A near-infrared narrow-waveband ratio to determine Leaf Area Index in orchards

An innovative index for LAI estimation over orchard canopies based on the combination of two near-infrared narrow wavebands (1050 nm and 1250 nm) is presented. The index is insensitive to physiological changes on the leaf level even under changing pheneological developmental stage. In the meantime, the index is highly sensitive for variations at the canopy level. Values of LAI estimated by means of this newly developed index, called sLAIDI (standardized LAI Determining Index), resulted in excellent agreements with in-situ measured LAI values of citrus trees, with determination coefficients (R2) of 0.83. Based on in-situ measured data and simulated data (PROSPECT and ACRM), the newly developed sLAIDI was also found to perform outstanding at high LAI values in contrary to well-known LAI related indices such as the Normalized Difference Vegetation Index (NDVI) and the Modified Chlorophyll Ratio Index 2 (MCARI2) that saturate at LAI values of 5 and 8 respectively. Moreover, the sLAIDI index was independent of changing pigment contents. Consequently, this index was regarded a valuable tool to more accurately extract biochemical parameters after normalizing the considered spectral data for LAI. By implementing sLAIDI in existing chlorophyll-related indices (e.g., GM index), the performance to estimate the leaf chlorophyll content from spectral citrus and peach canopy level data is increased (R2 increases from 0.68 to 0.77). Moreover, a correction factor was proposed to enable the application of sLAIDI on fields with strong variations of water content.