Workshop on Global Change Research – “Climate Change and Desertification: Monitoring, Modelling and Forecasting” (Wengen, 10-13 September 2007)

Poster on Analysis of long dry spell series for drought detection and monitoring in Central Italy

Drought is a recurrent feature of climate and its impacts on human activities depend on the interaction between this natural event and water demand for several purposes, from agriculture to civil and industrial needs.
The aim of this study is to analyze long time series of rainfall data for evaluating variability, tendencies and intensity of drought events in three different geographical contexts of Tuscany region (coast, hill and mountain) and for analyzing their trends along with Mediterranean SST long-term variability.
The drought analysis is carried out using daily records from three central Italy rain gauges, covering, respectively, the periods 1945-2005, 1928-2003 and 1930-2005.
The first step of our study is to define the dry spells and its characteristics. The rain threshold is computed taking into account the minimum amount of water needed for the vegetation growth; number and duration of the spells are considered at seasonal and yearly level. Furthermore, a two-parameter General Pareto Distribution (GPD) is applied to study the frequency of all the dry spell events over a chosen threshold (peaks over the threshold) for their duration.
Afterward we use the Standardized Precipitation Index (SPI) to quantify the precipitation deficit for multiple time scale (3, 6, 12, 24 months) and, consequently, to assess the effects of a drought event on different water-resources components: soil moisture, streamflow, reservoir storage, groundwater. The standardization of the values permits a comparison between meteorological stations climatically and geographically different.
The frequency analysis of the negative values of the SPI classification gives information on the extreme event increasing or decreasing.
The results obtained from the two investigations show a general increase of drought events in the last decades.
The SPI highlights, in particular, a negative trend for the long time scale (hydrological drought).
Moreover, thanks to the seasonal analysis, we detect an intensification of the phenomenon during the winter period.

Magno R.1, Guarnieri F.1, Gozzini B.2, Meneguzzo F.1, Maracchi G.1, Pasqui M.1

1IBIMET – CNR (Institute of Biometeorology of the National Research Council).
2LaMMA – Environmental modelling and monitoring laboratory for sustainable development.

Poster (pdf)