Cartography and use of wheat foliar index from radiometric data acquired by tractor-mounted sensors

Abstract

Estimating the leaf area index of wheat along the crop cycle allows to answer two important purposes: (i) to be able to predict the crop nitrogen needs by using measurements from early stages; (ii) to use the green leaf area along the whole cycle in order to establish, by means of previous soil and yield maps, a diagnosis on the state of the crop from the intrafield heterogeneousness observed. With this aim in view, a georeferenced radiometrical data system, mounted on a tractor was finalized. The system is made up of (i) four twin-channels radiometers (red and near infrared), mounted on a 9m-long boom and measuring the reflected light by the canopy, plus an hemisphere viewing radiometer measuring the incident light, (ii) an electronic interface, and (iii) a computer for data acquisition and storage. Geopositioned data were acquired at 3 dates over a 15 ha field where non uniform agronomical factors were applied. The data were used to cartography the multidate GLAI after anthesis. A linear model explaining the GLAI was tested (r2=0.87). The soil factor was found as having the most important contribution to the data variance. Globally the acquisition and processing methodology prooved its ability to deliver pertinent spatial and temporal information on crop in order to control the growth or post-anthesis vegetation status.