Dinaric Karst, Croatia)

water

Article An Intermittent Karst River: The Case of the Cikolaˇ River (Dinaric Karst, Croatia)

Ognjen Bonacci 1, Josip Terzi´c 2,* , Tanja Roje-Bonacci 1 and Tihomir Frangen 2 1 Faculty of Civil Engineering, Architecture and Geodesy, University of Split, Matice hrvatske 15, HR-21000 Split, Croatia; [email protected] (O.B.); [email protected] (T.R.-B.) 2 Croatian Geological Survey, Sachsova 2, HR-10000 Zagreb, Croatia; [email protected] * Correspondence: [email protected]; Tel.: +385-91-6144-706

Received: 22 September 2019; Accepted: 14 November 2019; Published: 17 November 2019

Abstract: Intermittent and ephemeral streams (IRES) are responsible for transporting about half of the water on Earth’s surface. Their hydrological behavior is different in various landscapes. IRES are found more often in karst terrains than in any other regions, as a consequence of strong and direct interaction between groundwater and surface water. This paper presents a hydrogeological and hydrological analysis of the intermittent Cikolaˇ River and Spring catchment, which is located in deeply karstified and developed parts of the Dinaric karst in Croatia. Hydrological calculations determined that the catchment area covers approximately 300 km2 and very probably changes in accordance with rapid variations in groundwater level. The karst spring of the Cikolaˇ River is a cave, extracted for a public water supply with four drilled extraction wells. The results of the interrelated hydrological and hydrogeological analysis show interesting phenomena from an intermittent karst spring (cave) and its catchment, flowing downstream through a karst polje with several smaller confluences, then entering a karst canyon (where the river sinks during certain periods), and ending in an estuary before contributing to the larger Krka River. The research presented was based on water balance calculations, climatic and hydrological time series analyses, spring pumping tests, and thorough hydrogeological interpretation.

Keywords: karst; carbonate rocks; intermittent stream; rainfall/runoﬀ; Croatia

1. Introduction In intermittent and ephemeral streams (IRES), surface water is absent during certain times of the year. This type of stream drains about half of the world’s land surface [1]. Phenomena of their drying (timing, frequency, and duration) strongly depend on climate change and anthropogenic actions. They are among the most common and most hydrologically dynamic freshwater ecosystems. Their planetary significance far exceeds the level of our knowledge of them. It should be stressed that streams that periodically cease to flow are not restricted to arid regions. Although IRES make up a high proportion of river networks on Earth, they have been understudied for a long period [2]. Datry et al. [3] emphasize: “After years of obscurity, the science of IRES has bloomed recently and it is being recognized that IRES support a unique and high biodiversity, provide essential ecosystem services and are functionally part of river networks and groundwater systems.” Intermittent streams do not have continuous flowing water year-round. Intermittence creates unique longitudinal dynamics in flow regimes. Generally, they have flowing water periods during the wet season and are dry during hot summer months. These streams are partially supplied by groundwater rising to the surface as stream baseflow. Intermittent streams dry up periodically as ground water levels decline in drier seasons. Ephemeral streams have less flow than intermittent streams. They are normally dry for most of the year and have flowing water for brief periods in response

