Agriculture is by far the most important economic activity in the Spanish autonomous region of Castilla y Leon. ´
Numerous factors influence crop development but one of the most related variables to the photosynthetic process
is Photosynthetically Active Radiation (PAR). Estimating Intercepted Photosynthetically Active Radiation (IPAR)
in different crops through the Beer-Lambert law could be a relevant factor in crop season planning by enabling
photosynthesis monitoring. The Beer-Lambert Law is applied in this study to the data for almost 2 million
hectares of wheat, barley, and maize cultivated in Castilla y Leon ´ in 2021. The fourteen-year data set of Global
Horizontal Irradiance (GHI) used to calculate the monthly PAR data in the region was collected at 93 meteo rological stations (46 in Castilla y Leon ´ and 47 in neighboring Spanish and Portuguese regions). Two previously
published global calibrated models were employed to calculate the PAR, with a relative Root Mean Square Error
(rRMSE) below 6%, for the measured daily mean values of PAR in Burgos. Processing the various NASA Terra and
Aqua satellite images yielded the monthly Leaf Area Index (LAI) and the literature review provided the light
extinction coefficient (k). The Geographic Information System (GIS) facilitated visualization of IPAR estimates for
the three cereal crops in all months of its growing season. Wheat and barley reach their IPAR peaks in June and
July, while maize peaks in July and August. In addition, the fraction of Intercepted Photosynthetically Active
Radiation (fIPAR) was calculated in different provinces to assess PAR interception for each cereal at different
growing stages. In June, almost 50% of the wheat area in Burgos, Palencia and Soria displayed fIPAR values
exceeding 45% while in the case of barley only the province of Burgos reached these percentages of area
and.fIPAR.
