Abstract
The spatial quantification of green leaf area index (LAIgreen), 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 LAIgreen index for the Sentinel-2 mission. Two large and independent multi-crop datasets of in situ collected LAIgreen measurements were used. Commonly used LAIgreen indices applied on the Sentinel-2 10 m × 10 m pixel resulted in a validation R2 lower than 0.6. By calculating all Sentinel-2 band combinations to identify high correlation and physical basis with LAIgreen, the new Sentinel-2 LAIgreen 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 R2 of 0.708 (root mean squared error (RMSE) = 0.67) and a R2 of 0.732 (RMSE = 0.69) were obtained with a linear fitting for the calibration and validation datasets, respectively, outperforming established indices. Sentinel-2 LAIgreen maps are presented.
Highlights
Leaf area index (LAI), or the total one-sided leaf area per unit of ground area (m2 leaf per m2 surface or dimensionless), can be distinguished in two types
Numerous vegetation indices (VIs) have been proposed for the estimation of green leaf area index (LAIgreen ) over various crop types, but the general problem appears when they are applied to multi-crop datasets, obtaining low estimation accuracies and at LAIgreen values higher than 2, saturation problems appear
Based on the availability of Sentinel-2 narrow red-edge bands, we explored new index possibilities for accurate LAIgreen estimations for heterogeneous agricultural areas, based on spectral areas influenced mainly by photosynthesis-related absorption regions
Summary
Leaf area index (LAI), or the total one-sided leaf area per unit of ground area (m2 leaf per m2 surface or dimensionless), can be distinguished in two types. There is the green leaf area index (LAIgreen ), representing the leaves which are photosynthetically active, being the most common type of LAI [1], and, on the other hand, there is the brown leaf area index (LAIbrown ), representing the leaf area normalized which is senescent and losing photosynthetic function [2]. Its determination is crucial for the understanding of biophysical processes of crop canopies, being the main morphological parameter used for determining crop growth through the correlation with crop productivity [1,9,10]. In the context of agricultural monitoring, there is a strong interest in estimate LAIgreen parameter
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