Giovanni Braca
The indicator provides, for each month of the year, the evaluation, using the Thornthwaite method, of the cumulative monthly potential evapotranspiration water height on natural soil, expressed in mm, compared to the surface of the national territory.
In 2025, the estimated value of total annual potential evapotranspiration on natural soil was equal to 775.8 mm, deviating by +11% from the average value for the period 1951–2025, estimated at 695.9 mm.
Evapotranspiration is defined as the quantity of water that transfers into the atmosphere due to the phenomena of direct evaporation from bodies of water, from the soil and from vegetation (interception) and transpiration from vegetation. This is called potential evapotranspiration ( potential evapotranspiration ) when the water content in the soil does not constitute a limiting factor and it depends only on the climatic characteristics (temperature, wind, relative humidity, etc.). Therefore, potential evapotranspiration represents the maximum quantity of water that can be transformed into vapor by the complex of atmospheric factors and vegetation. Potential evapotranspiration, therefore, is always greater than or equal to actual evapotranspiration.
The monthly scale indicator is evaluated with the Thornthwaite method which uses only the information relating to the average monthly temperature and the average monthly number of hours of daily sunshine, a function only of the latitude and the month of the year. The calculation of the indicator is carried out starting from the evaluation of potential evapotranspiration on a regular grid of 1 km resolution which covers the entire national territory, in turn calculated on the basis of the distribution of the average monthly temperature on the same grid. Potential evapotranspiration is the basis of models for estimating actual evapotranspiration. The difference between potential and actual evapotranspiration constitutes the so-called " water deficit ", i. e. The quantity of water that the vegetation would lack for its maximum development. For crops this deficit should be supplied in whole or in part artificially through irrigation.
Potential evapotranspiration is evaluated only on natural soil to be consistent with the estimate of real evapotranspiration, which, due to the calculation scheme adopted, refers only to natural soil.
Potential evapotranspiration is also used to calculate climate indices such as, for example, the aridity index ( Yea ) defined by United Nations Environment Programme (UNEP), adopted as the official index within the United Nations Convention to combat drought and desertification (UNCCD), which qualitatively summarizes the climatic characteristics of the territory. The aridity index is defined as the ratio between the annual precipitation ( P ) and potential evapotranspiration ( ET P ):
Ia = P / ET P
Provide, as part of the estimate of the available or potential water resource, an evaluation of the maximum quantity of water that would pass into the atmosphere, through the processes of evaporation and transpiration, if the quantity of water in the soil does not constitute a limiting factor.
Current legislation does not set specific environmental objectives.
- Braca, G., Mariani, S., Lastoria, B., Piva, F., Archi, F., Botto, A., Casaioli, M., Forte, T., Marchetti, G., Peruzzi, C., Tropeano, R., Vendetti, C., and Bussettini, M., 2023: National hydrological balance: focus on drought and natural availability of renewable water resources. Update to 2022. Reports no. 388/2023, Rome. Available online at: https://www. isprambiente. gov. it/it/pubblicazioni/rapporti/bilancio-idrologico-nazionale-focus-su-siccita-e-disponibilita-naturale-della-risorsa-idrica-rinnovabile-formazione-al-2022.
- Braca, G., Bussettini, M., Lastoria, B., Mariani, S., and Piva, F., 2021: The Gis BAsed Hydrological Balance at National Scale on Regular Grid – BIGBANG: methodology and estimates. Report on the natural availability of water resources. Reports no. S339/21, Rome. Available online at: https://www. isprambiente. gov. it/it/pubblicazioni/rapporti/il-bilancio-idrologico-gis-based-a-scala-nazionale-su-griglia-regolare-bigbang .
- Mariani, S., Braca, G., Lastoria, B., Tropeano, R., Casaioli, M., Piva, F., and Bussettini, M., 2024: “The hydrological balance, the availability of water resources and the water balance”, in Drought, scarcity and water crises, Emanuele Romano, Ivan Portoghese (ed.), Habitat signa 1, 29-46. Rome: Cnr Editions. Available online at: https://www. cnr. it/sites/default/files/public/media/attivita/editoria/SiccitaInterattiva_ver2. pdf .
