RUNOFF INDEX

    Panel 1
    Data aggiornamento scheda
    Autori

    Giovanni Braca, Robertino Tropeano

    Abstract
    Grafici interattivi
    Abstract

    The indicator provides on an annual basis the assessment, expressed as a percentage, of the ratio between the annual volume of the runoff (surface preselling) and the annual volume of precipitation. This indicator aims to provide an assessment of the amount of water that is directly transformed into a surface runflow compared to total precipitation and to assess the trend over the long term, also in relation to the possible impact due to climate change. The annual values of the indicator shall also be compared with the corresponding average value calculated over the entire period 1951–2024.

    In 2024, the value of the indicator was 26.1%, higher than the average value of 25.1% in the long term. This shows a trend reversal compared to 2023, in which there was a value of 23.7%, below the average. The comparison is even more marked with 2022, the year in which the indicator reached an all-time low of 18.5%.

    Descrizione

    The indicator evaluates the percentage ratio between the annual volume of surface runoff, i.e. the rate of precipitation that is directly transformed into a surface runoff, and the annual volume of precipitation summarised to the national territory. By runoff in particular we mean the rate of precipitation that does not be able to infiltrate the ground due to the saturation of the soil or the reduced infiltration capacity, flows on the surface of the ground until it reaches the hydrographic lattice producing the surface outflow.

    Scopo

    Provide an assessment of the amount of water that turns into surface runoff in relation to total precipitation. The indicator is specifically defined to highlight possible impacts of climate change on the precipitation and outflow regime. The possible trend, purified of the effect of the change in the water-recognizing of the soils, could be an indication of a change in the extent and distribution of precipitation over the year.

    Rilevanza
    È di portata nazionale oppure applicabile a temi ambientali a livello regionale ma di significato nazionale
    È in grado di descrivere la tendenza senza necessariamente fornire una valutazione della stessa.
    È semplice, facile da interpretare.
    È sensibile ai cambiamenti che si verificano nell'ambiente e/o nelle attività umane
    Fornisce un quadro rappresentativo delle condizioni ambientali, delle pressioni sull'ambiente e delle risposte della società.
    Fornisce una base per confronti internazionali.
    Solidità
    È ben fondato sul piano tecnico e scientifico.
    Presenta affidabilità e attendibilità dei metodi di misurazione e raccolta dei dati
    Comparabilità nel tempo
    Comparabilità nello spazio
    Misurabilità (dati)
    Adeguatamente documentati e di fonte nota
    Aggiornati a intervalli regolari e con procedure affidabili
    Facilmente disponibili o resi disponibili a fronte di un ragionevole rapporto costi/benefici
    Un’ “adeguata” copertura spaziale
    Un’ “idonea” copertura temporale
    Principali riferimenti normativi e obiettivi

    There is no regulatory reference.

    DPSIR
    Stato
    Tipologia indicatore
    Descrittivo (A)
    Riferimenti bibliografici
    Limitazioni

    The indicator is built at the annual scale from estimates of runoff and precipitation at the monthly scale made with the BIGBANG national hydrological budget model, version 9.0, of ISPRA based on official data produced at national and local level. Such time aggregations shall not take into account the effect of the precipitation intensity affecting infiltration and runoff. In addition, previously published estimates of the BIGBANG model may have slight deviations from the estimates of the latest version available as some data or schematizations underlying the model may be improved. Correct comparisons between the values of the indicator must therefore be made using the same version of the BIGBANG model.

    Ulteriori azioni

    A daily scale indicator processing would allow to adequately grasp the effect of the precipitation intensity on the infiltration of the soils and on the runoff.

    Fonte dei dati

    ISPRA

    JRC (Joint Research Centre)

    Regional hydro-meteorological services and autonomous provinces

    Frequenza di rilevazione dei dati
    Monthly
    Accessibilità dei dati di base

    The precipitation and temperature data used are mostly those collected and published by the regional and provincial structures to which, according to Article 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 precipitation data are collected directly from the regional structures and autonomous provinces, while for the temperature are used the maps produced under the SCIA system of ISPRA (https://scia.isprambiente.it). The precipitation and temperature data aggregated to the monthly scale on the regular BIGBANG grid of resolution 1 km, which covers the entire national territory, are available on the Groupware portal of ISPRA (https://groupware.sinanet.isprambiente.it/bigbang-data/library/bigbang-90). The data on the hydraulic characteristics of the soils are accessible (subject to registration) to the JRC/ESDAC portal (http://esdac.jrc.ec.europa.eu/content/lucas-2009-topsoil-data). For the permeability data of hydrogeological complexes, a new permeability paper produced by ISPRA was used compared to previous years.

