STANDARDIZED HYDRO-CLIMATE BALANCE

Drought is a natural and temporary meteorological condition in which there is a significant reduction in precipitation compared to the climatic conditions of a specific place of interest. Therefore, it should not be confused with the phenomenon of aridity, a term which indicates a permanent natural climatic condition in which the low quantity of annual precipitation, associated with high temperatures, does not provide the soil with the necessary degree of humidity to promote the development of life.

There is no single definition of drought, although everyone agrees that drought is a temporary but frequent phenomenon, which can generate environmental, social and economic impacts. The phenomenon of drought presents different characteristics in the components of the hydrological cycle, which, in turn, produce different impacts on water systems, cultures and socio-economic and environmental systems. In relation to the effects produced, drought is generally classified into four categories:

• meteorological drought in case of relative scarcity of precipitation;
• hydrological drought in the presence of a relatively low water supply in the soil, in watercourses, or in aquifers;
• agricultural drought in the event of a deficit in soil water content which causes stress conditions in crop growth;
• socio-economic and environmental drought understood as the set of impacts that manifest themselves as an imbalance between the availability of the resource and the demand for economic activities, for social aspects and for the conservation of ecosystems.

The impact of drought on the environment is then linked to the severity and persistence of the dry conditions. A prolonged lack of rainfall for many months (6-12 months) will have effects on river flow; while for a longer period (one or two years) it will affect the availability of water in the aquifers.

The Standardized Precipitation-Evapotranspiration Index (SPEI) is one of the indices used nationally and internationally to quantify drought on a given time scale. Compared to Standardized Precipitation Index (SPI), which considers only precipitation in the calculation ( P ), the SPEI also takes into account potential evapotranspiration ( ET _P or PET ) as the second element of the hydro-climatic balance (i. e., P – ET _P ). Therefore, the SPEI index also includes the effect of the evapotranspiration component in the monitoring of drought events, the impact of which has become increasingly relevant with the increase in temperatures due to climate change. High rates of evapotranspiration due to high temperatures can significantly influence the processes of soil-atmosphere interaction and therefore the mechanisms of formation of surface runoff, as well as infiltration and consequent recharge to underground water bodies.

Positive values ​​of the index highlight a hydro-climatic balance greater than the reference climatology, i. e. Humid conditions, while negative values ​​indicate a hydro-climatic balance lower than the climatology, i. e. More or less extreme dry conditions. This index is often used at a regional and/or hydrographic district level for monitoring and identifying dry periods, making use of the rainfall and temperatures recorded by the regional rainfall networks and the autonomous provinces for its calculation. Furthermore, it has been included both in the European context (from " Water Scarcity & Drought ” Expert Group from the Common Implementation Strategy for the Water Framework Directive 2000/60/EC) and international (since World Meteorological Organization ) as one of the tools for drought monitoring. Last but not least, the SPEI indicator was also included in the national guidelines which identify the common set of indicators to be adopted in the permanent district observatories for water uses (established in 2016 as a plan measure pursuant to the Water Framework Directive 2000/60/EC and body of the District Basin Authorities pursuant to Legislative Decree 39/2023) in the monitoring activities of drought situations and water scarcity.

For each area under consideration, the calculation of the SPEI is based on the normalization of the probability distribution of the variable P ‒ ET _P (hydro-climatic balance) aggregated over different time scales (e. g., 1, 3, 6, 12, 24 or 48 months). In doing so, regions characterized by different climatic regimes can be compared. For the purpose of a national and large-scale assessment of hydrological drought conditions, the 12-month SPEI maps are used, taking as precipitation and potential evapotranspiration data those on the regular 1 km resolution grid of the ISPRA BIGBANG hydrological balance model and as climatological reference the period 1952-2024 (reference baseline).