Predicting Turfgrass Root Zone Temperatures and Root Length Using the Ratio Vegetation Index.

Researchers have adopted spectral reflectance indices as viable options for objective turfgrass quality measurements. One challenge that limits the adoption of turfgrass reflectance as a management tool is an inability to relate reflectance patterns to measureable turfgrass attributes.  The objective of the current study was to demonstrate the utility of using the near-infrared index to predict relative root zone temperatures and root length at unsampled locations on in-play golf course greens.  The study was located at Willow Oaks Country Club in Richmond, Va on “Penn A1/A4” creeping bentgrass putting greens.  Ground truth samples of root zone temperatures and root length were collected on three greens along three 21 m arcs in 3 m increments and marked using an OmniStar corrected GPS.  Reflectance data were collected using a Holland Scientific ACS470 in the red (670nm) red-edge (730nm) and near-infrared NIR (760nm).   Reflectance data were transformed using the ratio vegetation index (RVI:  NIR/Red), and then individual bands as well as the RVI were correlated with root zone temperatures and root length.  For each green a random subset of ground truth samples were retained for accuracy assessments.  Preliminary results indicate a strong negative correlation exists between RVI and root zone temperatures (r = -0.91- -0.93) and moderate relationship exists between RVI and root length (r = 0.58 – 0.62).  Regression results explained from 59-86% of the variability in root zone temperatures and approximately 34% of the variability in rooting depth.  Results were used to create surface maps of root length and root zone temperatures for entire greens with a minimum of 15 ground truth samples collected per green.