UAV Assessment of Crop Evapo-transpiration Dynamics in Winter Wheat and Barley under Varying Pressures of Fungal Diseases

Early detection of the onset of disease in crops allows for more timely and more effective management. Often it is too late to treat diseases once the clinical symptoms are visible to the human eye. To this end, there is great potential to use sensors operating in the infra-red and thermal bands (outside our visible range) to detect diseases before they become visible. Vegetative indices based on near-infrared reflectance responses are often used for this. However, vigor is a delayed response to plant function. The rate of transpiration is a more immediate indicator of plant function and health. Estimations of daily ET rates from thermal satellite imagery have been shown in several studies. The general process-based physical surface energy balance (SEBS) method used was adapted and applied for the first time to Unmanned Aerial Vehicle (UAV) collected thermal and RGB imagery using off-the-shelf low cost camera. On-site COSMOS-UK weather station data were used for meteorological inputs. The ILWIS2 Surface Energy Balance System (SEBS) was used for the daily ET modeling. The daily wheat and barley ET measurements for early May and June 2015 were similar to values obtained from Landsat imagery over nearby cereal fields and closer to the Penman-Monteith calculations for the survey days. This indicates promising transformation from satellite to UAV imagery for estimating ET. Varied spatial patterns were visible in the imagery corresponding to environmental (soil), variety and treatment (fungicide) differences. Barley exhibited little disease pressure at any stage of the season. Disease pressure was not visible at mid-late season (early May) in wheat, but susceptible varieties had visible late season rust infections. ET was a better discriminator of non-visible infections in May than NDVI. Further, studies are required to further validate this proof of concept.