黄土高原冬小麦田光谱特征变化及其与二氧化碳日收支的相关分析

根据光谱辐射仪对黄土高原冬小麦整个生育期光谱反射率的连续观测数据及CO₂通量观测数据，对冬小麦田光谱特征变化及其与CO₂日收支的相关性进行了分析.结果表明：冬小麦田不同波长光谱反射率和归一化植被指数（NDVI）呈现明显的日变化和季节变化.同一天内，反射率随太阳高度角的变化而变化，变化最大的波段（550 nm左右、700～1 050 nm）表现为峰.不同生育期同一时刻，可见光波段（350～670 nm）反射率变化不大，近红外波段（700～1 050 nm）出现较大差异，在出苗期、分蘖期和越冬期后红边位置向长波方向“红移”；越冬期前出现向短波方向“蓝移” 的现象；但成熟期“蓝移”现象不明显，表现为突变；其他生育时期没有观测到波谱位移.NDVI的日变化呈U型，13：00左右最低，16：00后出现较大波动，与抛物线有较好的拟合效果，小麦生长旺盛时期，对其地面遥感观测应选择在NDVI变化不大的13：00左右进行；整个冬小麦生长季11:00反射率及NDVI以播种后第140天为中心对称，NDVI的季节变化表现为M型，可用四次多项式拟合；在整个小麦生育期中NDVI与CO₂的日收支呈极显著负相关，但正午左右的相关性稍差.

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 700-1 050 nm. At the same time period of different growth stages, the spectral reflectance changed little at 350-670 nm but varied greatly at 700-1 050 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.