Non-stationarity in MODIS fAPAR Time-series and its Impact on Operational Drought Detection

ABSTRACTA time-series of fAPAR (fraction of Absorbed Photosynthetically Active Radiation) data collected by the MODIS (MODerate Resolution Imaging Spectroradiometer) Terra satellite between 2001 and 2016 has been analysed at global scale in order to identify possible areas interested by a linear temporal trend. The main scope of this study is to quantify how the possible trend may affect estimates of drought events based on 10-day fAPAR standardized anomalies (z values). The trend analysis, performed according to the Theil–Sen approach, highlighted that about 10% of the globe shows a statistically significant trend (following the Mann–Kendall test at p = 0.05), mostly being positive (increasing fAPAR). The impact of such a trend on standardized anomalies has been formally expressed as a linear function of two quantities: 1) the z value itself, and 2) the year under investigation. This approach allows summarising the variations in z into just two parameters. In the case of negative anomalies (of greater in...