[Correlations between spectral characteristics and diurnal CO2 budget of winter wheat field on Loess Plateau].

Based on the continuous observation on the spectral reflectance and CO2 flux during the whole growth period of winter wheat, the correlations between the variations of spectral characteristics and the diurnal CO2 budget of winter wheat field on Loess Plateau were analyzed. The results showed that the spectral reflectance at different wavelengths and the NDVI changed with the growth stages of winter wheat, and the diurnal spectral reflectance changed with solar altitude angle, with the wavelengths at which the reflectance changed most being at 550 nm and 700-1050 nm. At the same time period of different growth stages, the spectral reflectance changed little at 350-670 nm but varied greatly at 700-1050 nm. The red edge position shifted to longer wavelengths at seedling and tillering stages and by the end of wintering while to shorter wavelengths at the beginning of wintering, but the shift to shorter wavelengths was not obvious at ripening stage. No spectrum shift was observed at other growth stages. The diurnal NDVI changed in U-shape, reaching to the minimum at about 13:00 and fluctuating after 16:00, and the change pattern could be fitted by a parabola. Therefore, for winter wheat field, the best time for remote sensing observation should be selected at about 13:00 when the NDVI changed little, except in winter. After the 140th day of seeding, the reflectance and NDVI at 11:00 were symmetrical, and NDVI changed in M-shape, being able to be simulated by a quartic polynomial. There was a significant negative correlation between the NDVI and the diurnal CO2 budget during the whole growth period of winter wheat, but the correlation was weaker around midday.