REMOTE SENSING OF REGIONAL CROP TRANSPIRATION OF WINTER WHEAT BASED ON MODIS DATA AND FAO-56 CROP COEFFICIENT METHOD

Crop evapotranspiration is one of the most important parameters of farmland water cycle, which consists of crop transpiration (T-c) and soil evaporation. As the efficient component for crop production, T-c and its accurate determination, especially on a regional scale, is very critical for scientific design of irrigation scheduling and high-efficiency utilization of water resources. In this work, the T-c of winter wheat over an irrigation area located in the lower Yellow River of China was estimated by combining MODIS data and FAO-56 crop coefficient method. Specifically, the relationships between the single crop coefficient (K-c), basal crop coefficient (K-cb) and canopy vegetation indices were investigated and compared based on field data. Then, the actual K-cb map of winter wheat over the study area was estimated with MODIS-derived soil adjusted vegetation index (SAW) using the relationship obtained from above field investigations. Finally, the T-c of winter wheat over the area was determined as the product of K-cb and reference crop evapotranspiration (ET0). ET0 was calculated from meteorological data, and then were spatially interpolated to obtain the regional map matching with the remotely sensed K-cb. It was found that compared with K-c, K-cb was much more closely related to the vegetation indices of NDVI, SAVI, and EVI, even in the presence of nitrogen and water stress, with the coefficients of determination (R-2) being 0.60, 0.67 and 0.68 respectively (n = 195) which could be even higher without the water-stress points that had not reached the severity to make obvious changes in canopies. Results also demonstrated that it was feasible to utilize the K-cb-SAVI relationship to derive the K-cb of winter wheat over a large area by means of satellite remote sensing, and that it was effective to determine regional crop T-c using the above approach. It would be useful in practical application due to the advantages of easy operation and separating soil evaporation effectively.