94 J.Bai,J.Li,and S.Li for each growing stage is 0.11,0.27.0.53 and 0.82.On May 24,the range of EVI distribution is narrow and it shows insignificant linear relationship between EVI and PDC.However,EVI distribute more widely on June 9 and June 25,and it shows the obvious linear relationship.EVI on June 25th is higher than on June 9,the correlation degree isn't kept increasing tendency as time goes on,EVI difference on July 11 is less than on June 25th and the linear isn't significant.From the above analysis,we could deduce the emergence time caused the difference of seedling size,which affects the linear between PDC and EVI.Furthermore,the mixed pixels with non-cotton information decrease the relation on May 24%h and July 11th,and the relations aren't nearly interrupted on July 9th and July 25 coefficients bety .the estima June9:N=2.61EV+1.22 R2=0.23 (6) June25:N=2.41EV+0.65 R2=0.30 。 · 12 A. 26 26 24 。 14 2 g。 04045 Fig.2.Relationship between PDC and EVI 94 J. Bai, J. Li, and S. Li for each growing stage is 0.11, 0.27, 0.53 and 0.82. On May 24th, the range of EVI distribution is narrow and it shows insignificant linear relationship between EVI and PDC. However, EVI distribute more widely on June 9th and June 25th, and it shows the obvious linear relationship. EVI on June 25th is higher than on June 9th, the correlation degree isn’t kept increasing tendency as time goes on, EVI difference on July 11th is less than on June 25th and the linear isn’t significant. From the above analysis, we could deduce the emergence time caused the difference of seedling size, which affects the linear between PDC and EVI. Furthermore, the mixed pixels with non-cotton information decrease the relation on May 24th and July 11th, and the relations aren’t nearly interrupted on July 9th and July 25th. The correlation coefficients between PDC and EVI haven’t reached significant level on May 24th and July 11th and reached significant level on June 9th and June 25th, and correspondingly R2 is 0.23 on June 9 and 0.30 on June 25. Therefore, the estima￾tion models on PDC with EVI as below: June 9th : Nu=2.61EVI+1.22 R2 =0.23** (6) June 25th : Nu =2.41EVI+0.65 R2 =0.30** (7) R2 1, R2 2 showed the coefficient of determination from three sowing dates and middle sowing date. △ the late sowing; × the early sowing; ■ the middle sowing; ◇ at the edge of farmland, and the same as below. Fig. 2. Relationship between PDC and EVI