Indirect Determination of Leaf Area Index to Calculate Evapotranspiration.

The plant integrates soil and environmental factors. Monitoring plant properties is challenging across the multiple plant types in organic agriculture. The purpose of this study was to use nadir photos to determine ground cover, leaf area index (LAI), and plant water use. The photos were 4.9 m above the ground, and limited micrometeorology measurements were included. Measurements were completed on plots comparing a four-year organic rotation as well as conventional corn-soybean. Monitoring wells (for water table depth) and neutron access tubes were installed on each of 30 plots. Soil samples from the neutron access tubes and well sites were saved to examine soil morphology, and determine bulk density and particle size. Starting mid July 2016, crop height was measured and photos were taken around every 20 days through senescence using a digital camera on 4.9 m pole, using remote control to take the picture. Preliminary photos were taken late June 2016 from a step ladder (with plant height). More detailed measurements were re-started in April 2017. The photos were manually analyzed using "SamplePoint" (Booth et al., 2006) software to categorize each grid (225 points) or random (199) point (green crop, dry, weed, soil). An equation was derived to determine LAI from plant height and fraction green crop cover. Potential evapotranspiration (ET) was calculated by the Priestley Taylor method. The LAI and ET showed influence of weed pressure and water table depth on crop growth and function.