Multi-Crop Green LAI Estimation with a New Simple Sentinel-2 LAI Index (SeLI)

The spatial quantification of green leaf area index (LAI(green)), the total green photosynthetically active leaf area per ground area, is a crucial biophysical variable for agroecosystem monitoring. The Sentinel-2 mission is with (1) a temporal resolution lower than a week, (2) a spatial resolution of up to 10 m, and (3) narrow bands in the red and red-edge region, a highly promising mission for agricultural monitoring. The aim of this work is to define an easy implementable LAI(green) index for the Sentinel-2 mission. Two large and independent multi-crop datasets of in situ collected LAI(green) measurements were used. Commonly used LAI(green) indices applied on the Sentinel-2 10 m × 10 m pixel resulted in a validation R(2) lower than 0.6. By calculating all Sentinel-2 band combinations to identify high correlation and physical basis with LAI(green), the new Sentinel-2 LAI(green) Index (SeLI) was defined. SeLI is a normalized index that uses the 705 nm and 865 nm centered bands, exploiting the red-edge region for low-saturating absorption sensitivity to photosynthetic vegetation. A R(2) of 0.708 (root mean squared error (RMSE) = 0.67) and a R(2) of 0.732 (RMSE = 0.69) were obtained with a linear fitting for the calibration and validation datasets, respectively, outperforming established indices. Sentinel-2 LAI(green) maps are presented.