To investigate the effects of land use change, within the catchments of rivers feeding major African lakes, on the longer term trends in variations of lake levels.

To develop a modelling strategy that requires minimal data inputs and which will predict the impacts of changes in land cover on the annual yield, flood response and low flows of rivers that feed the lakes.

Using the Lake Malawi catchment as a case study since measurements of rainfall and lake level extending back to the beginning of the 20th century were available, a process model was developed based upon a soil water balance approach that considers the catchment to be composed of three surface types: forest, agricultural land, or water. Values for the model parameters for the vegetation types were obtained from experimental studies in India. The model required average monthly estimates of potential evaporation and average monthly estimates of both land and lake rainfall as input data. Predicted runoff is translated into monthly lake level predictions.

Performance of the model was assessed by comparison with observed levels: from 1896 to 1967 the major fluctuations were well described is a constant value of 64% forest cover was adopted. This result indicates that variations in rainfall alone, without changes in either land use or in the hydraulic regime of the lake, are sufficient to explain lake level changes for this period. Land-use change during the period 1954 to 1994, however, is modelled best if a 13% decrease in forest cover is incorporated: this figure is consistent with the actual decrease in forest cover for this period. Without such a decrease, the model predicted that the lake level would have actually been 1 metre lower than that observed during the southern African drought of 1992.

The model development proceeded satisfactorily such that it was able to predict the impacts of land-se change on levels in Lake Malawi.

Calder, I.R. et al. 1995. The impact of land use change on water resources in sub-Saharan Africa; a modelling study of Lake Malawi. J. Hydrol., 170, 123-135.

Price, D.J., Calder, I.R. and Hall, R.L. 1998. Water balance of African lakes. DFID Report 98/1, Institute of Hydrology, Wallingford.

(1)Successful model development has resulted in take-up by the local Water Departments.

(2)Work to extend the application of the model to large river basins, using GIS techniques, has been carried out under the associated project R5848 Water resources modelling for large catchments.

Email: hmg@mail.nwl.ac.uk