Landsat Thematic Mapper indices for evaluating management and growth characteristics of soybeans and corn

Landsat satellite Thematic Mapper (TM) data from 25 August 19.88 were used to extract information on corn and soybean crops in Seneca County, Ohio. Specific objectives included the evaluation of satellite TM data indices to: (a) quantify field differences in corn and soybean crop parameters at critical and senescing growth stages; (b) determine if the ground truth and satellite TM data could be used to identify and quantify soil type and agricultural management practice influences on crop development; and (c) evaluate the suitability of the ground truth data and identify data collection modifications that would improve the practicality of using satellite data. The TM data were evaluated in terms of brightness values and vegetation indices for each farm at a spatial scale of 10 pixels. Discrete ranges of the normalized difference vegetation index, NDVI, and the stress-related vegetation index, STVI, were used to identify crop management and soil differences. A comparison was made between using the following methods for quantifying ground truth crop parameters and then correlating them to satellite data: (a) measured data on the day of the satellite overpass; (b) estimates of crop characteristics based on fitting polynomial equations to data collected throughout the growing season; and (c) estimates based on a 3-point mean of data from the day of the overpass and the sampling episodes prior to and following the overpass. NDVI and STVI data correlated best to the polynomial equation estimates of crop variables. The correlation of individual indices with crop variables was generally poor but combinations of bands and indices can be used to describe between 50 and 70% of the between field variability in leaf area index, plant biomass, and crop yield. Generally, TM vegetation indices can be used to identify significant differences in crop responses to different management systems. These differences were mainly a function of plant density, management problems associated with drought conditions, and tillage.