COBISS: 1.01 UNDERSTANDING THE TEMPORAL VARIATION OF FLOW DIRECTION IN A COMPLEX KARST SYSTEM (PLANINSKA JAMA, SLOVENIA) RAZUMEVANJE ČASOVNE SPREMENLJIVOSTI SMERI TOKA V KOMPLEKSNEM KRAŠKEM SISTEMU (PLANINSKA JAMA, SLOVENIJA) Georg KAUFMANN1,*, Cyril MAYAUD2, Blaž KOGOVŠEK3 & Franci GABROVŠEK4 Abstract UDC 556.3:551.444(497.4Planina) Izvleček UDK 556.3:551.444(497.4Planina) Georg Kaufmann, Cyril Mayaud, Blaž Kogovšek & Franci Georg Kaufmann, Cyril Mayaud, Blaž Kogovšek & Franci Gabrovšek: Understanding the temporal variation of flow di- Gabrovšek: Razumevanje časovne spremenljivosti smeri toka rection in a complex karst system (Planinska Jama, Slovenia) v kompleksnem kraškem sistemu (Planinska jama, Slovenija) Karst aquifers are abundant, but vulnerable water resources. Kraški vodonosniki so izdatni, a ranljivi vodni viri, zato je ra- Therefore, a deeper understanding of possible mechanisms zumevanje mehanizmov, ki vplivajo na pretoke kraških izvirov, that determine the properties of karst springs is crucial. In this zelo pomembno. V prispevku obravnavamo primer izvira reke work, we present an example of Unica Spring and Malni Spring, Unice v Planinski jami in izvira v Malnih, ki drenirata kom- the two main outlets of a large karst system in the Notranjska pleksen vodonosnik Notranjskega krasa. Čeprav imata izvira karst region, Slovenia. Although the two springs share same skupno zaledje, je porazdelitev pretoka med njima precej ne- catchment area, the flow distribution between them shows an navadna. Ob nizkem vodostaju prevladuje izvir v Malnih, izvir interesting behaviour: At low-flow conditions Malni Spring is reke Unice v Planinski jami pa od skupnega zaledja prejema le the main outlet, while Unica spring receives almost no water. majhen del vode. Ob visokem vodostaju je pretok izvira Malni During high water events, discharge of Malni Spring stays lim- omejen, izvir Unice pa postane glavni iztok sistema. V članku ited and Unica Spring becomes the main outlet. We relate these ta opažanja pojasnimo z lokalno porazdelitvijo kanalov in po- observations to the local geometry of the channels and break- dora povezanih s Skrivnostnim jezerom v Planinski jami. Tja se downs in the remote part of the Planinska Jama (Planina Cave), steka voda iz Rakovega Škocjana in vodonosnika Javornikov in called Mysterious Lake. There, waters from Rakov Škocjan and od tam odteka proti obema izviroma. Podorno območje pred Javorniki aquifer merge and further diverge to both springs. At Skrivnostnem jezerom ob nizkih vodah omejuje odtok proti low water conditions, the outflow towards the Unica Spring is izviru Unice, zato večino vode odteče proti izviru Malni. Ob restricted by the breakdown, so that most of the inflow is di- naraščanju dotoka se nivo v Skrivnostnem jezeru dvigne nad rected towards the Malni Spring. With increasing recharge, the podor, zato voda prosto odteče v velike rove Rakovega rokava level in Mysterious Lake rises until the water starts to flow over proti izviru Unice. Višina preliva omejuje hidravlični tlak in the breakdown along a system of large channels (Rak Branch pretok proti izviru Malni. Opisano dogajanje smo preverili s of Planinska Jama) to the Unica Spring. The breakdown level hidravličnim modelom, ki ga sestavljajo znani in predvideni 1 Georg Kaufmann*, Institute of Geological Sciences, Geophysics Section, Freie Universität Berlin, Malteserstr. 74-100, Haus D, 12249 Berlin, Germany, Address, e-mail: [email protected] 2 Cyril Mayaud, Karst Research Institute ZRC SAZU, Titov trg 2, 6230 Postojna, Slovenia. UNESCO Chair on Karst Education, University of Nova Gorica, Glavni trg 8, 5271 Vipava, Slovenia. e-mail: [email protected] 3 Blaž Kogovšek, Karst Research Institute ZRC SAZU, Titov trg 2, 6230 Postojna, Slovenia. UNESCO Chair on Karst Education, University of Nova Gorica, Glavni trg 8, 5271 Vipava, Slovenia. e-mail: [email protected] 4 Franci Gabrovšek, Karst Research Institute ZRC SAZU, Titov trg 2, 6230 Postojna, Slovenia. UNESCO Chair on Karst Education, University of Nova Gorica, Glavni trg 8, 5271 Vipava, Slovenia. e-mail: [email protected] * Corresponding Author Received/Prejeto: 01.04.2019 DOI: 10.3986/ac.v49i2-3.7373 ACTA CARSOLOGICA 49/2-3, 213-228, POSTOJNA 2020 GEORG KAUFMANN, CYRIL MAYAUD, BLAŽ KOGOVŠEK & FRANCI GABROVŠEK keeps the hydraulic head and the flow towards Malni Spring kanali ter pretoki dotokov v Skrivnostno jezero, pri čemer smo limited. To verify this scenario, a hydraulic conduit model was slednje izračunali iz arhivskih podatkov pretokov izvirov in made based on the known and predicted channels, and inflows ponornic. Izbrane parametre hidravličnega modela smo prila- calculated from the historical data of discharge measurements gajali z inverznim algoritmom in