Errors in remote sensing of intercepted photosynthetically active radiation: an example from HAPEX-Sahel

Canopy PAR interception (IPAR) is an important variable in many models of canopy photosynthesis and net primary production. IPAR can be estimated remotely using spectral vegetation indices (VI). However, IPAR estimates by this method are subject to errors (E) associated with the estimation of fractional interception (fPAR) from a VI measurement (EF), the accuracy of estimates of incident PAR (ET), and the interaction of these errors (EX). During the HAPEX-Sahel experiment in Niger in 1992 the temporal evolution of a VI was measured over the study sites with a radiometer mounted in a light aircraft. Ground measurements of fPAR and incident PAR on HAPEX West Central shrub fallow, grass fallow, degraded shrub fallow and millet sites were available. The fPAR measurements were used to determine the relationship between the VI and fractional interception. The aircraft measurements were then used, together with remote estimates of incident PAR from the TOMS satellite, to estimate IPAR through the season and to quantify the error sources. Both EF and ET can result in large absolute and relative errors in IPAR estimates. Ex was generally small. In ten-day summations on the shrub and grass fallow sites, the total errors were mostly less than 20% of the ground measurements. On the millet and degraded shrub fallow, the ten-day errors were more substantial. The errors at short time steps (1-10 days) cancel when summed for the whole growing season. In applications where the precise daily errors in IPAR estimates cannot be quantified, the statistical uncertainty of the estimates can be evaluated if the uncertainty in the input variables is known. The standard error of IPAR estimates is then dependent on the residual variance in the regression between vegetation index and fPAR and on the mean square error of the remote estimates of incident PAR.