Vodnobilan~ni ra~un temelji na kro`enju vode.Kaj je vodni krog?

Voda na Zemlji je v stalnem gibanju, hkra-ti pa spreminja fizikalna stanja – od teko~ega inplinastega do trdnega ter obratno. Neprestanopremikanje vode z vsemi stanji opisuje vodnikrog, ki deluje na Zemlji ̀ e milijarde let.Vse ̀ iv-ljenje temelji na njem.

Pri vodnem kro`enju na Zemlji lo~imo maliin veliki vodni krog. Kro`enje vode med oceaniin atmosfero predstavlja glede na koli~ine glav-nino vodnega kroga. Imenujemo ga tudi malivodni krog.Veliki vodni krog poteka tudi na kop-nem in je bolj razvejan in raznovrsten. Sevedase oba krogotoka prepletata.

Vodna bilanca ocenjuje koli~ine vode na dolo-~enem obmo~ju v dolo~enem ~asovnem obdob-ju. Upo{tevati mora vse dotoke in odtoke terspremembe zalog. Za preprost sistem, kakr{en jenpr. posoda ali vodni zbiralnik z merljivim doto-kom in odtokom, je bilanca preprosta in lahkorazumljiva. Shemati~no je preprosta tudi bilan-ca malega vodnega kroga, pri katerem so glavni»dotok« padavine, glavni »odtok« pa je izhlape-vanje (Ritter, 2006).Veliko bolj zapletena je vod-na bilanca izbranega obmo~ja, kot je npr. dr`ava,pokrajina itd.Tovrstna bilanca je vedno poeno-stavljen prikaz dejanskih razmer, smiselna pa, ~ezajame vse bistvene bilan~ne elemente in pra-vilno poda razmerja med njimi.

Osnovna ena~ba vodne bilance temelji nakro`enju vode med ozra~jem in povr{jem Zem-lje (Van Abs et al., 2000; Kolbezen et al., 1998):

Padavine so po definiciji atmosferska voda,ki po kondenzaciji in sublimaciji izhaja iz zrakain zaradi te`nosti pada proti tlem oz. pade na tla(DIN 1996 po Schöniger et al., 2003).S pojmom

The water balance calculation is based on thecirculation of water. What is the water cycle?

Water on Earth is in constant motion and, atthe same time, it changes its physical states – fromliquid and gaseous to solid and vice versa.The con-stant movement of water in all its physical statesis the water cycle that has been going on on Earthfor billions of years. All life is based on it.

We distinguish between the small water cycleand the large water cycle in the circulation ofwater on Earth.The circulation of water betweenthe oceans and the atmosphere represents themajor part of the water cycle because of thequantities involved.We also call it the small watercycle.The large water cycle takes place on landand is more diversified and differentiated. Bothcycles, of course, intertwine with one another.

The water balance assesses the quantities ofwater in a certain area and over a certain timeperiod. It must consider all the inflows and out-flows as well as changes in storage. For simplesystems, such as a container or water reservoirwith measurable inflow and outflow, the balanceis straightforward and easily understood. Thesmall water cycle schematic, where the main»inflow« is precipitation and the main »outflow« isevaporation, is also straightforward (Ritter, 2006).The water balance of a selected area, such asa country, region, etc., is much more complicated.Such a balance is invariably a simplified depic-tion of the actual conditions and is only prac-tical if it covers all the essential water balanceelements and accurately portrays the relationsbetween them.