Water 2019, 11, 2415; doi:10.3390/w11112415 www.mdpi.com/journal/water Water 2019, 11, 2415 2 of 18 to intensive rainfall. In the literature, there are many different, more or less similar, definitions and names of IRES, for example [1]: (1) Ephemeral; (2) episodic; (3) temporary; (4) intermittent; (5) seasonal; (6) dryland; (7) interrupted; (8) nonperennial; (9) near perennial; (10) torrential; (11) influence, etc. Flow cessation and drying can be caused by one or more of the following processes [4]: (1) Transmission loss (seepage through porous beds or through small swallow-holes at the karst stream’s bottom), (2) evapotranspiration, (3) downward shifts in water tables caused by natural and/or anthropogenic factors, (4) hillslope runoff recession, (5) freeze-up (cessation of melting or soil freezing), (6) cessation of karst spring discharge, (7) drops in lake level below outlet elevation, and/or (8) cessation of discharge from reservoirs. Flow intermittence is an extreme form of flow variation, which has strong and quick influence on different processes in its catchment, especially ecological. Specific hydrological, geomorphological, and ecological processes characterize the hydrological regime of any given IRES. Generally, an open stream’s flow regime is characterized by: (1) Magnitude—the total amount of flow at any given time, (2) frequency—how often flow exceeds or is below a given magnitude, (3) duration—how long flow exceeds or is below a given magnitude, (4) predictability—regularity of occurrence of different flow events, and (5) rate of change or flashiness—how quickly flow changes from one magnitude to another. Detailed knowledge of IRES hydrological regimes is a prerequisite to ensure their sustainable development. The most important prerequisite for interdisciplinary analyses of IRES is to understand how alterations in flow regimes impair IRES biodiversity, functions, and services. After interruption of flow in some IRES, surface waters can be isolated in pools and may continue to flow through the hyporheic zone below the streambed. After some time, this flow may also cease [5]. One of the main obstacles is that each case is individual, having very different environmental—as well as climatological and hydrological–hydrogeological—characteristics. Considerable work is needed to explain reactions of various ecosystems and/or species to fast and drastic hydrological changes, especially in timing and duration of flow cessation. These investigations are of special importance in the case of analyses of IRES in karst regions, where such changes occur more often than in other geological media. Because of their unique features, the karstic IRES represent especially endangered and vulnerable systems. Due to very high infiltration rates, overland and surface flows are rare in comparison with non-karst terrains [6]. Flow regimes in open streamflows in karst depend mostly upon the interaction between the groundwater and the surface water [7,8]. They are hydraulically connected through numerous karst forms, which facilitate and govern the exchange of water between the surface and subsurface [9]. Karst landforms in the analyzed catchment and along the rivercourse play predominant roles in occurrence and duration of flow cessation. Generally, karstic IRES dry up, with the number of zero-discharge days increasing from the spring towards the outlet. Open streamflow in karst very often disappears underground multiple times and emerges again in different karst springs, usually under a different name. Sinking, losing, and underground streams are frequent karst phenomena [10]. Bonacci et al. [11] described a case of one river in Dinaric karst, which, over a length of 106 km, switches among losing, sinking, and underground stream sections eight times. The occurrence of IRES in karst terrain is more the rule than the exception. A losing streamflow is a surface stream that contributes water to the karst groundwater system in localized areas. A sinking surface streamflow can be defined as a surface river or stream flowing onto or over karst that then disappears completely underground through a swallow-hole (ponor or sinkhole), and which may or may not rise again and flow as a resurgent surface river or stream. Infiltration from sinking streams into the karst groundwater system is the most rapid form of recharge for carbonate aquifers [12]. Underground or subterranean streamflows are subsurface karst passages that have the main characteristics of open rivers or streams. Karstic IRES cease flowing and dry at certain stream courses, strongly depending on surface and underground karst features and groundwater level oscillations. Rapid and substantial rising and falling of karst groundwater level (more than 100 m in a few hours) is a characteristic which Water 2019, 11, 2415 3 of 18 strongly and quickly influences their hydrological characteristics. The lack of techniques to assess the extent and significance of underground conduit networks, caves, and jamas (vertical or subvertical caves), as well as their interconnections with surface karst forms, presents a major problem in analyses, protection, and management of these systems [13]. Nevertheless, groundwater flow through the rock mass discontinuity networks should not be neglected, especially in regional scale studies, as it often represents a significant portion of the storage—even in karstic areas. To ascertain the vulnerability of water and ecological resources of IRES in karst is much more difficult than in non-karst regions. Karst IRES are among the least-known ecosystems on Earth. Long-term ecological studies and population size estimates in these systems are very rare [14]. It should be stressed that in all of Earth’s karst regions, there are many IRES. Some cases from Dinaric karst are described in the literature [11,15–23]. IRES are common to all Mediterranean karst areas, whereas the soil is quite thin and the infiltration is relatively high [15–23]. For Doglioni et al. [19], IRES are morphological elements of karstic low-relief areas, characterized by relatively large and flat transects. They occasionally drain runoff, in particular after extraordinary or extreme rainfall events. A karst IRES loses water as it flows downstream, and contributes to the karst groundwater systems in localized areas. These losses can be massive in particular river sections, whereas in others, they are small and difficult or even impossible to observe without performing especially precise measurements [11]. The main processes causing flow intermittence in karst areas are: (1) Transmission loss; (2) cessation of spring discharge; and (3) groundwater decline. It is also important to mention other environments of similar monitoring and time series were researched [24–27]. This paper presents a hydrogeological and hydrological analysis of the intermittent Cikolaˇ River and Spring catchment, which are located in deep and developed parts of the Dinaric karst in Croatia. The objective of this paper is to provide a comprehensive explanation of the hydrogeological and hydrological characteristics of one very characteristic intermittent karst river from the Dinaric karst area. The purpose is to improve the general understanding of flow processes within the analyzed intermittent river on the basis of existing data. We hope that this example will be useful for the broader scientific community dealing with complex interdisciplinary problems of karstic IRES.

