Changes in reflectance anisotropy of wheat crop during different phenophases

Abstract

A b s t r a c t. The canopy structure of wheat changes significantly with growth stages and leads to changes in reflectance anisotropy. Bidirectional reflectance distribution function characterises the reflectance anisotropy of the targets, which can be approximated. Spectrodirectional reflectance measurements on wheat crop were acquired using a field goniometer system. The bidirectional reflectance spectra were acquired at 54 view angles to cover the hemispheric span up to 60° view zenith. The observations were made during early growth stages till maturity of the crop. The anisotropy was not constant for all wavelengths and anisotropic factors clearly revealed spectral dependence, which was more pronounced in near principal plane. In near infrared, wheat canopy expressed less reflectance anisotropy because of higher multiple scattering. The broad hotspot signature was noticeable in reflectance of canopy whenever view and solar angles were close. Distinct changes in bidirectional reflectance distribution function were observed during booting to flowering stages as the canopy achieves more uniformity, height and head emergence. The function clearly reveals bowl shape during heading to early milking growth stages of the crop. Late growth stages show less prominent gap and shadow effects. Anisotropy index revealed that wheat exhibits changes in reflectance anisotropy with phenological development and with spectral bands.

Determination of Spectral and Broadband Albedos in Visible-near Infrared Bands for Different Phenophases of Wheat Using Hemispherical Directional Reflectance Measurements

“…Wheat crop was considered in this study as the crop has large acreage than any other crop over the world. The canopy architecture of wheat changes dramatically at different phenophases during life cycle (Lunagaria and Patel, 2017) of the crop and leads to change in albedo. Thus, evaluation of magnitude of changes in albedo with changes in canopy parameters at different phenophases of wheat is attempted in the study.…”

“…The measurements were made at 7-8 days' interval from early growth stage (tillering) to dough stage of wheat. Comprehensive methodology and phenophase wise results on reflectance anisotropy using these data sets are explained in Lunagaria and Patel, 2017. Illumination geometry (solar zenith ( i ) and azimuth (φ i ) angles) for the experimental site was computed with algorithm given in Reda and Andreas (2003).…”

“…Crop breeding efforts to increase crop albedo can help to improve crop performance in drought and heat stress conditions by reducing transpiration and canopy temperature. Crops like wheat go through distinct canopy architectural changes during crop life cycle and markedly influence the canopy reflectance and its anisotropy (Lunagaria and Patel, 2017). The spectral albedo strongly depends on crop type, age of the crop and season (Iqbal, 1983).…”

In-Field Wheat Reflectance: How to Reach the Organ Scale?

“…In the field, reflectance measured at the canopy scale depends not only on organ anatomy and physiology but also on sun-sensor view geometry, canopy architecture, and soil background [7][8][9][10][11][12][13]. To further complicate the problem, all those factors have different impacts depending on the development stage of the crop [14]. One approach to extract crop properties based on canopy reflectance is to exploit vegetation indices [15], i.e., mathematical operations combining reflectance from several wavebands.…”