Réf. Frantar & al. 2008 - P

Référence bibliographique complète

XXIVth Conference of the Danubian Countries. FRANTAR, P., DOLINAR, M., KURNIK, B. 2008. Water Balance of Slovenia 1971 – 2000. IOP Conf. Series: Earth and Environmental Science, 4,  012020, doi:10.1088/1755-1307/4/1/012020

Abstract: The water is becoming more and more valuable natural resource. The increasing water demand and climate changes are making water a precious and not always available valuable. The water balance is the most appropriate way to make a full overview of water cycle in Slovenia, to find general information about hydrological characteristics of drainage basins, precipitation, evaporation and runoff. The article presents the methodology and the results of the Water balance project of Slovenia. Slovenia has the geographical position at the juncture of 4 main European georegions: The Alps, the Panonian Basin, the Mediterranean and the Dinaric Mountains. This makes the territory very diverse also from a hydrological point of view. Our major watershed divides the precipitation runoff into two watershed areas – the Adriatic Sea and the Black Sea. Due to this watershed almost all the Slovenia’s rivers have headwaters in our territory. Water balance is calculation of water inputs and outputs over the defined area. The basic elements of the water balance include all the inflows and outflows for a given basin and serve for the computation of the water regime of a catchment area. It is defined by the parameters precipitation (P), evaporation (E), discharge (Q) and the change of the water reserves (dS). Main results of the water balance elements for the 1971 – 2000 period for Slovenia are: Average annual precipitation in Slovenia is 1579 mm, average annual evapotranspiration is 717 mm and calculated runoff is 862 mm. Compared to water amounts in the World, where the average precipitation is 750 mm, evapotranspiration is 480 mm and runoff is 270 mm, Slovenia shows an abundance of water quantities. Also the runoff coefficient with 55 % is much higher as 36 % of the world. The major questions remain if we are capable to live with this water amounts within the limits of sustainable development and what will be the effects of climate change to water balance.


Organismes / Contact
• Environmental Agency of the Republic of Slovenia, Ljubljana, Slovenia - Peter.Frantar@gov.si, Mojca.Dolinar@gov.si
• Joint Research Center, Institute for Environment and Sustainability, ISPRA, Italy - Blaz.Kurnik@jrc.it

(1) - Paramètre(s) atmosphérique(s) modifié(s)
(2) - Elément(s) du milieu impacté(s)
(3) - Type(s) d'aléa impacté(s)
(3) - Sous-type(s) d'aléa
Precipitation, Evaporation River discharge, Water reserves    

Pays / Zone
Massif / Secteur
Site(s) d'étude
Période(s) d'observation
Slovenia         1971-2000

(1) - Modifications des paramètres atmosphériques

A direct comparison with the 1961-1990 reference period water balance [6] shows that the quantities of precipitation were almost the same in the 1971-2000 period [see below].


Informations complémentaires (données utilisées, méthode, scénarios, etc.)

In the 1971-2000 reference period, the quantities of precipitation were measured at 394 stations. The authors used data from stations with at least 25 years of operation within the period treated for the analysis. To achieve a greater spatial coverage with the stations, they included 8 additional meteorological stations with shorter time series. Finally, there were 201 precipitation stations included in the analysis. For the calculation of the spatial distribution they also used measurements from totalizer rain gauges in 18 locations in the Julian Alps, the Karavanke Mountains and the Dinaric Alps available for the period treated. In addition to data from the stations in the territory of Slovenia, also the data on the daily precipitation from 29 stations located in Austria, Croatia and Italy were included.

The quantities of precipitation are measured in Slovenia using the Hellmann rain gauge. These measurements are partially underestimated the amount of precipitation due to various effects: evaporation, the wetting of the walls of the Hellmann rain gauge and the effect of the wind blowing the precipitation away from the rain gauge [11]. For the purpose of calculating the water balance, the measured precipitation were corrected, considering the effect of the wind, the precipitation intensity and the wetting of the Hellmann rain gauge [7][3]. The missing data was interpolated on a daily and monthly level based on the values from neighboring stations. The average correction factors for the precipitation stations are similar to those calculated for the 1961-1990 period [6]. Differences occurred at some of the individual precipitation stations, but no systematic deviations could be observed.

