Mapping Primary Production for the West African Sahel with Satellite Data

Abstract: A light Use Efficiency (LUE) model is developed that allows the mapping of total growing season Gross Primary Production (GPP) for the West African Sahel, using the Normalized Difference Vegetation Index (NDVI) together with other data. Image quality may be poor in monthly NDVI maximum value composites as shown by an improved geostatistical noise estimation technique. Quality may be improved by other compositing methods that use NOAA AVHRR-derived surface temperature and scan angle information to reduce residual cloud amount and off-nadir bias. These data are then used in conjunction with ancillary information to map total growing season GPP using the LUE approach, which reduces the complexities of plant growth to a simple parametric statement. To overcome the lack of ground data, NOAA AVHRR-derived CLAVR (CLouds from AVHRR) fields are used to derive several key parameters of energy balance, including Photosynthetically Active Radiation (PAR). Fraction of absorbed Photosynthetically Active Radiation (FPAR) is calculated from the NDVI and multiplied with PAR to yield Absorbed Photosynthetically Active Radiation (APAR). A water stress scalar is estimated with a two-layer hydrological model that treats separately bare soil evaporation and transpiration. This scalar is used to reduce potential photosynthetic capacity in the LUE model, as defined by the product of APAR and the potential growth efficiency. The absolute precision of GPP estimates decreases for dense vegetation while the relative precision increases. LUE primary production estimates are systematically higher for dense vegetation when compared to point estimates from the CENTURY model. This bias is not apparent when compared to previous work reported in the literature. The LUE model may be used to address issues related to desertification, food security, and climate change.