To develop a new capability for modelling the water resources of large catchments in relation to land use through (i) the use of GIS and the present knowledge of evaporation, and (ii) by modelling river flows and lake and reservoir levels. To apply the modelling developments to the Zambezi Basin to assess the relative impacts on water availability of key development activities.

A spatially distributed GIS version of the process-based water resource model developed under R5850 was applied to the whole Zambezi basin. To do this, the basin was split into sever major sub-catchments and the model applied to each of these in turn, using average monthly climatic data distributed according to a 0.5 degree grid. The model accounted for four types of land cover: low vegetation, forest, marsh, and water surfaces. The impact of land-use change on average annual runoff was investigated for each of the sub-catchments. A simple spreadsheet model was then developed to assess the impact that both deforestation and irrigation development will have upon the internal yields of the river network and thus influence the quantities of available water flowing through the Zambezi system.

Using the model, the predicted impact of converting 1% of the area of each sub-catchment from forest to low vegetation was a general increase in runoff of 2-3mm/yr, with lower values predicted in the more arid south-western region. In practical terms, for example, it could be that an increase in runoff resulting from 1% of the Zambezi basin being deforested would be balanced out by the losses associated with the development of 300,000 ha of irrigation across the basin.

A useful operational tool for planners has been developed with which to assess the impact of a range of possible development scenarios on water availability.

Price, D.J. and Calder, I.R. 1998. Water resource modelling for large catchments. DFID Reoport 98/1, Institute of Hydrology, Wallingford.

Email: hmg@mail.nwl.ac.uk