The differences between the corrected and measured precipitation values are highest in the mountainous regions and the smallest in western and south-western Slovenia.

Evaporation or evapotranspiration (ETP) is the transfer of water in the form of water vapour from the water surface, the ground and through plant stomata into the atmosphere (Allen, 1998). Evapotranspiration is a process that combines evaporation from un-vegetated ground and water surfaces with the transpiration from plants. We have calculated the actual, real evaporation for the 1971-2000 period, which is given in the average height of the water column in mm.

The real evapotranspiration depends on the variety of plant, the phenological phase of the plant, the ground moisture available to the plants and the meteorological conditions, among which the air temperature affects evaporation the most, followed by relative air humidity, wind speed and solar radiation.

The calculation of the real evaporation was performed for 37 climatological stations using the modified Hargreave’s method balanced for Slovenia with linear regression coefficients concerning their daily value of potential evaporation according to the Penman-Monteith method [1]. This calculation applies to the warm part of the year and to well-wetted ground covered with grass. Because of the difference in evapotranspiration from different types of land cover (forest, farmland, etc.), the values obtained for potential evaporation were corrected by using standard correction coefficients for individual layers of land cover with respect to the potential evaporation.

The calculated average annual values of the real evaporation for individual meteorological stations vary from 355 mm at the station on Kredarica meteorological station to 845 mm measured at the Bilje pri Novi Gorici meteorological station. Evaporation is strongly affected by elevation, so it also depends on the geographical position of the station. Using interpolation, we produced a map of the spatial distribution of the average evaporation.

(2) - Effets du changement climatique sur le milieu naturel

On the territory of Slovenia in the 1971-2000 period, there was an average of 1579 mm of annual precipitation, 717 mm of evaporation and the 862 mm of runoff.

In mm



Precipitation 1567 1579
Evaporation 650 717
Runoff (Q = P – E) 917 862
Runoff coefficient 58.5 % 54.5 %

Table 1. The water balance – a comparison of periods (1961-90 source: [6]) for the territory of the Republic of Slovenia

A direct comparison with the 1961-1990 reference period water balance [6] shows that the quantities of precipitation were almost the same in the 1971-2000 period, but that the quantity of evaporation has increased and runoff has decreased. It shows a significant change in the evaporation and runoffs. Evaporation is higher by 11%, and the runoff is lower by 6%.

[cf. Water balance of Slovenia 1971–2000]


Sensibilité du milieu à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)

The water balance calculation is based on the circulation of water. It assesses the quantities of water in a certain area over a certain time period. It must consider all the inflows and outflows as well as changes in storage. For simple systems, such as a container or water reservoir with measurable inflow and outflow, the balance is straightforward and easily understood. The small water cycle schematic, where the main “inflow” is precipitation and the main “outflow” is evaporation, is also straightforward [8]. The water balance of a selected area, such as a country, region, etc., is much more complicated. Such water balance is invariably a simplified depiction of the actual conditions that covers the essential water balance elements and accurately portrays the relations between them.

The basic water balance equation is based on the circulation of water between the atmosphere and the surface of the Earth [10] [6]: Precipitation (P) = Runoff (Q) + Evaporation (E) + Changes in the storage (dS)

According to the definition, precipitation is atmospheric water that exits from the air either by condensation or sublimation and falls to or towards the ground because of gravity [9]. The term evaporation signifies the transfer of water into vapor from areas of open water, while transpiration means the transfer through vegetation. The term evaporation here encompasses both forms, which are together called evapotranspiration. When analyzing the water balance, we must limit ourselves to the analysis of the areas – to hydrometric catchment areas – whose runoff can be measured as the water flow rate (Q) of the water gauging station.

In addition to precipitation, it is necessary to consider other inflows of water (Qi) when dealing with the water balance of a selected area. The equation is thus as follows: P (precipitation) + Qi (inflow) = Qo (runoff) + E (evaporation) + dS (changes in the storage)

In the balance for the 1971–2000 period the authors have not taken into account changes in water storage (dS) as we assume that these can be neglected over a long-time period [4].