The basic water balance equation is based onthe circulation of water between the atmosphereand the surface of the Earth (Van Abs et al.,2000, Kolbezen et al., 1998):

9

2. VODNA BILANCA / WATER BALANCE

2Vodna bilancaWater Balance

Marjan Bat, Peter Frantar

Padavine (P) = Odtok (Q) + Izhlapevanje (I) ++ Sprememba zalog (dS)

Precipitation (P) = Runoff (Q) + Evaporation (I) ++ Changes in the storage (dS)

10

VODNA BILANCA SLOVENIJE 1971–2000 / WATER BALANCE OF SLOVENIA 1971–2000

izhlapevanje ozna~imo prehajanje vode v paroz odprtih vodnih povr{in, kar pojmujemo kotevaporacijo, in preko rastlinstva, kjer prehajanjepoteka s transpiracijo. Izraz izhlapevanje zajemaobe obliki, ki ju zdru`eno imenujemo tudi eva-potranspiracija.Z odtokom zaobjamemo razli~neoblike pretakanja vode od ploskovnega povr-{inskega odtoka, premikanja (infiltracije in pro-nicanja) vode v preperelini (prst, tla) in tokovpodzemne vode, do pretoka rek.V na{ih razme-rah zaradi dolgega obdobja analize lahko zane-marimo sneg in led. V vodni bilanci se omejimona analizo obmo~ij – hidrometri~nih zaledij,katerih odtok lahko izmerimo kot pretok vode(Q) skozi merski profil vodomerne postaje.

Pri obravnavi vodne bilance izbranega obmo~-ja je potrebno poleg padavin upo{tevati {e osta-le dotoke vode (Qv).Tako ena~bo lahko zapi{emo:

Po priporo~ilih Svetovne meteorolo{ke orga-nizacije je povpre~ne podnebne in hidrolo{ke

According to the definition, precipitation isatmospheric water that exits from the air eitherby condensation or sublimation and falls to ortowards the ground because of gravity (DIN 1996according to Schöniger et al., 2003). The termevaporation signifies the transfer of water intovapour from areas of open water, while tran-spiration means the transfer through vegetation.The term evaporation in this book encompass-es both forms, which are together called eva-potranspiration.The term runoff covers variousforms of water transfer ranging from arial sur-face runoff, the movement (infiltration and per-colation) of water into the eluvium (soil, ground)and the flow of groundwater, to the dischargeof rivers. Snow and ice may be disregarded inour analysis due to longterm period concerned.When analysing the water balance, we limit our-selves to the analysis of the areas – to hydro-metric catchment areas – whose runoff can bemeasured as the water flow rate (Q) throughthe water-gauging cross-section of the watergauging station.

In addition to precipitation, it is necessaryto consider other inflows of water (Qv) whendealing with the water balance of a selected area.The equation is thus as follows:

According to the recommendations of theWorld Meteorological Organization, it is reason-able to treat the average climatic and hydrologi-cal conditions in a certain area covering a longer(30-year) period.To calculate the water balance,we covered precipitation, evaporation and run-offs in the 1971–2000 period.We have not con-sidered changes in water storage (dS) in the bal-ance for the 1971–2000 period, as we assumethat these can be neglected over a long-time peri-od (Frantar et al., 2005).

In a spatial sense, the main building block ofthe water balance is the hydrometric catchmentarea. This area (polygon) is delimited by thewater divide of balance cross-sections that arepredominantly the water divides of the watergauging stations used.Aside from precipitation,the headwater hydrometric catchment areas donot have other inflows. The precipitation sur-plus that does not evaporate and simply flowsout of them and is measured as a discharge (Q)on the water balance cross-section at the appro-priate water gauging station. Intermediate hydro-metric catchment areas receive water from bothprecipitation and inflow from the upstreamhydrometric catchment area. The inflow (Qv)and runoff (Qi) are measured as the discharges

http

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s.gov

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ercy

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i.htm

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Slika 3: Kro`enjevode na Zemlji

Figure 3:The circulation of water on Earth

P (padavine) + Qv (dotok) = Qo (odtok) + + I (izhlapevanje) + dS (sprememba zalog)

P (precipitation) + Qv (inflow) = Qo (runoff) + + I (evaporation) + dS (changes in the storage)

razmere na nekem obmo~ju smiselno obravna-vati v dalj{em (30-letnem) obdobju. Za izdelavovodne bilance smo obravnavali padavine, izhlape-vanje in odtoke v obdobju 1971–2000.V bilan-ci obdobja 1971–2000 sprememb vodnih zalog(dS) nismo upo{tevali, saj predpostavljamo, daso zaradi povpre~evanja zanemarljive (Frantaret al., 2005).

