Shortwave radiation budget of Sahelian vegetation .1. Techniques of measurement and results during HAPEX-Sahel

Shortwave radiative budgets of Sahelian savannas and a millet crop were measured during the 1992 HAPEX-Sahel field experiment, in Niger, West Africa. Measurements were conducted on four land cover types: shrub fallow, grass fallow, degraded shrubland, and a millet field. Each land unit was equipped with sets of sensors to measure the photosynthetically active radiation (PAR) and near-infrared (NIR) radiative fluxes within the canopies, and were operated throughout the entire growing season. Daily fractional PAR and NTR interception by vegetation was rather low (less than 60% and 30% for natural vegetation and crop, respectively). The sparse vegetation and bright sandy soils meant that the PAR absorption and interception were similar (they were equal at a value of approximately 20%). The albedo of the plots varied little diurnally and seasonally, and was strongly affected by the reflection from the soil. The interception and absorption and, to a lesser degree, the albedo exhibited distinct directional effects related to solar zenith angle.