2. Study Area: Geology, Hydrogeology, Hydrography, and Geomorphology The study area is located in the central part of the Dinaric karst in Croatia. Figure1 shows a location map of the study area, indicating the Cikolaˇ River and Spring, main tributaries (Vrba and Mahnitaš), two temporary springs (Velika and Mala Kanjevaˇca—tooclose to Cikolaˇ to be presented in maps), the permanent spring Torak, three hydrological stations (Ružić,Drniš, Kljuˇcice),meteorological station (Drniš), two rain gauging stations (Muć,Otavice), a water pumping site, and some major ponors. The Cikolaˇ is a typical karst intermittent river, while its main tributaries, Vrba and Mahnitaš, are ephemeral open streamflows. Mahnitaš is a typical torrential stream. The Cikolaˇ River, the largest tributary of the Krka River, emerges in the edge of Petrovo polje under the Svilaja Mtn., runs through the town of Drniš, and drains into the Krka River near Torak. Thanks to its remarkable beauty, the pristine environment of the Cikolaˇ River Canyon has been protected since 1965. Due to well-developed and deep karstification, catchment area boundaries of the Cikolaˇ River are not precisely defined. According to the analyses and calculations made by Bonacci et al. [28], the catchment area of Cikolaˇ at Ružićhydrological station covers approximately 300 km2, and very probably changes in accordance with very fast groundwater level variations. Dinaric karst is morphologically a typical karst terrain, known for its deep compressive tectonics, especially in the neotectonic phase, which cause intensive reverse faulting, fracturing of rock masses, and, consequentially, deep and very irregular karstification. Such natural conditions practically exclude a groundwater hydraulic modeling approach, especially on a regional scale, and simultaneous hydrological and hydrogeological analysis gives the best results for understanding the stream and its complex connections with the groundwater. Although in certain karst terrains hydraulic modeling is acceptable [29], for the Dinaric karst regional scale studies, it would be completely senseless. Calculation Water 2019, 11, 2415 4 of 18

Water 2019, 11, 2415 4 of 18 of the Dinaric karst aquifers hydraulic properties is acceptable only as a rough approximation [30], completely senseless. Calculation of the Dinaric karst aquifers hydraulic properties is acceptable in subregional or local scale studies. The investigation of karst aquifers is often limited to the only as a rough approximation [30], in subregional or local scale studies. The investigation of karst determinationaquifers is often of limited the geological to the determination setting, combined of the geological with the setting, research combined of surface with water the research occurrences, of assurface springs, water ponors, occurrences, surface as streams, springs, andponors, preferential surface streams, underground and preferential ﬂow paths underground [31,32]. Fromflow a hydrogeologicalpaths [31,32]. From point a hydrogeological of view, this middle point andof view, north this Dalmatian middle and karst north terrain Dalmatian was researched karst terrain on a regionalwas researched scale [33 on–35 a]. regional Several scal studiese [33–35]. have Several addressed studies similar have addressed problems similar in karst problems areas (especially in karst Dinaric)areas (especially [11,15–17, 20Dinaric),21,36 ,[11,15–17,20,21,36,37].37]. The data from several The data unpublished from several technical unpublished reports technical were used reports as well. were used as well.

