MEASUREMENTS OF SOIL, PLANT AND TOTAL EVAPORATION FROM MILLET IN NIGER

In sparse dryland crops such as millet in West Africa a knowledge of total evaporation and its soil and plant components is important for the prediction of growth and yield, yet currently such data are very scarce. This paper presents measurements of soil, plant and total evaporation from millet crops grown during 1985, 1986 and 1987 at the ICRISAT Sahelian Center, Sadore, Niger. Total evaporation was measured using an eddy correlation technique, transpiration using a porometer, and soil evaporation using micro-lysimeters. Soil evaporation was found to be a very important component of the crop water use, indicating that it requires explicit recognition in models of evaporation from millet. Transpiration appears to be more controlled by leaf area than stomatal conductance, which may make its prediction comparatively simple. Total evaporation, as measured by Hydra, generally agreed well with the sum of the independently measured soil and plant evaporation. However, there remains a discrepancy at high leaf area which is thought to be due to over-estimation of transpiration as a result of poor leaf area sampling in a very heterogeneous canopy. Data on total evaporation for three seasons 1985, 1986 and 1987 indicate that accurate prediction of evaporation from rainfed millet may not be achievable using the conventional crop coefficient approach. However, there is potential for developing a simple model which recognises soil and plant evaporation separately for a better prediction of both evaporation and crop yield.