Retrieving crop physiological parameters and assessing water deficiency using MODIS data during the winter wheat growing period

The potential of moderate-resolution imaging spectroradiometer (MODIS) time-series in monitoring crop condition was examined in the Weishan Irrigation Zone along a downstream reach of the Yellow River, China. First, MODIS-derived spectral indices, including the normalized difference water index (NDWI) and normalized difference vegetation index (NDVI), were employed to retrieve the crop physiological parameters (such as vegetation water content and dry matter) during the winter wheat growing season. NDWI was found to be less susceptible than NDVI to the directional effects induced by the changes of illumination and view angles. NDWI1640 and NDW2130, which were based on the shortwave infrared (SWIR) 1640 mn and 2130 mn bands, showed great potential for vegetation water content (VWC) estimation and performed better than NDVI and NDWI1240. Both NDW11640 and NDW12130 had a significant positive correlation with dry matter content before the blooming stage. A significant negative correlation between NDW12130 and dry matter content was found after the blooming stage. Second, NDWI was also employed to assess the canopy water deficit during the same period as a direct indicator of crop water content. The strong correlation between NDWI and soil moisture after the jointing stage showed that NDWI can also be utilized as an indicator of soil moisture changes. An integrated analysis of the temporal variation of NDWI and the observed VWC demonstrated that the wheat was under water stress to some extent during the later part of the growing period in 2006, and this was supported by a field experiment.