Tall tower based evaluation of the MOD17 model over heterogeneous agricultural vegetation

We used remote sensing data and the MOD17 model to estimate GPP of a region in Western Hungary covered by heterogeneous agricultural vegetation. Validation was carried out using tall tower eddy covariance measurements. The land cover in the investigated area can be characterized by small individual parcels of mainly maize and winter wheat. The evaluation of the original, 1 km spatial resolution MOD17 product against tall tower data showed discrepancies in certain years. Errors in meteorological data, together with the mismatch between the representativeness of tall tower and remotely sensed data can partially explain the differences. The real performance of the model can only be evaluated after the elimination of these effects.

The use of local meteorological measurements can help to overcome errors in reanalysis data originally used in the product. In order to spatially synchronize the model calculations and tower measurements, we used the results of a footprint model, and scaled down the model using 250 m resolution MODIS NDVI. The improved model utilizes the temporally varying footprint information (regridded to the same 250 m MODIS grid) and finer resolution NDVI (instead of 1 km MOD15 FPAR). GPP is calculated using NDVI data only from those pixels where the flux is originated from, according to the footprint model.

Given the crop coverage information, this setup provides the possibility of crop specific model calculations. The results can be compared to crop specific tower measurements. It is clear from the results, that although both errors in meteorology and the spatial mismatch affect modeling results, appropriate calibration of the model is necessary.