FigureFigure 1. 1.Research Research areaarea position map map with with most most important important features. features.

InIn most most of of the the DinaricDinaric ka karstrst of of Croatia, Croatia, the the strike strike of regional of regional geological geological structures structures is NW–SE, is NW–SE, the theso-called so-called Dinaric Dinaric strike. strike. Small-scale Small-scale “Basic “Basic geological geological maps” maps” [38,39] [38 ,cover39] cover geological geological construction construction of ofthe the research research area. area. In In the the hydrogeological hydrogeological map map (F (Figureigure 2),2), lithological lithological and and stratigraphic stratigraphic members members werewere grouped grouped according according to to their their hydrogeological hydrogeological features:features: QuaternaryQuaternary deposits—on deposits—on thethe surface,surface, mostly al alluvial,luvial, with with intergranular intergranular porosity, porosity, connected connected withwith karst karst poljes. poljes. Permeability Permeability dependsdepends on clast size sizess (usually (usually mixtures mixtures of of terra terra rossa rossa clays clays and and silts silts withwith clasts clasts of of base base rocks, rocks, mostly mostlycarbonates). carbonates). These deposits deposits are are of of low low relevance relevance for for deeper deeper karst karst groundwater studies, but in Petrovo polje, there is influence on Čikola stream and spring. groundwater studies, but in Petrovo polje, there is influence on Cikolaˇ stream and spring. Water-tight rock masses—mostly covered by Quaternary deposits of karst in Petrovo polje, Water-tight rock masses—mostly covered by Quaternary deposits of karst in Petrovo polje, various various rock types from Permian igneous rocks (diabase), Triassic sandstones, and similar clastites rock types from Permian igneous rocks (diabase), Triassic sandstones, and similar clastites to Pliocene to Pliocene marls and limestone-marls (lake sediments). marls and limestone-marls (lake sediments). Low permeability carbonates—mostly dolomites and dolomitic limestones, which are less karstified.Low permeability Cretaceous, Jurassic, carbonates—mostly and Triassic rocks. dolomites and dolomitic limestones, which are less karstified.Intermediate Cretaceous, permeability Jurassic,and carbonates—dolomitic Triassic rocks. limestones and marly limestones, which are tectonicallyIntermediate fractured permeability and karstified carbonates—dolomitic to a certain extent. Cretaceous, limestones Jurassic, and marly and limestones, Triassic rocks; which are tectonicallyHigh fracturedpermeability and carbonates—highly karstified to a certain fractured extent. and Cretaceous, intensively Jurassic, karstified and rocks, Triassic mostly rocks; from theHigh Upper permeability Cretaceous carbonates—highly (rudist limestones) fracturedbut also andmost intensively Eocene–Oligocene karstified “Promina” rocks, mostly rocks from (all the Upperexcept Cretaceous marl beds), (rudist and othe limestones)r karstified but limestones. also most Eocene–Oligocene “Promina” rocks (all except marl beds), and other karstified limestones.

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FigureFigure 2. 2. HydrogeologicalHydrogeological map map of of the the research research area. area.

