MEASUREMENTS OF ALBEDO VARIATION OVER NATURAL VEGETATION IN THE SAHEL

This paper reports ground-based measurements of albedo at two different sites in south-western Niger: agricultural fallow at the ICRISAT Sahelian Center (ISC), Sadore, and a type of open natural forest (tiger-bush), 6 km south-west of ISC. The vegetative cover at each site consisted of two clearly defined components, for which separate measurements were made. The four different component surfaces spanned the entire range of plant cover density normally occurring in the southern Sahel, ranging from zero (bare soil) to a dense, closed canopy of woody shrubs. Continuous automated observations of albedo were made over a period of 15 months, from July 1989 to September 1990, including both wet and dry seasons. The data are analysed on hourly, monthly, and annual time-scales in order to demonstrate how albedo varies in response to the density of plant cover, soil-surface moisture content, solar zenith angle, and the proportion of diffuse light in the incoming solar radiation. Large annual variation in monthly mean albedo was observed at both sites (increases from the wet to dry season of 0.065 and 0.057 for the fallow and tiger-bush, respectively). At the fallow site the annual variation in albedo resulted mainly from the wet to dry season cycle of leaf growth and loss. At the tiger-bush site the primary cause of annual variation in albedo was the frequent wetting of the extensive bare soil component that occurred during the rainy season. The significance of these results for global climate modelling is assessed briefly.