Vegetation exerts primary control of the rate of water loss from land surfaces. In the Sahel its degradation by human over-exploitation has been proposed as a possible cause of decreased rainfall which threatens the sustainability of future food production. This idea is supported by simulation studies using general circulation models but their credibility is undermined by lack of data on evaporation from Sahelian vegetation and their consequent crude representation of the evaporation process. Direct measurements were made of transpiration from vegetation and evaporation from the soil in two widespread vegetation types of the Sahel region of West Africa to aid the development of more physically realistic models of evaporation from sparsely-vegetated land surfaces.

Measurements of water use by the herbs and bushes in fallow savannah regions were made with a venetilated evaporation chamber and sap flow gauges. Leaf-area measurements were also obtained for all the vegetation components and total evaporation flux measurements using the 'Hydra', an IH-developed eddy correlation device.

Hourly fluxes of evaporation from a large patch of bare soil within the tiger-bush were measured with a Bowen ratio system. The data showed how soil evaporation varies after rainfall as the surface dries out. Hourly and daily comparisons of actual and potential evaporation were made and used to calibrate a model aided at calculating the soil evaporation component of the water balance over a number of seasons with different rainfall. This analysis showed that over the whole tiger-bush area, annual soil evaporation is normally ~28% of annual rainfall but that this percentage increases markedly in dry years. The implications are that in dry years runoff from these bare soil areas will decrease by a greater percentage than rainfall because a greater proportion of rainfall is lost as soil evaporation.

The model development proved satisfactory and the impact of land degradation on regional climate has been modelled successfully. Simulated land degradation reduced rainfall and increased surface temperature, producing climate anomaly patterns consistent with the longer-term droughts observed in these regions.

Allen, S.J. and Grime, V.L. 1995. Measurements of transpiration from savannah shrubs using sap flow gauges. Ag. & For. Meteorol., 75, 23-41.

Huntingford, C., Allen, S,J. and Harding, R.J. 1995. An intercomparison of single and dual-source vegetation-atmosphere transfer models applied to transpiration from Sahelian savannah. Bound.-Layer Meteorol., 74, 397-418.

Verhoef, A. et al. 1996. CO2 and water vapour fluxes from a Sahelian savannah. Agric. For. Meteorol., 80, 231-248.

Wallace, J.S. and Holwill, C.J. 1997. Soil evaporation from tiger-bush in Niger. J. Hydrol., 188-189, 426-442.

Roberts, J.M. et al. 1998. Water use by vegetation in the Sahel. DFID Report 98/6

Email: hmg@mail.nwl.ac.uk