V prostorskem smislu je osnovni gradnik vod-ne bilance hidrometri~no zaledje.To je obmo~je(poligon) omejeno z razvodnicami bilan~nih pro-filov, ki so v ve~ini kar razvodnice uporabljenihvodomernih postaj. Povirna hidrometri~na zaled-ja razen padavin nimajo dotokov, prese`ek pada-vin, ki ne izhlapi, pa iz njih odte~e in ga kotpretok (Q) izmerimo na bilan~nem profilu naustrezni vodomerni postaji. Preto~na hidrome-tri~na zaledja prejemajo vodo s padavinami indotokom iz gorvodnega hidrometri~nega zaled-ja. Dotok (Qv) in odtok (Qi) merimo kot pretokna gorvodnih in dolvodnih vodomernih posta-jah. Razlika med njima je neto odtok hidrome-tri~nega zaledja (Qi – Qv = Qn). Pri povirnihhidrometri~nih zaledjih je pretok, ki ga izme-rimo ne merskem profilu `e neto odtok(Q = Qn). Pravilna prostorska razmejitev hidro-metri~nega zaledja je torej zelo pomembna.

Za bilan~ne analize so bile izbrane postaje,ki imajo popolne podatkovne nize pretokov alipa jih je bilo s pomo~jo statisti~nih metodmogo~e dopolniti. Nepopolne nize vodomernihpostaj smo dopolnili s pomo~jo Pearsonovegakoeficienta linearne korelacije na podlagi sred-njih mese~nih pretokov obdobja 1961–2001.Postaj, ki imajo ustrezne podatkovne nize, nima-jo pa zanesljivo omejenih hidrometri~nih zale-dij, v bilanci nismo upo{tevali (npr. vodomernapostaja Hasberg na Unici ima kra{ko vodozbir-no zaledje, katerega obseg se spreminja v odvi-snosti od hidrolo{kih razmer).

Za izbrana obmo~ja smo analizirali in uskla-jevali ~lene vodne bilance – padavine, izhlape-vanje in odtok. Odtoke, ki so izvedeni izpretokov vodomernih postaj (Q), smo primer-jali z odtoki, ki jih izra~unamo z bilan~no ena~-bo (Qb = Q = P – I).

Pravilnost razmerij med elementi vodne bi-lance smo pregledali s pomo~jo analize bilan~-ne napake. Bilan~na napaka je oblika kontroleosnovnih elementov vodnega kroga in medse-bojnega vpliva. Vzroki za odstopanja med nji-mi so lahko razli~ni, od napake meritev pa donezanesljivih razvodij na vodonosnikih z velikovodoprepustnostjo (kra{ki razpoklinski ali vodo-nosniki z medzrnsko poroznostjo).

V vodni bilanci pretoke (Q) pravilomapodajamo v m3/s, padavine (P) in izhlapevanje(I) pa v mm. Zaradi medsebojne primerjave pasmo lahko tudi padavine in izhlapevanje izrazili

at the upstream and downstream water gaug-ing stations.The difference between them is thenet runoff of the hydrometric catchment area(Qi – Qv = Qn).The discharge measured at thewater-gauging cross-section in the headwaterhydrometric catchment area is the net runoff(Q = Qn).The correct spatial delimitation of thehydrometric catchment area is therefore veryimportant.

The water balance analysis required theselection of water gauging stations with com-plete data sets for discharges or with data setsthat could be supplemented with the aid of sta-tistical methods. Incomplete data sets weresupplemented with the aid of the Pearson lin-ear correlation coefficient based on the meanmonthly discharges in the 1961–2001 period.Stations with suitable data sets but without reli-ably delimited hydrometric catchment areaswere also not taken into account in the waterbalance (e. g. the Hasberg water gauging stationon the Unica River has a karstic water basincatchment area whose size changes dependingon the hydrological conditions).

