Drought Variability and Land Degradation in Semiarid Regions: Assessment Using Remote Sensing Data and Drought Indices (1982–2011)

Sergio Vicente-Serrano,Natalia Martín-Hernández,Cesar Azorin-Molina,Ahmed Kenawy,Miquel Tomás-Burguera,Santiago Beguería,Daniel Cabello

doi:10.3390/rs70404391

Abstract

We analyzed potential land degradation processes in semiarid regions worldwide using long time series of remote sensing images and the Normalized Difference Vegetation Index (NDVI) for the period 1981 to 2011. The objectives of the study were to identify semiarid regions showing a marked decrease in potential vegetation activity, indicative of the occurrence of land degradation processes, and to assess the possible influence of the observed drought trends quantified using the Standardized Precipitation Evapotranspiration Index (SPEI). We found that the NDVI values recorded during the period of maximum vegetation activity (NDVImax) predominantly showed a positive evolution in the majority of the semiarid regions assessed, but NDVImax was highly correlated with drought variability, and the trends of drought events influenced trends in NDVImax at the global scale. The semiarid regions that showed most increase in NDVImax (the Sahel, northern Australia, South Africa) were characterized by a clear positive trend in the SPEI values, indicative of conditions of greater humidity and lesser drought conditions. While changes in drought severity may be an important driver of NDVI trends and land degradation processes in semiarid regions worldwide, drought did not apparently explain some of the observed changes in NDVImax. This reflects the complexity of vegetation activity processes in the world’s semiarid regions, and the difficulty of defining a universal response to drought in these regions, where a number of factors (natural and anthropogenic) may also affect on land degradation.

Highlights

Semiarid vegetation is well adapted to long periods of water scarcity, through various phenological and morphological mechanisms [1]
The NDVImax provided information on the maximum potential vegetation activity in each region, and it may not be fully indicative of all annual vegetation growth, and the month corresponding to this value can change from year to year; we have found a strong one-month autocorrelation in the monthly Normalized Difference Vegetation Index (NDVI)
We investigated the influence of drought events on semiarid land degradation processes using a multi-scalar drought index, which took into account both precipitation and the atmospheric evaporative demand, and was able to be calculated at different time scales to take account of the varied times of response of vegetation activity to water availability

Summary

Introduction

Semiarid vegetation is well adapted to long periods of water scarcity, through various phenological and morphological mechanisms [1]. Semiarid vegetation should not be severely damaged by extreme drought events because it is markedly resilient and has the capacity to recover. The majority of semiarid regions worldwide is subject to anthropogenic perturbations, and is far from the natural equilibrium state. As a consequence of anthropogenic climate forcing and intensive historical land uses (e.g., grazing, agriculture, etc.), semiarid regions are prone to land degradation processes. Current climate change processes have been recognized as important factors in land degradation [6,7,8] Climate and human forcing can trigger positive feedback between the biota and the physical environment, leading the ecosystem towards a degraded state; if taken beyond a threshold, return to the initial state is very difficult [2,3,4,5].

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