Evaluation of vegetation indices for rangeland biomass estimation in the Kimberley area of Western Australia

Abstract

The objective of this paper is to test the relationships between Above Ground Biomass (AGB) and remotely sensed vegetation indices for AGB assessments in the Kimberley area in Western Australia. For 19 different sites, vegetation indices were derived from eight Landsat ETM+ scenes over a period of two years (2011-2013). The sites were divided into three groups ('Open plains', 'Bunch grasses' and 'Spinifex') based on similarities in dominant vegetation types. Dry and green biomass fractions were measured at these sites. Single and multiple regression relationships between vegetation indices and green and total AGB were calibrated and validated using a "leave site out" cross validation. Four tests were compared: (1) relationships between AGB and vegetation indices combining all sites; (2) separate relationships per site group; (3) multiple regressions including selected vegetation indices per site group; and (4) as in 3 but including rainfall and elevation data. Results indicate that relationships based on single vegetation indices are moderately accurate for green biomass in wide open plains covered with annual grasses. The cross-validation results for green AGB improved for a combination of indices for the Open plains and Bunch grasses sites, but not for 'Spinifex' sites. When rainfall and elevation data are included, cross validation improved slightly with a Q² of 0.49-0.72 for 'Open plains' and 'Bunch grasses' sites respectively. Cross validation results for total AGB were moderately accurate (Q² of 0.41) for 'Open plains' but weak or absent for other site groups despite good calibration results, indicating strong influence of site-specific factors.