Diurnal Trends of Maize Canopy Cover Under Water Stress

Highlights RGB trail cams above maize crop collected images every 15 minutes, day and night, to monitor leaf curl. Fractional canopy cover ( fc ,%) was constant for full irrigation but reduced during the day for limited irrigation. Techniques were developed to estimate reductions in water use (i.e., evapotranspiration) using images of fc. Measurements of fc prior to wilting represent transpiration demand ( K cb ), and reductions of fc represent stress ( K s ). Abstract. By differentiating green and non-green pixels of RGB images of crop canopy, the fractional canopy cover (fc,%) can be estimated and subsequently used to estimate crop transpiration demand (e.g., K cb , basal crop coefficient, following FAO-56 methodology). While unstressed maize crops maintain consistent fc through the day, leaves of heat or water stressed crops will curl, thereby reducing the instantaneous fc and the associated transpiration. This paper reports the effects of diurnal leaf curl and subsequent fc reduction on full and limited water treatments of irrigated maize through nadir images obtained every 15 minutes. Results suggest that fc images obtained in the early morning (fc NS ) are typically representative of a non-stressed condition and can be used to estimate K cb . Reduction of fc throughout the day (i.e., fc/fc NS ) was compared to water stress coefficient K s obtained from neutron probe and TDR measurement and showed that fc/fc NS from hours 11 a.m. to 2 p.m. had a high correlation (R 2 = 0.773), indicating that mid-day fractional canopy cover reduction could be used as a proxy for K s . This method creates new opportunities for estimating crop water stress during vegetative growth but also highlights potentially unknown challenges regarding lower resolution remote sensing during the same growth stages. Keywords: Basal crop coefficient, Canopy cover, Crop coefficient, Evapotranspiration, Stomatal conductance, Stress crop coefficient, Transpiration, Water stress.