We analysed and reconciled the water bal-ance elements of precipitation, evaporationand runoff for the selected areas. The runoffsderived from the discharges at water gauging sta-tions (Q) were compared with the runoffs cal-culated using the water balance equation(Qb = Q = P – I).

The correctness of the relationships betweenthe elements of the water balance was reviewedwith the help of balance error analysis.The bal-ance error is a form of control over the basic ele-ments of the water cycle and their interaction.The causes of deviations between them arediverse, ranging from an error in the measure-ment to unreliable water divides in aquifers withhigh permeability (karstic fissured aquifers oraquifers with intergranular porosity).

11

2. VODNA BILANCA / WATER BALANCE

Slika 4: Dero~avoda U~je

Figure 4: Stormwater in the U~jaRiver

AR

SO

12

VODNA BILANCA SLOVENIJE 1971–2000 / WATER BALANCE OF SLOVENIA 1971–2000

The discharges (Q) in the water balance are,as a rule, expressed in m3/s, while precipitation(P) and evaporation (I) are expressed in mm. Forthe purpose of comparison, we also expressedprecipitation and evaporation in m3/s and thedischarge in mm. The surface area of a sectoris required for the conversion. Specific runoffsgive the average quantities of water that ran outof a certain sector and these are expressed inl/s/km2 or in mm. They are derived from thereference period mean discharges (Qs) – Hq orare calculated based on the balance equationwith the difference between the precipitationand evaporation – Kq.

The runoff coefficients give the share ofprecipitation that runs out of a certain area andare obtained based on the measured hydrologicaldischarges and precipitation (KH – hydrologi-cal runoff coefficient) or on the basis of the cal-culated runoffs (KK – climatic runoff coefficient).

Except in the Trends chapter, the dataalways refers to the mean annual values forthe 1971–2000 period if not stated otherwise.

#

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Ve{ter

Slika 5: Primer povirnega hidrometri~nega zaledja jevodozbirno zaledje Sel{ke Sore gorvodno od vodomernepostaje @elezniki, primer preto~nega pa vodozbirno zaledjeSel{ke Sore med vodomernima postajam @elezniki inVe{ter. Hidrometri~na zaledja so omejena z razvodnico.

Figure 5: An example of a headwater hydrometriccatchment area is the watershed area of the Sel{kaSora River upstream from the @elezniki water gaugingstation, while an example of an intermediate hydrometriccatchment area is the area of the Sel{ka Sora Riverbetween the water gauging stations of @elezniki andVe{ter. The hydrometric catchment areas are delimitedby a drainage divide.

v m3/s in pretok v mm. Za pretvorbo je potre-ben podatek o povr{ini obmo~ja.Specifi~ni odto-ki podajajo povpre~no koli~ino odtekle vodez dolo~enega obmo~ja in so izra`eni v l/s/km2 aliv mm. Izvedeni so iz obdobnih srednjih letnihpretokov (Qs) – Hq ali pa so izra~unani na osno-vi bilan~nega ra~una z razliko padavin in izhla-pevanja – Kq.

Odto~ni koli~niki podajajo dele` padavin, kiodte~ejo z dolo~enega obmo~ja, in so pridob-ljeni na osnovi izmerjenih hidrolo{kih pretokovin padavin (KH – hidrolo{ki odto~ni koli~nik)ali na osnovi izra~unanih odtokov (KK – klima-tolo{ki odto~ni koli~nik).

Razen v poglavju o trendih podatki ved-no pomenijo srednje letne vrednosti obdob-ja 1971–2000, ~e ni druga~e navedeno.

13

2. VODNA BILANCA / WATER BALANCE

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sta

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on

surf

ace

wat

ers

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08

Slika 7: Predaselj / Figure 7: The Predaselj gorge

PET

ER F

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