Analysis of vegetation and land cover dynamics in north-western Morocco during the last decade using MODIS NDVI time series data

摘要

Vegetation phenology as well as the current variability and dynamics ofvegetation and land cover, including its climatic and human drivers, areexamined in a region in north-western Morocco that is nearly 22 700 km2big. A gapless time series of Normalized Differenced Vegetation Index (NDVI)composite raster data from 29 September 2000 to 29 September 2009 isutilised. The data have a spatial resolution of 250 m and were acquired bythe Moderate Resolution Imaging Spectroradiometer (MODIS) sensor.The presented approach allows to compose and to analyse yearly land covermaps in a widely unknown region with scarce validated ground truth data byderiving phenological parameters. Results show that the high temporalresolution of 16 d is sufficient for (a) determining local land cover betterthan global land cover classifications of Plant Functional Types (PFT) andGlobal Land Cover 2000 (GLC2000) and (b) for drawing conclusions onvegetation dynamics and its drivers. Areas of stably classified land covertypes (i.e. areas that did not change their land cover type) showclimatically driven inter- and intra-annual variability with indicatedinfluence of droughts. The presented approach to determine human-driveninfluence on vegetation dynamics caused by agriculture results in a morethan ten times larger area compared with stably classified areas. Changedetection based on yearly land cover maps shows a gain of high-productivevegetation (cropland) of about 259.3 km2. Statistically significantinter-annual trends in vegetation dynamics during the last decade couldhowever not be discovered. A sequence of correlations was respectivelycarried out to extract the most important periods of rainfall responsiblefor the production of green biomass and for the extent of land cover types.Results show that mean daily precipitation from 1 October to 15 December hashigh correlation results (max. r2=0.85) on an intra-annual time scale to NDVIpercentiles (50 %) of land cover types. Correlation results of mean dailyprecipitation from 16 September to 15 January and percentage of yearlyclassified area of each land cover type are medium up to high (max. r2=0.64). In all, an offset of nearly 1.5 months is detected betweenprecipitation rates and NDVI values. High-productive vegetation (cropland)is proved to be mainly rain-fed. We conclude that identification,understanding and knowledge about vegetation phenology, and currentvariability of vegetation and land cover, as well as prediction methods ofland cover change, can be improved using multi-year MODIS NDVI time seriesdata. This study enhances the comprehension of current land surface dynamicsand variability of vegetation and land cover in north-western Morocco. Itespecially offers a quick access when estimating the extent of agriculturallands.