For the main building block of the water balance the hydrometric catchment area was used. This area is delimited by the water divide of the balance cross-sections. Aside from precipitation, the headwater hydrometric catchment areas do not have other inflows. The precipitation surplus (part of precipitation that does not evaporate) simply flows out of them and is measured as a discharge (Q). Intermediate hydrometric catchment areas receive water from both precipitation and inflow from the upstream hydrometric catchment area. Therefore the correct spatial delimitation of the hydrometric catchment area is very important.

The authors analyzed and reconciled the water balance elements of precipitation, evaporation [see above] and runoff for the selected areas. The runoffs derived from the discharges at water gauging stations (Q) were compared with the runoffs calculated using the water balance equation. The correctness of the relationships between the elements of the water balance was reviewed with the help of balance error analysis.

Runoff describes or represents the movement of a certain part of precipitated water into channelled streams or the discharge of water within it [2][10]. It is the surplus of precipitation that does not evaporate and is not used for transpiration. This surplus flows away as runoff. When this surplus is large enough, the runoff is collected into streams, which represent the majority of the runoff from a certain catchment area. In places where the majority of the runoff is collected the runoff can be measured as the discharge. In general, measured discharges are the most reliably measured elements of the water cycle. In suitably located water gauging stations, great majority of water from a certain catchment area runs through the cross-section of the water gauging station.

The characteristics of a discharge at a certain point are a reflection of the entire catchment area [11]. Because of this, knowledge of the physical-geographical space, especially of the gauging profiles and divides is of key importance, as it is only in this way that we can obtain comparable data and analyse smaller river basin units. Discharge data are linked to the space via the hydrometric catchment areas.

Data gaps cause difficulties in the analysis of hydrological data. When reviewing the data on discharges for the 1971-2000 period, it was found that 65 water gauging stations had complete data sets. On other stations, data gaps were supplemented with the use of a statistical method – with the Pearson linear correlation coefficient based on the mean monthly averages of the 1961-2001 period.

Specific Runoff in the 1971-2000 Period:
The specific runoff shows how many mm of water runs off per year on average. The average runoff can be estimated from the measured discharge values at individual water gauging stations or by using the water balance equation – precipitation minus evaporation (Q = P – E). In Slovenia the runoffs measured are very similar to those calculated using the water balance formula, which indicates the correctness of the calculations of precipitation and evapotranspiration. The prevailing characteristic of the specific discharge in Slovenia is that it is highest in the upstream part and gradually decreases downstream [6]. The most important effect on the specific runoff has the climate or, more precisely, the precipitation, which is also reflected in the geographical distribution. The quantity of the specific runoff decreases as you move from the Alps and the Dinaric belt toward the north-east and south-west, which is also shown by the chart of specific runoffs that was produced based on the water balance equation using precipitation and evaporation data.

(3) - Effets du changement climatique sur l'aléa

Paramètres de l'aléa
Sensibilité du paramètre de l'aléa à des paramètres climatiques et du milieu
Informations complémentaires (données utilisées, méthode, scénarios, etc.)

(4) - Remarques générales

(5) - Préconisations et recomandations

Références citées :

[1] Allen R G, Perreira L S, Reas D and Smith M 1998 Crop evapotranspiration – guidelines for computing crop water requirements FAO irrig. and drain. p.

[2] Davie T 2004 Fundamentals of hydrology Routledge Fundamentals of Physical Geography

[3] Dolinar M, Ovsenik-Jeglič T, Bertalanič R 2006 Izračun korigiranih padavin v obdobju 1971– 2000

[4] Frantar P, Hrvatin M 2005 Pretočni režimi v Sloveniji med letoma 1971 – 2000 Geog. vest. 77- 2 115–27

[6] Kolbezen M, Pristov J 1998 Površinski vodotoki in vodna bilanca Slovenije HMZ RS

[7] Nespor V, Sevruk B 1999 Estimation of wind-enduced error of rainfall gauge measurements using a numerical simulation J. of Atm. and O. Tech. 16 450-64

[8] Ritter M 2006 The Water Balance

[9] Schöniger M, Dietrich J 2003 Hydrologie Hydroskript

[10] Van Abs D J, Stanuikynas T J 2000 Water Budget in the Raritan River Basin

[11] World Meteorological Organisation 1994 Guide to hydrological practicies WMO – No. 168