Estimation of leaf area index in onion (Allium cepa L.) using an unmanned aerial vehicle

Leaf area index (LAI) is one of the most common indices in agronomy, being a parameter associated with physiological processes. Canopy cover and structure are related to LAI and they have effects on the interaction between crops and the environment. The aim was to evaluate a non-destructive method to measure canopy cover in an onion crop using an unmanned aerial vehicle (UAV). A field experiment was conducted in a commercial onion plot irrigated with a centre pivot system during the 2010 irrigation season. Several data sampling events were carried out in order to determine leaf area in eight experimental plots. In each one of these plots, aerial photographs were taken using a vertical take-off and landing (VTOL) quadrotor aircraft. Canopy cover (CC) was obtained by means of software developed for this study. The maximum value of LAI represents a CC of 56%, which is high for the characteristics of this crop. Three models were used to analyse the relationship between leaf area index and canopy cover. According to the results, a more linear relationship was found between both parameters during early growth stages than during more advanced stages. For the linear model, which best fitted all growth stages; the slope that relates CC with LAI was 2.877 with a coefficient of determination of 0.837.