Testing the LI-strahler four-component canopy reflectance model in the hapex-sahel shrub savanna sites using ground reflectance data

The Li-Strahler [1] discrete-object canopy reflectance model was tested in two sites, a shrub grass savanna and a degraded shrub savanna on bare soil, in the proposed HAPEX II/Sahel study area in Niger, West Africa. Average site reflectance was predicted for each site from the reflectances and cover proportions of four components: shrub canopy, background (soil or grass and soil), shaded canopy and shaded background. Component reflectances were sampled in the SPOT wavebands using a hand-held radiometer. Predicted reflectance was compared to average site reflectance measured using the same radiometer mounted on a backpack with measurements recorded every 5 m along two 1 km transects, also in the SPOT bands. Measurements and predictions were made for each of three days during the summer growing season, approximately two weeks apart Red, near infrared reflectance and the normalized difference vegetation index (NDVI) were all predicted with a high degree of accuracy for the shrub/grass site and reasonable accuracy for the degraded shrub site. The cause of interdate variation in the accuracy of the predictions has not been determined, but may indicate the sensitivity of the model to the component reflectances used as parameters.