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 wer e measured during the 1992 HAPEX-Sahel field experiment, in Niger, Wes t Africa. Measurements were conducted on four land cover types: shrub fallow, grass fallow, degraded shrubland, and a millet field. Each lan d unit was equipped with sets of sensors to measure the photosynthetic ally active radiation (PAR) and near-infrared (NIR) radiative fluxes w ithin the canopies, and were operated throughout the entire growing se ason. Daily fractional PAR and NTR interception by vegetation was rath er low (less than 60% and 30% for natural vegetation and crop, respect ively). The sparse vegetation and bright sandy soils meant that the PA R absorption and interception were similar (they were equal at a value of approximately 20%). The albedo of the plots varied little diurnall y and seasonally, and was strongly affected by the reflection from the soil. The interception and absorption and, to a lesser degree, the al bedo exhibited distinct directional effects related to solar zenith an gle.