Estimating leaf area index of maize using UAV-based digital imagery and machine learning methods.

Leaf area index (LAI) is a fundamental indicator of crop growth status, timely and non-destructive estimation of LAI is of significant importance for precision agriculture. In this study, a multi-rotor UAV platform equipped with CMOS image sensors was used to capture maize canopy information, simultaneously, a total of 264 ground-measured LAI data were collected during a 2-year field experiment. Linear regression (LR), backpropagation neural network (BPNN), and random forest (RF) algorithms were used to establish LAI estimation models, and their performances were evaluated through 500 repetitions of random sub-sampling, training, and testing. The results showed that RGB-based VIs derived from UAV digital images were strongly related to LAI, and the grain-filling stage (GS) of maize was identified as the optimal period for LAI estimation. The RF model performed best at both whole period and individual growth stages, with the highest R (0.71-0.88) and the lowest RMSE (0.12-0.25) on test datasets, followed by the BPNN model and LR models. In addition, a smaller 5-95% interval range of R and RMSE was observed in the RF model, which indicated that the RF model has good generalization ability and is able to produce reliable estimation results.