Retrieval of equivalent water thickness and information related to biochemical components of vegetation canopies from AVIRIS data

Remote sensing of water status and biochemical components of vegetation can have important applications in the fields of agriculture and forestry. Reflectance of fresh, green vegetation in the 1.0–2.5 μm region is dominated by liquid water absorption and also weakly affected by absorption due to biochemical components, such as protein, lignin, and cellulose. We have developed both the nonlinear and linear least squares spectrum-matching techniques for deriving equivalent water thickness (EWT) of vegetation from AVIRIS data in the 1.0 μm and 1.6 μm regions. EWT values are compared with in situ canopy measurements in Harvard Forest, Massachusetts. Seasonal variations of EWTs over an agricultural area in Greeley, Colorado are determined. EWTs from the 1.0 μm region are generally greater than those from the 1.6 μm region. This is because the absorptivity of water near 1.0 μm is much less than that at 1.6 μm—resulting in a greater mean absorption path length in the 1.0 μ, region. After fitting the AVIRIS data with a reflectance spectrum of water, a weak lignin-cellulose absorption feature centered at 1.72 μm is seen in the difference spectra. We map the depth of the 1.72-μ feature, which can be considered as an index of abundance of those compounds in the canopy. AVIRIS data for the Harvard Forest, Massachusetts were analyzed and compared with laboratory chemical analysis of the foliage. The results obtained with the linear least squares spectrum-matching technique are comparable to the stepwise linear regression results obtained from first difference spectra.