Statistical Analysis of Crop Water Stress in Rainfed Rice (Oryza sativa L.) Using Spectral and Non-spectral Indices

Abstract

The use of vegetation indices derived from wavelengths known to be sensitive to plant water status is a fast, reliable, and non-destructive method of identifying the spatial and temporal distribution of crop water requirements. A pot experiment was conducted in a screenhouse to assess the potential of vegetation indices in detecting water stress in rainfed rice during the reproductive growth phase. Rainfed lowland (PSB Rc14) and upland (UPL Ri7) rice varieties were subjected to non-water stress [2.0 irrigation water (IW) per cumulative pan evaporation (CPE)], mild water stress (1.5 IW/CPE), and severe water stress (1.0 IW/CPE) treatments during the reproductive growth stage. Leaf reflectance measurements were taken using a portable field spectrometer (Jazz Spectral Sensing Suite, Ocean Optics, Inc.) with a spectral range of 650–1050 nm and 0.36 nm bandwidth. Vegetation indices – namely, water index (WI), normalized water index-1 (NWI-1), normalized water index-2 (NWI-2), normalized water index-3 (NWI-3), and normalized water index-4 (NWI-4) – were then calculated from the leaf spectral reflectance measurements and correlated with leaf relative water content (RWC), crop water stress index, and grain yield. The leaf reflectance in the NIR region (700–1050 nm) of both rice varieties under severe water stress conditions (1.0 IW/CPE) was lower compared to those under mild water stress (1.5 IW/CPE) and non-water stress treatments (2.0 IW/CPE). Significant differences in the vegetation indices were detected at the flowering stage when the onset of water stress was also identified by the crop water stress index (CWSI). NWI-2 had the strongest correlation with grain yield of PSB Rc14 (r = –0.7815), whereas WI and NWI-1 correlated well with grain yield for UPL Ri7 (r = –0.876). These results suggest the potential of using hyperspectral vegetation indices as indicators of water stress during the flowering stage of rice.