Vegetation response to droughts: The case of northern Italy

Abstract

Drought is a normal and recurrent climate feature, but one of the most important climate hazards that can produce negative effects on natural and socioeconomic systems. The northern part of Italy is historically rich in water resources and a very fertile and productive area. Recently, however, drought events increased their influence on the hydrological behaviour of the main rivers in this region. In this work, we analyse the spatial distribution of drought events and their effects on vegetation greenness in northern Italy during the 2000–2020 period, using MODIS images at 1 km spatial resolution. Correlation maps between fields of bi-weekly vegetation indices (normalized difference vegetation index [NDVI] and enhanced vegetation index [EVI]) and drought indices (standardized precipitation index [SPI] and standardized precipitation evapotranspiration index [SPEI]) were estimated. To generate the SPEI index, 150 daily precipitation and temperature ground station series were collected, aggregated at bi-weekly scale, reconstructed, homogenized and spatialized at 1 km resolution by mean of Universal Kriging with auxiliary variables. The NDVI and EVI indices (MOD13A2) and the land surface temperature (LST; MOD11A2) were collected from MODIS images and quality checked, removing pixels with clouds and low-quality data. Drought events were detected by means of the SPEI and SPI at medium (12-months) and long (24 and 36 months) timescales. The events were characterized by drought intensity, duration and percentage of affected area. For each pixel, the Pearson's correlation between NDVI/EVI and SPEI/SPI at different timescales was estimated. On the basis of the correlation maps and the CORINE Land Cover 2020 (MCD12Q1), we estimated the drought impact on different vegetation types as the percentage of vegetation undergoing drought stress. The investigation on drought indices has revealed nine main summer and spring events in the study period. The events of summer 2003 and 2017 were especially remarkable, with respectively 50% and 40% of the total area under drought according to the SPEI index at 12 months. The correlation between drought and vegetation indices revealed that in the Po Valley and Tuscan hills, the vegetation communities perturbed by agriculture are those that are most affected by droughts, with 13% of vegetation under drought on long timescales. This study also indicated that in the last two decades, the main trigger of droughts in northern Italy was the lack of precipitation rather than pure temperature increase.