    Copertura spaziale

    national

    Copertura temporale

    1951–2024

    Descrizione della metodologia di elaborazione

    The estimate of the indicator series from 1951 to 2024 is carried out through the national model of hydrological budget on a monthly scale developed by ISPRA, called BIGBANG-Billionological Impact Gis BAsed on a National Scale on Regular Grid, version 9.0, which evaluates the runoff as the end of the hydrological soil balance with the Thornthwaite and Mather method and the precipitation as a spatial interpolation of data recorded by the networks.

    The indicator is calculated from the monthly evaluation of the runoff and precipitation on a regular grid of 1 km resolution that covers the entire national territory and aggregated to the annual scale. For the year i-th, the IRFi indicator (in %) is calculated as the ratio of the annual RFI runoff to the annual TPi precipitation:

    IRFi = (RFi/TPi) x 100.

    Periodicità di aggiornamento
    Annuale
    Data quality

    The data used for the indicator are reliable, as they come from the bodies responsible for the law of hydro-meteorological monitoring, and present a homogeneous coverage on a national scale.

    Stato
    Buono
    Trend
    Negativo
    Valutazione/descrizione dello stato

    In 2024 the runoff index was 26.1% and is higher than the long-term 1951–2024 average, or 25.1%. This means that only 26.1% of the total annual precipitation (estimated at 318.8 billion cubic meters) has directly turned into surface runflow (83.3 billion cubic meters).

    In terms of assessing the amount of water that turns into surface runoff in relation to total precipitation, resulting in the runoff index above the long-term average, the state in 2024 can be defined as positive as it indicates greater availability of surface water resources.

    Valutazione/descrizione del trend

    The runoff index has a slight, statistically significant trend based on the Mann-Kendall test, with a significance level of 5%. The indicator takes into account the effect of the temporal variability of the soil sealing.

    The trend of the indicator is attributable to two factors, the increase in temperature and the waterproofing of the soils, which act in the opposite direction. On the one hand, the average temperature has an increasing trend, due to climate change, which produces, due to the consequent increase in the share of evapotranspiration, a reduction in the availability of renewable water (internal flow) and therefore the rate of precipitation that turns into runoff. The national series of average temperature anomalies, compared to the average value over the 30-year climate 1991–2020, shows a significant increase in recent decades. At the national level, 2024 was the hottest year since 1961 (+1.33 C) and the eleventh consecutive year with a positive anomaly compared to the climatological average of the last thirty years of reference. On the other hand, the increase in the waterproofing of the soils produces the opposite effect on the runoff.

    Commenti

    The runoff index, in the period 1951–2024, varies between a minimum value of 17.5%, estimated for 1989, and a maximum value of 29.9%, estimated for 1960 (Figure 1). This means that, from 1951 to 2024, no more than 30% of the annual precipitation in Italy has turned into surface runoff, to which must be added the basic outflow, deriving from underground water circulation, to obtain the total annual runoff in watercourses.

    In 2024, the runoff index (26.1%) recorded a temporary reversal of the trend compared to 2023, the year in which it was equal to 23.7% and below the long-term average (25.1%), and compared to 2022, in which the deviation was even greater from the average resulting in the index of 18.5% (second value smaller than the series after the 1989 low).

    Analyzing the historical series of the long-term runoff index remains, however, a negative trend to be attributed to the increase in temperature that involves a reduction in the natural availability of renewable water resources (see the "Internal Flow" indicator present in the Environmental Theme "Water sources and Balance" of the Environmental Indicators Database) of which the runmanship constitutes a rate.

    The series of values of the components of the hydrological balance, and therefore also of the runoff index, are recalculated every year on the basis of new data sets and new schematizations, so a correct comparison in time and space must be made with respect to the same version of the model.

    Allegati
    Thumbnail
    Titolo

    Figure 1: Runoff index in the period 1951–2024

    Fonte

    ISPRA processing through the BIGBANG national hydrological budget model on data from regional hydro-meteorological offices and autonomous provinces and historical data of the SIMN suppressed

    English
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