Figures (1)  Tables (3)

    • Figure 1. 

      Network visualization of vegetation indices (VIs) and water-use efficiency (WUE) components. WUE is the central node, surrounded by its physiological traits: carbon isotope discrimination (Δ13C, in green), chlorophyll (in brown), photosynthesis (A, in red), stomatal conductance (gs, in teal), leaf area index (LAI, in blue), and water content (in purple). Vegetation indices are arranged in the outer ring and connected to specific physiological traits by lines. Lines connect each VIs to its corresponding WUE component. VIs associated with multiple WUE components are displayed in the color of the additional traits they relate to and are summarized separately in Table 2 to highlight their versatility.

    • Indices Formula Sensor type Ref.
      NDVI (Normalized Difference Vegetation Index) (R800 − R720)/(R800 + R720);
      (R830 − R660)/(R830 + R660);
      (R900 − R680)/(R900 + R680);
      (R800 − R600)/(R800 + R600);      		 Multispectral [20−23]
      DVI (Difference Vegetation Index) R810 − R680;
      R800 − R720      		 Multispectral [22,24]
      RVI (Ratio Vegetation Index) R830 / R660 Multispectral [23,25]
      SR (Simple Ratio) R900/R680 Multispectral [23,26]
      CVI (Chlorophyll Vegetation Index) NIR* RED /GREEN2 Multispectral [27]
      CIred-edge (Chlorophyll Index - Red Edge) (RNIR/Rred-edge) − 1 Red-edge multispectral [22,28,29]
      SAVI (Soil Adjusted Vegetation Index) (1 + 0.5)*(RNIR − Rred)/(RNIR + Rred + 0.5) Multispectral [22,23,30]
      MSAVI (Modified SAVI) (2*RNIR + 1)−(2*RNIR + 1)2−8*(RNIR − Rred)2 Multispectral [22,31]
      PRI (Photochemical Reflectance Index) (R531 − R570)/(R531 + R570) Hyperspectral/narrowband multispectral [32,33]
      SIPI (Structure Insensitive Pigment Index) (R800 − R445)/(R800 − R680) Multispectral [23,34]
      PSRI (Plant Senescence Reflectance Index) (R680 − R500)/R750 Multispectral/hyperspectral [35]
      TSAVI (Transformed Soil Adjusted Vegetation Index) (−1.4735*R780 + R650 + 1.4735*1.3681) Multispectral [36]
      RDVI (Red Difference Vegetation Index) (R800 − R670)/sqrt(R800 + R670) Multispectral [37]
      Lo (The minimum spectral reflectance corresponding
      to the chlorophyll absorption well)      		 Min(R680 − R780)
      		 Hyperspectral [38,39]
      EVI-1 (Enhanced Vegetation Index-1) 2.5*(R860 − R645)/(1 + R860 + 6*R645 − 7.5*R470) Multispectral [39,40]
      Readone R415/R695 Red-edge multispectral [41]
      NDDAig (Normalized Difference of the Double-peak
      Areas based on REPig [red edge positions derived by
      maximum inverted Gaussian fitting] division)      		 (R755 + R680 − 2*R705)/(R755 − R680) Hyperspectral [39,42]
      WEI (Water Use Efficiency Index) NDDAig/FWBI;
      [(R755 + R680 − 2*R705)]*Min(R930 − 980)/
      [(R755 − R680)*R900]      		 Hyperspectral/multispectral [39]
      WBI (Water Band Index)
      		 R970/R900;
      R950/R900      		 Hyperspectral/narrowband multispectral [43−48]
      SRWI (Simple Ratio Water Index) R860/R1240 SWIR sensor [49,50]
      NDWI (Normalized Difference Water Index) (R860 − R1240)/(R860 + R1240);
      (R860 − R2270)/(R860 + R2270);
      (R970 − R880)/(R970 + R880);
      (R970 − R920)/(R970 + R920);
      (R858 − R1640)/(R858 + R1640);
      (R858 − R2130)/(R858 + R2130);      		 SWIR-enabled multispectral [33,50−53]
      NDI (Normalized Difference Index)
      		 (R602 − R598 − R600)/(R602 + R598 + R600);
      (R644 − R630 − R652)/(R644 + R630 + R652);
      (R648 − R662 − R624)/(R648 + R662 + R624)      		 RGB/multispectral [54]
      GNDVI (Green Normalized Difference Vegetation Index) (R780 − R550)/(R780 + R550) Multispectral [33,39,45,55,56]
      NDRE (Red Edge NDVI) (RNIR − Rred-edge)/(RNIR + Rred-edge)

