Soil evaporation from tiger-bush in south-west Niger

• Published in: Journal of hydrology (Amsterdam) 1997-02, Vol.188 (1-4), p.426-442
• Main Authors: Wallace, J.S., Holwill, C.J.
• Format: Article
• Language: English
• Subjects: soil chemistry; soil physics; soil science
• ISSN: 0022-1694; 1879-2707
• DOI: 10.1016/S0022-1694(96)03185-X

A previous study of evaporation from an area of patterned woodland (tiger-bush) in Niger by Culf et al. (1993) has demonstrated the need to determine the contribution from the bare soil strips which occupy 67% of the surface area. They measured total evaporation from the entire land surface using eddy correlation, but not the individual contributions from the soil and vegetation components. This distinction is necessary to create accurate models of evaporation from the tiger-bush as different processes operate in the two components. The previous study in the same area relied upon untested modelling of the bare soil evaporation as no direct measurements were available. In the present study, carried out during HAPEX-Sahel, hourly fluxes of evaporation from a large patch of bare soil within the tiger-bush were measured using a Bowen ratio system. The data obtained show in detail how soil evaporation varies after rainfall as the surface dried out. Comparison is made on an hourly and daily basis of actual and potential evaporation. A two-phase model based on the Ritchie (1972) approach is calibrated using these data and the model is used to calculate the soil evaporation component of the water balance over a number of seasons with different rainfall. This analysis shows that over the entire tiger-bush area, annual soil evaporation is normally ∼28% of annual rainfall, but 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.