Determining transpiration coefficients of Sahelian vegetation barriers

Vegetation barriers along contour lines are an appropriate measure for runoff management. An essential element of their design is minimisation of competition for water between the agricultural crop and the vegetation barrier. This requires the evapotranspiration of the vegetation barrier to be known. In 1998 FAO (the Food and Agricultural Organisation of the United Nations) presented a method to estimate the evapotranspiration of natural vegetation. The method includes a dual crop coefficient approach, distinguishing soil evaporation and canopy transpiration. The transpiration coefficient can be obtained from either the LAI (leaf area index) or the fractional ground cover. This paper reports the results of a study conducted in Burkina Faso in 1998. Transpiration coefficients of three vegetation barriers were obtained by directly measuring transpiration and soil evaporation during the mid-season growing stage. Measured values were compared with those obtained from the FAO estimation method, using both the LAI and the ground cover approach. In most cases the FAO approach overestimated the transpiration coefficient. To overcome overestimation FAO proposes a reduction factor based on the stomatal resistance of vegetation with high stomatal control. This factor is difficult to obtain, however, and complicates the relatively simple method of estimation. Even if this reduction factor is taken into account, an overestimation of transpiration coefficients when using the LAI approach should still be acknowledged.