Effect of wetness on turfgrass canopy reflectance

Wetness is a key factor in the development of various foliar pathogens on turfgrass. A spectroradiometric approach was used to measure dew and precipitation in short turfgrass (creeping bentgrass and bluegrass) canopies, because of the unavailability of sensors representing the leaf size and the need not to disturb such a short canopy environment. Measurements of reflectance, between 400 and 2250 nm with a field portable spectroradiometer, showed that the erectophile rough bluegrass (about 4 cm height) trapped more radiation within the canopy, hence lowering the reflectance, than the planophile homogeneous bentgrass (about 1 cm height). For both dew and rain-covered bentgrass canopies, reflectance increased in the visible (VIS), about 50 and 16%, respectively, and decreased in the middle-infrared (MIR), about 45 and 28%, respectively. However, changes in dew reflectance on the bluegrass canopy were mainly observed in the near-infrared (NIR) and MIR (9 and 19% decrease, respectively), as the reflectance in the VIS was almost unaffected (about 4% increase). The light precipitation affected the bluegrass reflectance by mainly decreasing NIR (765–960 nm) wavelengths. Relationships were generally weak for the dew-covered bluegrass reflectance, but a strong quadratic relationship at 1165 nm reflectance was found under rainy conditions. Strong quadratic relationships were found between dew-covered bentgrass and MIR/green and MIR/red ratios. The reflectance at 1165 nm (r2=0.95) showed the greatest response to increasing wetness either for dew or rainfall, suggesting that the reflectance at this wavelength may be used on a remote sensor to measure wetness on bentgrass canopies.