# Regional Crop Gross Primary Productivity and Yield Estimation Using Fused Landsat-MODIS Data
## 3 Results  
### NDVI Fusion  
An example comparison showing the baseline Landsat and MODIS NDVI records relative to the fused 30-m NDVI record developed from this study is shown for the Valley County sub-region of MT (Figure 2). The Landsat 5 (or 7) NDVI record has extensive gaps in spatial and temporal coverage relative to the MODIS record due to less frequent Landsat temporal sampling (16-day revisit cycle) and data loss from atmosphere aerosol and cloud contamination effects, as well as the SLC-off issues in Landsat 7 (Figure 2a). Nevertheless, the Landsat data provide approximately eight-fold improved spatial resolution NDVI observations relative to MODIS, which enhances the delineation of agricultural fields and heterogeneous crop types. However, the MODIS (MOD09Q1) NDVI record (Figure 2b) provides complete spatial coverage and continuous eight-day sampling. The resulting fused NDVI record (Figure 2c) benefits from the combined qualities of both Landsat and MODIS, while minimizing the limitations of the individual sensor records. For Valley County and other MT areas, the fused NDVI 30-m record shows similar spatial patterns and magnitudes as the component Landsat and MODIS NDVI records. The fused NDVI record also provides complete spatial coverage with continuous eight-day sampling at 30-m resolution over the entire MT study domain and 2008 to 2015 record. Figure 2: Examples of (**a**) 30-m NDVI derived from Landsat 5 surface reflectance data after removing cloud-contaminated pixels; (**b**) 250-m NDVI from the MODIS MOD09Q1 eight-day temporal composite product; and (**c**) 30-m NDVI created by fusing Landsat and MODIS data from (**a**,**b**). The NDVI metric ranges from zero to one between respective low to high levels of vegetation greenness. The grey area represents no NDVI data. All three images are obtained for the selected period during 1 to 8 July 2008 over Valley County, Montana. For the entire MT domain, the Landsat 5 and MODIS NDVI records were favorably correlated (\(r\) = 0.78 \(\pm\) 0.22, \(p\) < 0.05) for the 2008 to 2011 portion of the study period overlapping with Landsat 5. Similar favorable correlations were also found between Landsat 7 and MODIS NDVI records for the 2012 to 2015 portion of the study period (\(r\) = 0.77 \(\pm\) 0.21, \(p\) < 0.05). These results indicate suitable conditions for deriving a fused NDVI record using the linear regression model relationship between the Landsat and MODIS NDVI records. One pixel for each crop type, including alfalfa, barley, maize, peas, durum wheat, spring wheat and winter wheat, was randomly selected within the MT domain and over the 2008 to 2015 study period to illustrate the NDVI seasonal variations from Landsat 5 (LS5), Landsat 7 (LS7), MODIS and the fused NDVI records derived from MODIS and Landsat 5 (LS5_fused) and Landsat 7 (LS7_fused) inputs (Figure 3). The NDVI time series from the different sensor products in each plot in Figure 3 have consistent geographic coordinates and overlapping time periods, but have varying footprints due to the different pixel sizes represented from each data record. All of the NDVI records show similar seasonality for each of the seven crop types, with seasonal maximum NDVI values generally occurring in the summer (June to August) and early spring (February to April) and seasonal minimum NDVI values occurring in late autumn (September to October) and winter (December to January) periods. The LS5 and LS7 records have relatively sparse growing season temporal coverage from 2008 to 2015 due to cloud contamination and long revisit cycle (16-day), while MODIS NDVI and the two fused NDVI records provide continuous eight-day time series spanning the entire study period. The LS5 and MODIS NDVI records were well correlated for each MT crop type during the overlapping period from 2008 to 2011 (0.91 \(\leq r\leq\) 0.98, \(p\) < 0.05). The LS7 and MODIS NDVI records for the seven MT crop types also showed strong correspondence for the overlapping period from 2012 to 2015 (0.93 \(\leq r\leq\) 0.99, \(p\) < 0.05).