Detecting Crop Lodging Stage using SAR-Derived Crop Angle of Inclination

Lodging is one of the yield-reducing factors in wheat, causing losses of up to 80%. Lodging also has several knock-on effects such as deterioration in grain quality, slower harvest and increased drying costs. The severe effects of lodging can be minimised by detecting its incidence in time so that decisions related to expected crop yield, crop price or its insurance can be made timely. The traditional methods of lodging assessment require the on-site assessment to rate lodging state visually or through manual measurements of the inclination angle of the plant base from the vertical (lodging angle). However, such field estimates can be biased and subjective since they depend upon the skill of the observer or the complexity of the observation. A rapid and quantitative evaluation of crop lodging at field/farm scale can contribute to more accurate assessment of yield loss. The aim of this study is to apply remote sensing (RS) information to estimate the crop angle of inclination (CAI) and detect the stage of lodging (slight lodging, moderate lodging,…). Such quantitative estimates related to crop condition can facilitate prediction of yield losses. Studies show that yield losses incurred at 80o can be almost 2-4 times than those at 45o in wheat.

This study investiagtes the ability of multi-temporal Sentinel-1 along with multi-incidence angle and fully polarimetric Radarsat-2 SAR data for estimating CAI and to assess the lodging stage. Ground measurements of CAI, height and other structural/biophysical properties (e.g. LAI, biomass) were made on wheat in the Bonifiche Ferraresi farm (Jolanda di Savoia, Italy) from October 2017 to July 2018. The crop lodging condition (lodged/non-lodged and stages) was also recorded. A total of 35 Sentinel-1 and 20 corresponding time-series Radarsat-2 single look complex SAR images were acquired (coincident to the field measurements) covering all the growth stages of wheat. The Radarsat-2 data was acquired in four beam modes: FQ3, FQ8, FQ21 and FQ26. Both empirical and semi-empirical approaches were tested to test the angular responses of radar backscatter and SAR polarimetric parameters derived from Sentinel-1 and Radarsat-2 for the wheat canopy. Significant correlations were observed between SAR polarimetric parameters/backscatter ratios and the CAI of wheat from both sensors. From this analysis it was possible to assess crop condition in a scale comprising of five different lodging stages: upright (angle up to 4o from vertical); leaning crop/slight lodging (5-40o); moderate lodging (41-60o); severe lodging (61-75o); and very severe lodging (76-90o). This study is a step towards quantitative evaluation of lodging stages and further illustrates the capability of SAR data for lodging assessment.