TheThe tectonics tectonics of theof areathe area is extensive, is extensive, and the and rocks the are rocks highly are karstified highly karstified [40–46]. Due[40–46]. to compressive Due to tectonicscompressive dominated tectonics by the dominated reverse faulting by the of reverse geological faul structures,ting of geological karstification structures, reaches karstification deep horizons in thereaches underground deep horizons and isin very the underground irregularly distributed. and is very On irregularly the other distributed. hand, the Middle On the Dalmatian other hand, plateau the whereMiddle Krka Dalmatian and Cikolaˇ plateau canyons where were Krka cut is and characterized Čikola canyons by relatively were cut mild is characterized tectonics. The byCikolaˇ relatively canyon is upmild to tectonics. 130 m deep The and Čikola is steeply canyon cut, is being up to present 130 m in deep a karst-plateau and is steeply with cut, low being tectonic present disturbance, in a builtkarst-plateau mostly in with Eocene–Oligocene low tectonic disturbance, “Promina” built or Uppermostly Cretaceousin Eocene–Oligocene limestone “Promina” rock mass. or ProminaUpper is aCretaceous series of mostlylimestone thick rock (meter-sized) mass. Promina beds is a series of carbonates, of mostly thick conglomerates, (meter-sized) and beds marly of carbonates, limestones. Theseconglomerates, rocks are mostly and marly subhorizontal limestones. and These are rocks some are of themostly most subhorizontal important factors and are in thesome creation of the of most important factors in the creation of the recent relief. Because these marly beds are less present, the recent relief. Because these marly beds are less present, the Promina complex has mostly karstic the Promina complex has mostly karstic hydrogeological behavior and resembles neighboring hydrogeological behavior and resembles neighboring Upper Cretaceous rudist limestones. Carbonates Upper Cretaceous rudist limestones. Carbonates within Promina are calcitic and karstified to about within Promina are calcitic and karstified to about the same extent as neighboring limestones. the same extent as neighboring limestones. According to such spread of lithological composition, most of the terrain is covered by pervious According to such spread of lithological composition, most of the terrain is covered by pervious karstifiedkarstified carbonate carbonate rocks. rocks. InIn PetrovoPetrovo polje, un underder the the Quaternary Quaternary deposits, deposits, there there are are practically practically imperviousimpervious clastic clastic (marly) (marly) rocks rocks (Figure (Figure3 ).3).

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Figure 3. Hydrogeological and geomorphological profile of the Čikola riverbed (a) and the spring’s catchment ((b) modified after [35]). FigureFigure 3. 3.Hydrogeological Hydrogeological and and geomorphologicalgeomorphological profile profile of of the the ČCikolaˇikola riverbed riverbed (a ()a and) and the the spring’s spring’s catchment ((b) modified after [35]). Incatchment Figure 4, ((thereb) modified is a detailed after [35 cross-section]). of the Čikola spring zone. The main spring is a cave, speleologically explored several tens of meters. In continuation, there is also a depression with InIn Figure Figure4, there4, there is is a detaileda detailed cross-section cross-section of of thethe ČCikolaˇikola spring spring zone. zone. The The main main spring spring is a is cave, a cave, several estavelles (suffosion sinkholes), which are, during high groundwater levels, springs of speleologicallyspeleologically explored explored several several tens tens of meters.of meters. In continuation,In continuation, there there is also is also a depression a depression with severalwith Čikola. When the spring dries up, these estavelles are either dry or act as ponors—for some shorter estavellesseveral (suestavellesffosion (suffosion sinkholes), sinkholes), which are, which during are, high during groundwater high groundwater levels, springs levels, of Cikola. ˇsprings When of periodČikola. of time, When groundwater the spring driesfrom up, alluvial these Quaterna estavellesry are sediments either dry of or Petrovo act as ponors—for polje sinks intosome the shorter karst the spring dries up, these estavelles are either dry or act as ponors—for some shorter period of time, underground.period of time, As groundwatershown, there from is an alluvial extraction Quaterna sitery with sediments four boredof Petrovo wells, polje which sinks intowere the drilled karst groundwater from alluvial Quaternary sediments of Petrovo polje sinks into the karst underground. directlyunderground. into the cave As undershown, the there lowest is angroundwate extractionr sitelevel with and fourare used bored for wells, public which water weresupply drilled with As shown, there is an extraction site with four bored wells, which were drilled directly into the cave a maximaldirectly pumping into the cave rate under of 180 the L/s. lowest Extraction groundwate fromr thelevel four and close-byare used forwells, public out water of the supply same withcave under the lowest groundwater level and are used for public water supply with a maximal pumping whicha maximalacts as estavelle,pumping andrate numerousof 180 L/s. Extractionsmaller estavelles from the nearby,four close-by makes wells, any attemptout of the of same hydraulic cave rate of 180 L/s. Extraction from the four close-by wells, out of the same cave which acts as estavelle, modelingwhich impossible, acts as estavelle, even ifand the numerous results would smaller be presentedestavelles nearby,within anmakes order any of attemptmagnitude. of hydraulic andmodeling numerous impossible, smaller estavelles even if the nearby, results makeswould anybe presented attempt of within hydraulic an order modeling of magnitude. impossible, even if the results would be presented within an order of magnitude.

