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Development of the shallow alluvial groundwater of the Hadejia-Jama'are-Yobe River valley flood plain (northern Nigeria) has been taking place with increasing intensity over the past decade. However little has previously been known about the nature of the Yobe River-aquifer interaction, including the recharge mechanisms. This thesis reports on a study of the river-aquifer interaction in the middle section of the basin, centred on a field site near Gashua, Yobe State. Detailed field studies were undertaken over a period of 14-months, which involved geoelectical sounding, drilling and water level monitoring. The results of the field study show that the Yobe basin is underlain by a sand and gravel aquifer, which is covered by an average of 1-3m of clay. The Yobe River is in hydraulic continuity with the adjacent alluvial aquifer and variations in aquifer storativity have been recognised as an important factor in understanding the hydraulic behaviour of the Yobe River-alluvial groundwater system. A confinedunconfined groundwater regime exists within the present site and is an inherent characteristic of the alluvial groundwater system. The recognition of this state enables a conceptual flow model of the system to be developed. A multi-layer, spatially distributed model is proposed, in which transitions between confined and unconfined conditions can be realistically represented. A numerical model needs to be designed with these concepts in mind in order to simulate the system. The research techniques employed in the study are appropriate for the assessment of the Yobe basin system where detailed data is currently not available. The combination of detailed geophysical survey, water level monitoring and conceptual modelling has led to a good understanding of the Yobe River-alluvial aquifer interaction. It is for this reason that the techniques employed in this study can be adapted for investigating the remainder of the basin downstream of the present site.

This research applies, evaluates and compares approaches to hydrological modelling and stream flow forecasting within a GIS environment. Three different approaches to modelling stream flow were investigated, namely; TOPMODEL, a regression approach and a GIS-based model, HydroGrid. TOPMODEL is a parametrically simple, physically-distributed model that allows the topological modelling of catchment processes. Regression modelling is a statistical technique that derives an empirical equation based on the assumption that the values of a dependent variable will depend upon the values of the independent variables. HydroGrid is a purpose-built GIS-based model for catchment modelling using the functionality that GIS offers for modelling the spatial variations of catchment characteristics. All three approaches were evaluated using readily available data for a lowland catchment, in Staffordshire, U.K. Model validation used six years of data covering the period 1991-92 through to 1996-97 - with years running from March-February. Five performance indicators were used to assess the models enabling both for detailed evaluation of the models and comparisons to be drawn with other research. The performance of the three models tested showed great similarities, with all approaches tending to over-predict stream flow. Model performance was also evaluated using three different evapotranspiration models - the Penman formula, the Crowe-irrigation method and the sine curve method. All three models performed best during wet years or wetter seasons indicating a common weakness in the accurate modelling of low stream flows. Despite similarities in performance, clear benefits of hydrological modelling within a GIS framework are identified. Overall, the results show that although the methods used here can help in daily flow modelling, there is a major need to improve methods for catchment modelling with routine data sources. An important development could be to loose-couple hydrological models with a GIS to improve their ability to use available information but also, as shown in this work, to model catchment processes directly within a GIS.