharge a substantial amount of water. In the midlands, few springs exist, yielding a substantially lower amount of discharge, with poorer quality. Abundance of spring water from the highlands is both used in the highlands and the midlands for irrigation. However, due to the many diversions, the rivers do no longer reach the outlet of the catchment, as they do not exceed the infiltration capacity of the alluvium. Only large flash floods reach the spateirrigation system in the lowlands

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