Normalized Difference Spectral Index Research Articles

Normalized difference spectral indices for estimating photosynthetic efficiency and capacity at a canopy scale derived from hyperspectral and CO 2 flux measurements in rice

We explored simple and useful spectral indices for estimating photosynthetic variables (radiation use efficiency and photosynthetic capacity) at a canopy scale based on seasonal measurements of hyperspectral reflectance, ecosystem CO 2 flux, and plant and micrometeorological variables. An experimental study was conducted over the simple and homogenous ecosystem of an irrigated rice field. Photosynthetically active radiation absorbed by the canopy (APAR), the canopy absorptivity of APAR (fAPAR), net ecosystem exchange of CO 2 (NEE CO 2 ) gross primary productivity (GPP), photosynthetic capacity at the saturating APAR ( P max), and three parameters of radiation use efficiency ( ε N: NEE CO 2 /APAR; ε G: GPP/APAR; φ: quantum efficiency) were derived from the data set. Based on the statistical analysis of relationships between these ecophysiological variables and reflectance indicators such as normalized difference spectral indices (NDSI[ i, j]) using all combinations of two wavelengths ( i and j nm), we found several new indices that would were more effective than conventional spectral indices such as photochemical reflectance index (PRI) and normalized difference vegetation index (NDVI = NDSI[near-infrared, red]). ε G was correlated well with NDSI[710, 410], NDSI[710, 520], and NDSI[530, 550] derived from nadir measurements. φ was best correlated with NDSI[450, 1330]. NDSI[550, 410] and NDSI[720, 420] had a consistent linear relationships with fAPAR throughout the growing season, whereas conventional indices such as NDVI showed very different relationships before and after heading. Off-nadir measurements were more closely related to the efficiency parameters than nadir measurements. Our results provide useful insights for assessing plant productivity and ecosystem CO 2 exchange, using a wide range of available spectral data as well as useful information for designing future sensors for ecosystem observations.

Foliage moisture content estimation from one‐dimensional and two‐dimensional spectroradiometry for fire danger assessment

Moisture content in live fuels is a critical variable in fire ignition and fire behavior assessment, but it is very difficult to estimate using traditional methods. Remote sensing measurements provide adequate temporal and spatial coverage to be used operationally in fire danger estimation. Before such applications can be routinely undertaken, reflectance changes caused by water content variations must be clearly separable from other factors in reflectance variation (leaf area index, chlorophyll content, dry matter, observation‐illumination angles, etc.). This paper analyzes the effect of these variables in a controlled experiment using young Holm oak (Quercus Ilex) plants, a common Mediterranean species. Spectro‐radiometric measurements were performed systematically in laboratory conditions using a GER 2600 spectroradiometer (point measurements, one‐dimensional (1‐D)) and a Specim hyperspectral camera system (matrix measurements, 2‐D). Leaf and canopy biophysical parameters were sampled to control their influence on reflectance as water stress increased. Equivalent water thickness (EWT) and fuel moisture content (FMC) were correlated against several variations of a new index: the Normalized Difference Spectral Index, (NDSI(λ1,λ2)). Results indicate that the choice of suitable spectral bands is critical to control plant physiological effects on reflectance. Strong correlations were obtained between NDSI and both EWT and FMC by applying operational bands at 1108 and 1242 nm (NDSI(1108,1242): r > 0.93), while the shorter near infrared bands (1000 and 860 nm) show a stronger influence of external factors. A combination of 2‐D pixel selection techniques and first derivative spectroscopy successfully solved absolute reflectance inconsistencies between 2‐D and 1‐D measurements, which may prove to be a helpful means of scaling up from leaf to canopy level, and from the ground to airborne observation.

Use of second derivatives of canopy reflectance for monitoring prairie vegetation over different soil backgrounds

Spectral derivative indices were compared with the commonly used near-infrared to red reflectance ratio ( NIR red ) and the normalized difference (ND) spectral indices. Two “windows” of second derivatives, ρ″ 0.69 μm and ρ″ 0.74 μm , exist in the visible and NIR spectral regions, which can be used to estimate vegetation characteristics such as percent cover and leaf area index (LAI). Second derivatives centered on the red and NIR wavebands (MSS Bands 3 and 4) were calculated using broadband radiometer data for grassland sites which had been previously burned or had been left unburned for several years. The second derivative indices minimized the effect of the soil background differences caused by the prior burning treatments, and clearly showed the advantages of the derivative technique over the conventional NIR red ratio and ND spectral indices.

SIMULATED RELATIONSHIPS BETWEEN SPECTRAL REFLECTANCE, THERMAL EMISSIONS, AND EVAPOTRANSPIRATION OF A SOYBEAN CANOPY1

ABSTRACT: A canopy reflectance model is incorporated into a routine for simulating water and energy flows in the soil‐plant‐atmosphere system. The reflectance model is structured tocalculate canopy albedo throughout each simulation period and to determine spectral reflectances at a specified time during the day. Spectral vegetation indices are then calculated from the reflectances and related to the evapotranspiration and thermal response of the canopy. The canopy reflectance model is also used to establish the photo‐sytheticaily active radiation load at various depths in the canopy. Stomatal resistances are calculated using these radiation values and integrated to give the minimum canopy resistance. Actual canopy resistance is obtained by adjusting minimum canopy resistance for environmental stresses such as leaf water potential and leaf temperature. Using data for a soybean canopy, canopy evapotranspiration and temperatures are simulated for a range of leaf area index values and compared with the corresponding spectral vegetation indices. The resuits indicate that the normalized difference spectral index has an inverse linear relationship with canopy temperature, concurring with results obtained from satellite observations. The possibility of using a spectral vegetation index and thermal observations together to parameterize surface moisture availability for evapotranspiration is considered.