Monitoring Vegetation Change in Tozeur Oases in Southern Tunisia by Using Trend Analysis of MODIS NDVI Time Series (2000–2016)

Abstract

Oasis ecosystems are highly vulnerable to environmental changes. To determine the state of vegetation in these ecosystems, monitoring systems must be provided with data on cultivated areas. These data can be obtained in part by using satellite observation systems with high and moderate spatial resolution and high temporal repetitiveness; these systems offer a synoptic vision that makes them a particularly appropriate information source for effectively estimating such data. In this study, we describe an approach to monitor the changing dynamics of Tozeur oases in southwestern Tunisia. To this end, we used a time series decomposition method (seasonal and trend decomposition using loess) to extract the trends from a multi-year time series at the scale of a geographical point (250 m × 250 m pixel) across the MOD13Q1 time series (2000–2016) of the Moderate Resolution Imaging Spectroradiometer (MODIS) sensor, at a 250-m spatial resolution time series. These methods were tested with the final aim of setting up an oasis monitoring system based on the analysis of time signatures obtained from MODIS images. The results showed that it was possible to identify the main types of irrigated perimeters present in the Djerid region and retrospectively trace their recent development history.