Refining Medium Resolution Fractional Cover For Arid Australia To Detect Vegetation Dynamics And Wind Erosion Susceptibility On Longitudinal Dunes

Medium resolution satellite-derived fractional cover estimates of bare soil (f(BS)), photosynthetic vegetation (f(PV)), and non-photosynthetic vegetation (f(NPV)) provide a powerful means to study arid ecosystem dynamics. This paper employed remote sensing estimates of f(PV) and f(NPV) from five case study sites from Australia's vegetated dunefields to observe (a) vegetation growth response to rainfall 'pulses' and subsequent transition to nonphotosynthetic dormancy or senescence; (b) multiple time scales of antecedent climatic influence on vegetation cover; (c) the susceptibility of dunes to wind-blown sand drift during periods of low cover; and (d) the implications of image resolution choice when ground cover is heterogeneous.A spectral unmixing model for Australia's arid zone (termed 'AZN') was first developed by generating endmembers from a dataset of 1405 field surveys; Landsat time series estimates of f(PV) and f(NPV) were subject to a Seasonal-Trend decomposition by Loess (STL); Time series components were correlated with rainfall (P) and aridity at various accumulation periods; Fire maps were used to compare the climatic response of unburnt and burnt vegetation; Landform maps were used to isolate dune vegetation cover from the adjacent interdunes; and Landsat estimates of erodible area were compared with Sentinel-2 and WorlView-3 data.The new AZN model yielded Root Mean Square Error (RMSE) estimates of 14.5% (f(BS)), 6.5% (f(PV)) and 15.8% (f(NPV)) during cross validation. The AZN model also compared favourably to an existing continental-scale model when evaluated with independent reference data. Rainfall pulse responses of dune vegetation were detected initially as f(PV), and 3-9 months later as a peak in f(NPV). Components of f(PV) responded to P accumulated over 3-9-months (intra-annual), and 12-15-months (trend). The long-term build-up of f(NPV), if left unburnt, was influenced by rainfall patterns over the preceding 45-114 months. Fires reduced both the depth and strength of antecedent rainfall's influence on vegetation, and vegetation was often more sensitive to P than to aridity. Erodibility (total cover <14%) and partial erodibility (cover <35%) were more common at the driest sites but did not universally match aridity levels, due to fires and differing vegetation. The targeting of dune crest regions highlighted their enhanced susceptibility to sand drift (in most cases), and, given their occurrence on relatively narrow ridges (similar to 30 m), the importance of estimating cover at Landsat resolutions or better (e.g. Sentinel-2).