Estimating Vegetation Water Content Based On Ground Observations And Modis Data In North China

The quantitative study of crop water content is important for estimating crop drought. In this paper, the characteristics of canopy spectrum of winter wheat under different soil moisture levels had been studied from the point of crop physiological drought using experimental data in the winter wheat field of Gucheng Station, Hebei Province, China.The main parameters describing the water content (WC) in vegetation which were usually investigated by remote sensing are Fuel Moisture Content (FMC, %) and leaf Equivalent Water Thickness (EWT, g/cm2 or cm). The red edge parameters, vegetation indices and double ratio spectral indices were computed from reflectance measurements, and then were used to establish the empirical models for the retrieval of water content. Also, the models had been evaluated using correlation coefficient, standard error and relative error. The models were validated using reflectance measurements at different growth stages of winter wheat. Then, correlation coefficient and relative error were also calculated. Finally, the crop moisture spatial distributions of winter wheat in North China were mapped using MODIS09A1 data according to the established model.The main conclusions in this study are as follows:(1) Comparing the relationships between water content and of water absorption band reflectance and red edge parameters, many discoveries were obtained. For the water absorption band, the correlation between FMC and R1450 was highest of all (R=-0.698), and the correlation between FMC and R1940 take the second place (R=-0.695).The correlation between R970 and EWT was strongest of all (R= 0.531). The relationship between FMC and red edge parameters showed a more steadily and significantly feature, the relative errors were less than 2%;The correlation between EWT and red edge parameters were not as better as that of FMC,but as a whole, the correlation coefficients were higher (the significance level was 0.01) . The results showed that red edge parameter is a good indicator to estimate WC.(2) In all of the vegetation indices (for the single index type), NDVI and SR can estimate both the FMC (R=0.837 for NDVI, RFMC=0.834 for SR)and EWT(R=0.683 for NDVI, REWT=0.672 for SR) with higher correlation coefficient. However, the NDII was the best indicator for estimate FWC (R=0.725) and EWT (R=0.378) in all of water indices. Whether the relationship between vegetation indices and water content good or not largely depended on the sensitive band that were used to build the indices. Vegetation indices can estimate FMC more successfully than EWT.(3) The correlation coefficients (R) between double ratio indices type and EWT (or FMC) showed significant improvement. In double ratio indices types, for the same water index (CwVI), the significant statistical relationships were also found for CwVI/SR while poorer correlations were achieved with CwVI/NDVI spectral indices. Double ratio indices may lead a way to improve the estimation of water content (WC).