Monitoring water stress in buffelgrass using hand-held radiometers

Abstract

Hand-held MARK-II radiometric measurements were used to monitor levels of water stress in buffelgrass (Cenchrus ciliaris L.) plots. Three levels of irrigation treatments were applied: dry, medium and wet. The radiometric RED (630 to 690 nm) and NIR (760 to 900 nm) measurements obtained from the plots were converted to reflectance factors and to perpendicular and normalized difference vegetation indices (PVI and NDVI respectively). Reflectance and vegetation indices were monitored over the growing season to trace buffelgrass development and response to rainfall and irrigation treatments. Higher than normal rainfall in 1986 moderated the water stress effects. However, we found that a range manager, using a hand-held radiometer, could detect (1) initial plant cover development due to increases in chlorophyll amounts at first of season in RED band, (2) continuing plant structure development in the NIR band, (3) maximum biomass in both RED and NIR bands, (4) initial onset of water stress due to plant structure changes in NIR band and (5) prolonged water stress effects due to chlorophyll concentration changes in the RED band. Water stress onset was detected in the NIR and PVI 24 days before being apparent in the RED or to visual observations; stress might have appeared sooner in the RED band if rainfall had been below normal or for soil with a lower water holding capacity. The NIR and PVI vegetation index provided better separation among the three levels of water stress treatments due to the effect on plant structure than did the RED and NDVI. These results indicated that hand-held radiometers may be useful tools that range managers can use to monitor water stress in grasses.