Monitoring Crop Growth Using Narrow-Band Multispectral Imagery Acquired from an Unmanned Aerial Vehicle (UAV).

Affordable unmanned aerial vehicles (UAVs) become increasingly popular as a remote sensing platform for vegetation monitoring due to the flexibility of flying and consequent customizations of imagery data in temporal, spatial and spectral resolutions. Recent work has shown the potential of UAV-based remote sensing as a cost-effective way for real-time monitoring of crop growth conditions. While many studies focused on the UAV monitoring for a single period or part of crop growing seasons, this study evaluates a 6-band multispectral sensor on board a lightweight UAV for monitoring the growth of wheat and rice crops over the entire phonological cycle.

Aerial imagery was collected for the critical growth stages of wheat and rice crops from December 2013 to September 2014. A Tetracam Mini-MCA6 multispectral camera was mounted on the eight-motor ARF-MikroKopter OktoXL UAV. The camera has five channels with FHWM values of 10 nm and center wavelengths at 490, 550, 680, 720 and 800 nm. The sixth channel is equipped with Incident Light Sensor for capturing downwelling radiation and converting pixel values in the five channels to reflectance values. A total of 36 plots (5×7 m each) were randomly distributed in an experimental field to represent three nitrogen fertilizer rates, two varieties, two planting density patterns and three replicates. Concurrent with aerial imagery acquisition, ground-level measurements were collected for canopy spectral reflectance and crop biophysical and biochemical properties including leaf area index (LAI), aboveground biomass, leaf chlorophyll content, leaf nitrogen content and leaf nitrogen accumulation. Vegetation indices calculated from the multi-temporal aerial imagery will be related to crop biophysical/biochemical variables for model construction. A number of spatially explicit maps will be produced for assessing the spatial and temporal variation in the growth status of crops. These maps are useful for guiding nitrogen fertilizer applications and developing informed strategies for crop management.