- Moisello, U., 1998: Technical hydrology, La Goliardica Pavese.
- SNPA, 2026: The climate in Italy in 2025. forthcoming .
- Thornthwaite, C. W., 1948: An approach towards a rational classification of climate, Geogr. Rev., 38, 55–89.
The indicator is estimated on the basis of a mathematical model, implemented in the national hydrological balance model BIGBANG, version 10.0, of ISPRA, which uses average temperature grids obtained from the spatial interpolation of a set of thermometric stations which may not be homogeneously distributed over the territory and whose number may differ from one year to the next. These circumstances, together with the different spatial estimation method used and the different formulation of the indicator, suggest particular attention and caution when comparing the indicator in space and time.
Thornthwaite's mathematical model only takes into account the average temperature and does not take into account the type of vegetation. Furthermore, previously published estimates referring to previous versions of the BIGBANG model may present slight deviations compared to the estimates of the latest available version since some data or schematizations underlying the model may have changed. Correct comparisons between the indicator values must therefore be made using the same version of the BIGBANG model.
Use of models that take into account other climatic variables (wind, relative humidity, etc.) and the type of vegetation.
ISPRA
Regional hydro-meteorological services and those of the autonomous provinces
The temperature data used are mostly those collected and published by the regional and provincial structures to which according to art. 92 of Legislative Decree no. 112 of 31 March 1998, the functions and tasks of the peripheral offices of the National Hydrographic and Mareographic Service (SIMN, now merged into ISPRA) of the Department for National Technical Services were transferred. The temperature data used in the development of the indicator can be accessed by consulting the institutions' websites. The temperature data are also accessible via the ISPRA SCIA portal ( https://scia. isprambiente. it ). The temperature data and potential evapotranspiration data calculated on natural soil, aggregated to the monthly scale on the regular BIGBANG grid of 1 km resolution, which covers the entire national territory, are available on the ISPRA Groupware portal ( https://groupware. sinanet. isprambiente. it/bigbang-data/library/bigbang100).
National
1951–2025
The calculation method used is Thornthwaite's as implemented in the BIGBANG-Gis BAsed Hydrological Balance model at a national scale on a regular grid, version 10.0, developed and implemented in a GIS environment by ISPRA. Thornthwaite's formula provides potential evapotranspiration ( ET P or PET ) for the month the -th based only on the average monthly temperature T the and some coefficients ( b the , THE , to ) which depend only on the monthly temperature, the month of the year and the latitude:
ET P = 16 x bi x (10 T the / THE )^ to
The annual value of the indicator in 2025 was higher than the average for the period 1951–2025.
The environmental situation is negative as it tends towards an increase in the water deficit for vegetation.
Using the Mann-Kendall test, an increasing trend is detected, statistically significant at 5%, in the period 1951–2025.
The increase in potential evapotranspiration leads to a reduction in the aridity index which highlights a trend towards drier climates. The trend of potential evapotranspiration, as it is calculated, is closely linked to the temperature trend.
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Figure 1: Height of monthly potential evapotranspiration, calculated on natural soil, compared to the national territory for 2025 and average for the period 1951–2025 Fonte
ISPRA processing on data from regional hydro-meteorological offices and autonomous provinces and historical data from the SIMN suppressed |
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In 2025, at a national level, the estimated value of total annual potential evapotranspiration on natural soil was equal to 775.8 mm, deviating by +11% from the average value relating to the period 1951–2025, estimated at 695.9 mm. In the various months of the year, the percentage deviation from the average for the period 1951–2025 was always positive, except for the month of October, with a maximum of 43% in the month of December (Figure 1).
Positive values of the deviation indicate that in recent months temperatures have been higher than average. In fact, potential evapotranspiration is a climate parameter which, evaluated with the Thornthwaite method, depends only on temperature, so its trend over the year reflects the trend of this quantity.