dosegli dobro ujemanje mod- at related springs and ponors. An inversion procedure was used elskih rezultatov in arhivskih meritev ob različnih vodostajih. to obtain a satisfactory fit to the observed discharge data and to Ključne besede: kraški vodonosnik, hidravlika podzemne constrain the selected model parameters. The model accurately vode, speleohidrologija, modeliranje, Planinska jama. reproduced the observed discharge behaviour under low- and high-flow conditions. Key words: Karst aquifer, groundwater hydraulics, speleohy- drology, modelling, Notranjski kras, Slovenia. INTRODUCTION Karst aquifers provide freshwater to about 50 % of the 2008; Ravbar et al. 2012; Koit et al. 2017). While these population of Slovenia (Turk 2010). In the Slovene Clas- techniques are very efficient to prove connections -be sical Karst, large springs often present an outflow from tween the aquifer sinks and sources, they are solely appli- active cave systems, evolved in complex tectonic/struc- cable at accessible points such as springs, ponors or wa- tural settings and catchments typically characterised by ter-active caves. As the effect of overflow processes may mixed allogenic and autogenic recharge and networks vary spatially and temporally (Wagner et al. 2013; May- of large conduits (Gabrovšek & Peric 2006; Gabrovšek aud et al. 2016), the combination of the above mentioned et al. 2010; Mihevc et al. 2010; Turk 2010; Ravbar et al. methods with indirect techniques such as groundwater 2012; Kaufmann et al. 2016). This makes their hydraulic modelling is needed to assess the geometry of aquifer un- response very variable in time: strong and pronounced explored parts. Groundwater modelling presents the ad- flood peaks, steep recession limbs and a minimal base vantage to combine all data available in a given area and flow during dry periods (Ravbar 2013). The water is to be very versatile: many hypotheses inferred from field transmitted rapidly through the aquifer, and is poten- observation and results of monitoring techniques can be tially very vulnerable to contamination (Ravbar & Gold- therefore tested. scheider 2007; Ravbar & Goldscheider 2009). Moreover, Up to now, only Mayaud et al. (2014) used ground- the geometry of caves (i.e., shape and distribution of water modelling in combination with event-based times- water active channels) as well plays an important role in series analyses to identify the location of an overflow the aquifer hydraulic behaviour. Restrictions may cause within an Austrian karst aquifer. While the method backflooding and activation of higher positioned over- proved to be reliable and identified properly the overflow flow passages, which may as well divert the flow and -af location, it was not possible to estimate geometrical pa- fect the size of active catchment (Wagner et al. 2013). rameters controlling the system’s hydrogeological behav- Flow diversions along overflow passages can influ- iour. This was due to the continuum nature of the model ence both quality and quantity of the water resources employed, which was not considering conduit flow. In available, making the management of the aquifer re- addition, as the model was highly simplified and aimed serves more challenging. While overflow processes are solely to reproduce the overflow behaviour, only numeri- commonly observed in karst aquifers (Herman et al. cal data were used as input without any calibration step. 2008; Ravbar et al. 2012; Birk et al. 2014; Mayaud et al. Therefore, the use of another modelling approach able to 2014; Mayaud et al. 2016; Gabrovšek et al. 2018; Koit et consider the conduit nature of karst aquifers would be al. 2017), they have been up to now rarely investigated as recommended to allow both calibration and parameter their own (Mayaud et al. 2014; Koit et al. 2017). There- estimation. fore, it is crucial to deepen our understanding of their This work focuses to the temporary flow diversion functioning, in order to better assess the hydraulic re- between the two main springs of the Unica River at the sponse of the aquifer system. Common methods used southern rim of Planinsko Polje, Slovenia. The Malni in characterisation of karst aquifers include continuous Spring and Unica Spring share a common recharge area, monitoring of physico-chemical parameters of water and but the distribution of flow between them heavily de- dye-tracing techniques, which are able to assess the flow pends on the hydrogeological situation within the aqui- directions and velocities during the conditions of injec- fer. During low-flow, the Malni Spring is the main outlet tion. These methods were used to investigate karst aqui- of the system, whilst most of the discharge emerges at the fers where an overflow was also present (Herman et