Panel 1
Giovanni Braca
The indicator provides, for each month of the year, the evaluation, by means of the Thornthwaite method, of the water height of per capitapiratospiration of monthly cumulated potential for the natural soil, expressed in mm, as follows to the surface of the national territory.
In 2024, the estimated value of the total potential annual subpostering on natural soil was 806,0 mm, deviating by +16% from the average value for the period 1951–2024, estimated at 695.4 mm.
Evapotranspiration is defined as the amount of water that moves into the atmosphere for evaporation phenomena directed by the bodies of water, soil and vegetation (interception) and perspiration of vegetation. We talk about potential evapotranspiration (potential evapotranspiration) when the water content in the soil is not a limiting factor and it depends only on climatic characteristics (temperature, wind, relative humidity, etc.). Therefore, potential evapotranspiration represents the maximum amount of water that can be transformed into steam by the atmospheric factor complex and vegetation. Potential evaporation, therefore, is always greater than or equal to the actual evapotranspiration.
The monthly scale indicator is evaluated with Thornthwaite’s method that uses only information about the average monthly temperature and the average monthly number of daily insolation hours, a function only of latitude and the month of the year. The calculation of the indicator is carried out from the evaluation of the potential evapotranspiration on a regular grid of resolution 1 km covering the entire national territory, in turn calculated on the basis of the distribution of the average monthly temperature on the same grid. Potential evapotranspiration underlies the models for estimating real evapotranspiration. The difference between potential and real evapotranspiration constitutes the so-called "water deficit", that is, the amount of water that would be lacking in the vegetation for its maximum development. For crops, this deficit should be artificially supplied in whole or in part by irrigation.
Potential evapotranspiration is only evaluated on natural soil to be consistent with the estimation of real evapotranspiration, which, for the calculation scheme adopted, refers only to the natural soil.
Potential evapotranspiration is also used for the calculation of climate indices such as the aridity index (IA) defined by the United Nations Environment Programme (UNEP), adopted as an official index under the United Nations Convention on the Fight against Drought and Desertification, which summarizes the climatic characteristics of the territory. The aridity index is defined as the ratio of annual precipitation (P) to potential evapotranspiration (ETPs):
Ia - P / ETP
Provide, within the estimation of the available or potential water resource, an assessment of the maximum amount of water that would pass into the atmosphere, through evaporation and transpiration processes, if the amount of water in the soil is not a limiting factor.
Existing legislation does not set specific environmental targets.
Panel 2
- Braca, G., Mariani, S., Lastoria, B., Tropeano, R., Casaioli, M., Piva, F., Marchetti, G., e Bussettini, M., 2024: Bilancio idrologico nazionale: stime BIGBANG e indicatori sulla risorsa idrica. Aggiornamento al 2023. Rapporti n. 401/2024, ISPRA, Roma. https://www.isprambiente.gov.it/it/pubblicazioni/rapporti/bilancio-idrologico-nazionale-stime-bigbang-e-indicatori-sulla-risorsa-idrica-aggiornamento-al-2023.
- Braca, G., Mariani, S., Lastoria, B., Piva, F., Archi, F., Botto, A., Casaioli, M., Forte, T., Marchetti, G., Peruzzi, C., Tropeano, R., Vendetti, C., e Bussettini, M., 2023: Bilancio idrologico nazionale: focus su siccità e disponibilità naturale della risorsa idrica rinnovabile. Aggiornamento al 2022. Rapporti n. 388/2023, Roma. Disponibile online all’indirizzo: https://www.isprambiente.gov.it/it/pubblicazioni/rapporti/bilancio-idrologico-nazionale-focus-su-siccita-e-disponibilita-naturale-della-risorsa-idrica-rinnovabile-aggiornamento-al-2022.
