Estimating Crop Canopy Coverage of Cotton Plants Within the FOV of an Infrared Thermometer Using a Two Band Photodiode Sensor.

 The majority of irrigated area in the Southern High Plains region of Texas is by center pivot systems; all are drawing water from the Ogallala Aquifer. Automating the center pivot system to schedule irrigations automatically based on crop canopy temperature measured with infrared thermometers (IRT), may improve water use efficiency and reduce Ogallala withdrawals. Although canopy-temperature-based methods have been used with success, there exists the possibility of false positive irrigation cues early in the growing season due to the contribution of high temperatures from background soil. The composite surface temperature must be partitioned into its soil and canopy components based on the fraction of vegetation appearing in the IRT field of view. The objectives of this study were to use reflectance measured with a two band photodiode sensor (680 nm and 970 nm) to calculate the normalized difference vegetative index (NDVI) within the same FOV as an IRT measuring crop canopy temperature, and to estimate the fraction of vegetation using NDVI. Preliminary results demonstrated that photodiode sensors constructed in-house provided stable and linear measurements (r² = 0.99) with respect to daily incident solar radiation (Wm^-2) in the field.