      		 Red-edge multispectral [29,39,57]
      NGRDI (Normalized Green Red Difference Index) (Rgreen − Rred)/(Rgreen + Rred)
      (R780 − R670)/(R780 + R670)

      		 RGB [56,58]
      EVI (Enhanced Vegetation Index) 2.5*(RNIR − R680)/(1 + RNIR + 6*R680–7.5*R450);
      2[R830 − R660]/[1 + R830 + 6R660 − 7.5R460]      		 Multispectral [23,25,56]
      SWWI (Shortwave Infrared Water Index) R1650/R850 SWIR sensor [23,25]
      NWI3 (Normalized Water Index -3) (R970 − R920)/(R970 + R920) SWIR sensor [23,25]
      NDMI (Normalized Difference Moisture Index) [R2200 − R1100]/[R2200 + R1100] SWIR-enabled multispectral [23,25]
      NDSI (Normalized Difference Snow Index) (R518 − R676)/(R518 + R676);
      (R620 − R623)/(R620 + R623);
      (R620 − R637)/(R620 + R637)      		 SWIR sensor [23,26]
      MTVI (Modified Triangular Vegetation Index) $ \dfrac{1.5[1.2({R}_{800}-{R}_{550})-2.5({R}_{670}-{R}_{550})]}{\sqrt{{(2{{R}_{800}}+1)}^{2}-\left(6{R}_{800}-5\sqrt{{R}_{670}}\right)-0.5}} $ Multispectral [23,33,59,60]
      OSAVI (Optimal Soil Adjusted Vegetation Index) (1 + 0.16) × (R750 − R705)/(R750 + R705 + 0.16) Multispectral [13,23,61]
      TVI (Triangular Vegetation Index) 0.5 [120(R750 − R550)-200(R670 − R550)] Multispectral [23,33,59,60]

      Table 1. 

      A summary of vegetation Indices for physiological traits associated with water use efficiency.

    • Vegetation index Leaf area index (LAI) Chlorophyll content Photosynthesis (A) Stomatal conductance (gs) Water status Carbon isotope
      discrimination (Δ13C)
      NDRE √ √
      NDSI √ √ √
      PRI √ √
      R701 √ √
      SIPI √ √
      TSAVI √ √
      WBI √ √ √
      OSAVI √ √ √
      MSAVI √ √
      MTVI √ √
      SAVI √ √
      SR √ √
      NDMI √ √
      SWWI √ √
      CVI √ √
      GNDVI √ √ √
      NDVI √ √ √
      Bold indicates vegetation indices (NDSI, OSAVI, GNDVI, and NDVI) that were associated with three traits.

      Table 2. 

      Vegetation indices linked to two or more traits of WUE.

    • Drought stage VI category VIs Related physiological traits Limitations
      Early/mild drought Pigment-related PRI Xanthophyll cycle,
      photosynthetic efficiency      		 Affected by illumination, canopy structure; species-specific response[89,91,109]
      Pigment-related CIred-edge Chlorophyll content Requires narrow spectral bands, saturates in high canopy density[95,110]
      Pigment-related SIPI Ratio of carotenoid/ chlorophyll Saturates at high chlorophyll levels, requires high spectral resolution[39,111]
      Structural/
      pigment hybrid      		 GNDVI Chlorophyll, nitrogen Influenced by canopy structure[55,83,112]
      Water-related NDWI Leaf water content Less sensitive under mild stress, Sensitive to atmospheric conditions[50,83,113,114]
      Prolonged/
      severe drought      		 Structural NDVI Canopy greenness, biomass Saturates at high LAI; insensitive to early stress[80]
      Structural EVI Canopy structure, biomass Higher data quality acquisition, overcompensate in high biomass density[80,115]
      Structural SAVI Vegetation vs soil contrast Saturates in high canopy density[82,116,117]
      Water-related NDWI Leaf/canopy water content Sensitive to atmospheric conditions[19,113,114]
      Water-related WBI Leaf water content Sensitive to canopy structure and chlorophyll levels[95,118]
      Pigment-related MCARI Chlorophyll degradation Sensitive to canopy structure and soil background at lower or higher chlorophyll levels[16,111]

      Table 3. 

      Vegetation indices linked to WUE under early and prolonged/severe drought.