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Intercepted photosynthetically active radiation (IPAR) is an important variable in vegetation processes such as photosynthesis, and water and energy exchange. Measurement of this quantity and the related fractional interception efficiency (f(PAR)) can be time consuming because of the need to sample for spatial and temporal variability. A method is presented for estimation of IPAR and f(PAR) using a commercially available hemispherical radiation sensor (Li-Cor LAI-2000) originally designed for the estimation of canopy leaf area index. This instrument provides sufficient information on the structure of the canopy and the angular dependence of light interception for single measurements to be interpreted for all solar zenith angles. These data can be used to infer instantaneous direct and diffuse interception as well as the daily integrated values.

The method was tested in a millet crop and a shrub fallow area in semi-arid West Africa. LAI-2000 estimates were compared with a more traditional method using PAR sensor arrays. The errors in estimation of instantaneous IPAR were small at most times of day, rising to a maximum of 15-30 W m-2 at solar noon, when incident PAR was more than 400 W m-2. Instantaneous f(PAR) estimates exhibited some bias towards underestimation at small solar zenith angles, and overestimation at larger angles, particularly on the shrub fallow site. These errors were generally small (much less than 0.1 unit of f(PAR)). Daily IPAR was estimated to within 7% of the direct PAR sensor measurements.