- Braca, G., Bussettini, M., Lastoria, B., Mariani, S., e Piva, F., 2021: Il Bilancio Idrologico Gis BAsed a scala Nazionale su Griglia regolare – BIGBANG: metodologia e stime. Rapporto sulla disponibilità naturale della risorsa idrica. Rapporti n. s339/21, Roma. Disponibile online all’indirizzo: 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., Bussettini, M., 2024: “Il bilancio idrologico, la disponibilità di risorsa idrica e il bilancio idrico”, in Siccità, scarsità e crisi idriche, Emanuele Romano, Ivan Portoghese (a cura di), Habitat signa 1, 29-46. Roma: Cnr Edizioni. Disponibile online all’indirizzo: https://www.cnr.it/sites/default/files/public/media/attivita/editoria/SiccitaInterattivo_ver2.pdf.
- Moisello, U., 1998: Idrologia tecnica, La Goliardica Pavese.
- 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 9.0, of ISPRA, which uses medium temperature grids obtained from the spatial interpolation of a set of thermometric stations that could be not homogeneously distributed on the territory and whose number could be different from one year to another. 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 takes into account only the average temperature and does not take into account the type of vegetation. In addition, previously published estimates referring to earlier versions of the BIGBANG model may have slight deviations from the estimates of the latest version available as some data or schematizations at the base of the model may have changed. Correct comparisons between the values of the indicator 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.
Data quality assessment
ISPRA
Regional hydro-meteorological services and autonomous provinces
he 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 Tire Service (SIMN, now merged into ISPRA) of the Department for National Technical Services, were transferred. The temperature data used in the processing of the indicator shall be accessible by consulting the websites of the entities. The temperature data are also accessible via the SCIA portal of ISPRA (https://scia.isprambiente.it). The temperature and potential evapotranspiration data calculated on natural soil, aggregated to the monthly scale on the regular grid of the 1 km resolution BIGBANG, which covers the entire national territory, are available on the Groupware portal of ISPRA (https://groupware.sinanet.isprambiente.it/bigbang-data/library/bigbang_90).
national
1951–2024
Indicator assessment
The calculation method used is that of Thornthwaite as implemented in the BIGBANG-Billion Cycle Gis BAsed model at National Scale on Regular Grid, version 9.0, developed and implemented in GIS environment by ISPRA. Thornthwaite’s formula provides potential evapotranspiration (ETP or PET) for the i-th month as a function of only the average monthly Ti temperature and some coefficients (bi, I, a) that depend only on the monthly temperature, the month of the year and the latitude:
ETP = 16 x bi x (10 Ti / I)^a
The annual value of the indicator in 2024 was higher than the 1951-2024 average.
The environmental situation is negative as it tends towards an increase in water deficits for vegetation.
It is noted, through the Mann-Kendall test, an increasing trend, statistically significant at 5%, in the period 1951–2024.
The increase in potential evapotranspiration leads to a reduction in the aridity index that highlights a trend towards more arid climates. The trend of potential evapotranspiration, as it is calculated, is closely linked to the temperature trend.
Data
Figure 1: Monthly potential evapotranspiration height, calculated on natural soil, in relation to the national territory for 2024 and average for the period 1951-2024
ISPRA elaboration on data from regional hydro-meteorological offices and autonomous provinces and on historical data of the Suppressed SIMN
Dati per Figura 1
Elaborazione ISPRA su dati degli uffici idro-meteorologici regionali e delle province autonome e su dati storici del soppresso SIMN
In 2024, at national level, the estimated value of the total annual potential evapotranspiration on natural soil was 806.0 mm, deviating by +16% from the average value for the period 1951–2024, estimated at 695,4 mm. In the various months of the year, the percentage deviation from the average for the period 1951–2024 was always positive between a maximum of 92% in February and a low in September of 1% (Figure 1).
Positive values of the deviation indicate that in recent months temperatures have been higher than average. In fact, potential evapotranspiration is a climatic parameter that, evaluated by the Thornthwaite method, depends on the temperature alone, so its trend over the year reflects the trend of this magnitude.