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Evaluation of Hyperspectral Reflectance Indexes to Detect Grapevine Water Status in Vineyards

¹ Universidad de León, Área de Ingeniería Cartográfica, Geodésica y Fotogrametría, Avda. de Astorga, s/n, 24400 Ponferrada León, Spain; ² Center for Spatial Technologies and Remote Sensing and ³ Department of Viticulture and Enology, University of California, Davis, CA 95616.

^* Corresponding author (email: drsmart{at}ucdavis.edu)

Irrigation scheduling is critical as it affects both fruit yield and composition. We examined the potential to use field-measured hyperspectral remote sensing data (reflectance and transmission over the 350–2500 nm wavelength region) to estimate leaf water content, equivalent water thickness (EWT), and leaf water potential () in a commercial vineyard of Vitis vinifera cv. Pinot noir. The data allowed us to evaluate a number of reflectance patterns to estimate vine water status through correlations using two spectral approaches: direct measurement of vegetation indexes (VIs) and continuum removal analysis (CRA). Continuum removal analysis was applied to obtain the maximum band depth (MBD) and the band area (BA) of several absorption features sensitive to water content. Correlations were high for EWT at the leaf level using a modification of the Simple Ratio VI (SR2; R² = 0.916) and for CRA with MBD₉₇₀ (R² = 0.917) and BA₁₁₆₀ (R² = 0.897). Correlations with EWT and water potential at the canopy level for SR2 were nonsignificant, which was characteristic for many VIs. For predawn water potential (_PD) and midday stem water potential (_stem) at the canopy level, best fits were realized for Modified Triangular VI (MTVI2; R² = 0.360) and Red/Green VI (RGI, R₆₉₅/R₅₅₄; R² = 0.462), respectively. For canopy level water status, the best results were obtained using the difference between the midday stem water potential and the pre-dawn leaf water potential (_stem - _PD) with R² = 0.619 for RGI and R² = 0.541 for Structure Intensive Pigment Index (SIPI, R₈₀₀-R₄₄₅/R₈₀₀-R₆₈₀), while for CRA R² = 0.477 for BA₁₆₀₀ and R² = 0.509 for MBD₉₇₀. Results suggest that noninvasive monitoring using hyperspectral data could improve current methods for estimating water status in individual vines. Applications of similar measurements could be produced from airborne hyperspectral imagers to provide spatially resolved estimates of water stress for use in water management of large-scale commercial vineyards.

Key words: vineyard remote sensing, vine water stress, leaf reflectance, canopy reflectance, vegetation index

Abbreviations: _leaf, midday leaf water potential (MPa); _PD, predawn leaf water potential (MPa); _stem, midday stem water potential (MPa); _s, osmotic potential (MPa); _t, turgor pressure (MPa); g_s, stomatal conductance to water vapor (mmol m^–2 s^–1); E, transpiration rate (mmol H₂O m^–2 s^–1); A, net photosynthetic rate (µmol CO₂ m^–2 s^–1); LA, leaf area (cm²); LAI, leaf area index (m² m^–2); Wt, total leaf weight (g); Wd: oven dry leaf weight (g); RWC, relative water content (%); WCd, water content as percent of dry mass (%); WCt, water content as percent of total fresh mass (%); EWT, equivalent water thickness (g cm^–2); SLW, specific leaf weight dry mass (kg m^–2); TSLW, total specific leaf fresh weight (kg m^–2); CRA, continuum removal analysis; BA, band area of the absorption feature in CRA; D, band depth in CRA; R, measured reflectance; MBD, maximum band depth of the absorption feature in CRA; lMBD, wavelength of the maximum band depth of the absorption feature in CRA (nm); FWHM, full band width at half maximum (nm); REIP, red-edge inflection position (nm); REIPv, derivative value of REIP (nm); ASD, Analytical Spectral Device (350–2500 nm); VIS, visual light spectrum (400–700 nm); IR, infrared light (750 nm–1 mm); NIR, near-IR (700–1300 nm); MIR, mid-IR (1300–2500 nm); SWIR, shortwave IR reflectance (1400–3000 nm); Rs, reflectance (relative units); Is, leaf reflectance (relative units); Id, leaf reflectance in the dark (relative units); Ir reference reflectance (relative units); VI, vegetation index(es) (waveband ratios)