Assessing Vegetation Dynamics Over Mainland Australia With Metop ASCAT

Abstract

Recently, the slope and curvature estimation of the backscatter–incidence angle relationship within the TU Wien retrieval algorithm has been improved. Where previously only climatologies of the slope and curvature parameters were available, i.e., one value for every day of year, slope and curvature are now calculated for every day. This enables the retrieval of time series of vegetation optical depth ( $\tau _a$ ) from backscatter observations. This study demonstrates the ability to detect interannual variability in vegetation dynamics using $\tau _a$ derived from backscatter provided by the advanced scatterometer on-board Metop-A. $\tau _a$ time series over Australia for the period 2007–2014 are compared to leaf area index (LAI) from SPOT-VEGETATION by calculating the rank correlation coefficient ( $r_s$ ) for original time series and anomalies. High values for $r_s$ are found over bare soil and sparse vegetation in central Australia with median $r_s$ values of 0.78 and 0.58, respectively. Forests and ephemeral lakes and rivers impact the retrieval of $\tau _a$ , and the negative values for $r_s$ are found in these areas. Looking at the annual averages of $\tau _a$ , LAI, and surface soil moisture, significantly high values are found for the anomalously wet years 2010 and 2011. Patterns in the increased $\tau _a$ correspond to regions with increased soil moisture and LAI. Values for $\tau _a$ and LAI are anomalous especially in sparsely vegetated regions, where the flush of grasses increases $\tau _a$ and LAI. Regions with enough precipitation and higher woody vegetation component show a smaller increase in 2010 and 2011. This study demonstrates the skill of $\tau _a$ , and subsequently of scatterometers, to monitor the vegetation dynamics thanks to the multiincidence angle observation capability.