JIA-2019-11

2633 ZHANG Xi-wang et al. Journal of Integrative Agriculture 2019, 18(11): 2628–2643 and the larger the proportion of other types, the smaller the slope. Therefore, the winter wheat acreage proportion in each MODIS pixel has a significant positive correlation with the slope of period CD. Winter wheat abundance assessment model Based on the above analysis, a regression analysis is employed to assess the abundance of winter wheat at the MODIS scale using temporal information. First, a correlation analysis is performed between abundance samples and time series curve slopes to rationally choose the periods for optimal abundance assessment. Then, the periods with significant correlation coefficients are selected to build regression models. For example, if the period XY is selected; the winter wheat abundance model will be constructed using eq. (2): − − = = + = X Y X Y XY XY XY XY / / b a TT NDVI NDVI ΔT ΔNDVI Slope Slope WWAF (2) where WWAF is winter wheat abundance, or the winter wheat acreage fraction in a MODIS pixel; Slope XY is the slope of period XY in the NDVI time series curve; NDVI X and NDVI Y are the corresponding NDVI values at times corresponding to points X and Y , respectively; T X and T Y are the corresponding times of points X and Y , respectively; X Winter wheat 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 289305 321337353 1 17 33 49 65 81 97 113 129145 161177 DOY NDVI 2003–2004 2004–2005 2005–2006 2006–2007 2007–2008 2008–2009 2009–2010 Forest 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 289305 321337353 1 17 33 49 65 81 97 113 129145 161177 DOY NDVI Shrub 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 289305321 337353 1 17 33 49 65 81 97 113 129 145161177 DOY NDVI Grass 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 289305321337353 1 17 33 49 65 81 97 113 129145161177 DOY NDVI Water –0.10 –0.05 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 289305321337353 1 17 33 49 65 81 97 113 129145161177 DOY NDVI Other 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 289305321 337353 1 17 33 49 65 81 97 113 129 145161177 DOY NDVI Fig. 3 Moderate resolution imaging spectroradiometer (MODIS) normalized difference vegetation index (NDVI) time series of different land cover types during the winter wheat growing season (from day 289 in the first year to 177 in the following year). DOY, the day of a year.