Abstract
Variations in hydrological regimes triggered by warming conditions are expected to occur in the future at global scale. In particular, higher temperatures could increase the frequency and severity of extreme events, in terms of both intense precipitation and droughts, with relevant negative impacts on human activities and community safety. Since the magnitude and features of climate change and impacts could be highly spatially variable, high-resolution studies are essential to evaluate local current trends in the climate signals extracted from long records of meteorological observations and to assess their influence on specific regions of interest.
In particular, the Alps have experienced a temperature arise of around 2°C between the late 19th and the early 21st century, which is twice as much the global average warming, enhancing the potential impacts on local ecosystems and socio-economic sectors. Trentino – South Tyrol is the northernmost Italian region, largely characterized by the Alpine mountain environment. Agriculture represents a key regional sector which rely on water resources and is managed over a large grassland variety extending over broad elevation ranges. The drought of summer 2003 revealed the vulnerability of the sector to water scarcity during which relevant production losses where registered.
The present contribution analyzes the evolution of temperature and precipitation together with that of two of the most used meteorological drought indices, the Standardized Precipitation Index (SPI) and the Standardized Precipitation Evapotranspiration Index (SPEI), for Trentino – South Tyrol to evaluate long and short-term variations of the hydrological cycle over the last 60 years. To this aim a large database of meteorological observations, from the station networks managed by the Provinces of Trento and Bolzano, was set up and all the series were checked for quality and homogeneity. In order to analyze the spatial variability of the climate signal over the complex orography of the territory, monthly temperature and precipitation data from stations were interpolated onto a regular grid of 750 m resolution by means of an anomaly-based spatialization approach. SPI and SPEI series were then computed for each grid cell and for different aggregation periods. Theil-Sen trends and Mann-Kendall significance of the series were assessed at both mean regional and grid cell levels. While no significant trends in the SPI and SPEI regional series were pointed out for the whole 60-year period, a greater variability was depicted by exploiting trends over shorter time moving windows. Moreover, the trend assessment over the gridded values allowed pointing out the features of the different climatic zones which were masked in the regional averages. Furthermore, variations in drought characteristics such as frequency, severity and duration over the whole period will be discussed.
Finally, time series of remotely-sensed data, including Leaf-Area-Index (LAI), acted as a basis for the investigation of the impact of meteorological variability and extremes, such as drought, on Alpine grasslands. LAI was retrieved from the Moderate Resolution Imaging Spectroradiometer (MODIS) bands in visible and infrared electromagnetic regions at 250 m spatial resolution, and an optimized (for mountainous terrains) inversion of the radiative transfer model PROSAIL. LAI time-series corresponding to grasslands were compared to water stress evolution derived from meteorological indices and observations over the period 2004–2019.