Estimating GPP for the Sahel region
Physical Geography and Ecosystem Science, Lund University, SWEDEN
MOD17A2 provides operational gross primary production (GPP) data globally at 1 km spatial resolution and 8-day temporal resolution. MOD17A2 estimates GPP according to the light use efficiency (LUE) concept assuming a fixed maximum rate of carbon assimilation per unit photosynthetically active radiation (PAR, [MJ]) absorbed by the vegetation (εmax, [g C MJ-1 ]). Minimum temperature and vapour pressure (VPD) deficit derived from meteorological data down-regulate εmax and constrain carbon assimilation. This data is useful for regional to global studies of the terrestrial carbon budget, climate change and natural resources.
Recent studies, based on eddy covariance (EC) flux data, suggest that current εmax may be too low, especially for drier sites in Africa, hence resulting in underestimations of MOD17 GPP. Uncertainties in driver data (temperature, VPD and incoming PAR) may cause further uncertainties in estimated GPP within the MOD17 algorithm.
This study quantifies the effect of using current MOD17 εmax versus using increased εmax while estimating GPP for savannahs and grasslands in the Sahel region (10-20 ° N). Observed GPP in the Sahel region, based on eddy covariance flux data is about 1.5 x MOD17 GPP for Savannas and about 2.5 x MOD17 GPP for grasslands. Here, GPP are summarized per land cover type for the Sahel region and the effects of εmax, ranging from 1.21 and 1.66 [g C MJ-1] for savannahs and ranging from 0.86 and 2.01 [g C MJ-1 ] for grasslands are quantified. Assuming EC measured GPP to be correct, the effect of 1.5 x MODIS GPP for Savannahs and 2.5 x MODIS GPP for grasslands were quantified, GPP for other land cover types remaining unchanged. Applying this assumption spatially uniformly suggest an increased GPP, versus current MOD17 estimate for Sahel with about 1.34 Pg 45%, from an average (2000-2011) of 2.91 Pg yr-1 to an average of 4.25 Pg yr-1 .
In addition to low εmax, uncertainties in the fraction absorbed PAR (FAPAR) may contribute to biased and uncertain GPP. Collection 5 of MOD17 include a quality assessment control (QC), describing the percentage of growing days that use artificially filled FAPAR and LAI (leaf area index) due to cloud cover to when calculating annual GPP. For most areas of Sahel are QC >40%, indicating potential uncertainties originating from FAPAR and LAI.
Additionally is MOD17 GPP compared to GPP originating from an Ecosystem model (LPJ-GUESS), providing GPP at spatial resolution of 0.5 by 0.5 degrees and monthly temporal resolution. Differences and similarities of included estimates of GPP for the Sahel region are discussed, including both spatial and temporal aspects as well as potential errors and sources of uncertainty.