Estimating equivalent water thickness in a conifer forest using Landsat TM and ASTER data: a comparison study

Abstract

Remotely sensed data from the Landsat thematic mapper (TM) and the advanced spaceborne thermal emission and reflection radiometer (ASTER) can be used to make physically based estimates of equivalent water thickness (EWT) in a Pinus ponderosa forest ecosystem. EWT is an important parameter for studying ecosystem dynamics and fire potential and is likely to be of interest in monitoring biological responses to climate change. Near infrared (NIR) and shortwave infrared (SWIR) reflectances were simulated using the LIBERTY and GeoSAIL leaf and canopy reflectance models; the results were used to calculate a NIR/SWIR ratio and a normalized NIR/SWIR index. Index–EWT relationships were modeled and inverted for EWT derivation. Landsat TM and ASTER images were able to produce EWT estimates with average errors of ±17.3% and 19.4%, respectively. TM band 5 and ASTER band 4 contained the most valuable information among the SWIR bands for EWT estimation. Exclusion of plots with dense understory vegetation significantly reduced point scatter, especially with Landsat (r2 = 0.85, ±13% average error), indicating that this method can provide robust EWT quantification where conifer trees predominate.