ˇ FigureFigureFigure 4. 4.Schematized 4.Schematized Schematized hydrogeological hydrogeologicalhydrogeological profile profile proﬁle of of ofthe the the Č ČikolaCikolaikola spring spring spring site.site. site.

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3. Study Area Climate The climate of the study area isis predominantlypredominantly Mediterranean,Mediterranean, with influenceinfluence of continental and mountain climates at the northern upper part of the catchment. In the ČCikolaˇ ikola River catchment, there are:are: (1)(1) A A meteorological meteorological station, station, Drniš Drni (1957–2018,š (1957–2018, missing missing 1991–1997), 1991–1997), and (2) and two (2) rain two gauging rain gaugingstations, stations, Muć(1991–2018) Muć (1991–2018) and Otavice and(1998–2018). Otavice (1998–2018). The annual rainfallrainfall measuredmeasured at at the the meteorological meteorological station station Drniš Drniš in in the the available available period period (Figure (Figure5a) varied5a) varied within within a very a very large large range, range, between between 1581.8 1581.8 mm (1976) mm and(1976) 630.1 and (2011), 630.1 with(2011), an with average an valueaverage of 1027.1value of mm. 1027.1 The mm. linear The regression linear regr lineession in Figure line in5a Figure shows 5a a decreasingshows a decreasing trend. Due trend. to missing Due to datamissing for dataseven for years seven (1991–1997), years (1991–1997), there is nothere utility is no in utility testing in its testing statistical its statistical significance. significance. Areal rainfall distribution follows a well-developed morphology. Precipitation events are most abundant at Mućstation.Muć station. Over the 21-year period (1998–201 (1998–2018),8), when all three rain gauging stations were active,active, averageaverage annual annual rainfall rainfall at at Mućwas Muć was 1261.6 1261.6 mm, mm, at Otaviceat Otavice 991.1 991.1 mm, mm, and and at Drniš at Drniš 951.9 951.9 mm. Julymm. andJuly August and August are the are driest, the driest, while Novemberwhile Nove andmber December and December are the wettestare the months.wettest months. The rainfalls The rainfallsfrequently frequently (few times (few in thetimes year) in the reach year) a greater reach a intensity greater intensity than 50 mm than/h. 50 mm/h. The time series of mean annual air temperaturestemperatures measured at Drniš meteorological station (1957–2018, missing 1991–1996) depicted in in Figure 55bb indicateindicate thatthat airair temperaturestemperatures variedvaried overover aa broad range, between 12.2 °C (1976 and 1978) and 14.6 °C (2018), with a mean value of 13.3 °C. broad range, between 12.2 ◦C (1976 and 1978) and 14.6 ◦C (2018), with a mean value of 13.3 ◦C. Linear Linearand second-order and second-order polynomial polynomial trend lines trend show lines existence show existence of an increasing of an increasing trend, which trend, is verywhich intensive is very intensivein the recent in the period recent of period 22 years of (1996–2018).22 years (1996–2018). If this increasing If this increasing trend continues, trend continues, it could haveit could a strong have ainfluence strong influence on the Cikolaˇ on the hydrological Čikola hydrological regime, especially regime, in especially combination in withcombination declines inwith annual declines rainfall. in annual rainfall. Minimum and maximum measured mean daily air temperatures vary between 9.8 ◦C and Minimum and maximum measured mean daily air temperatures vary between −9.8− °C and 32.9 32.9 ◦C, respectively. Maximum instantaneous temperature can reach 37 ◦C. The hottest month is July, °C,and respectively. the coldest is Maximum January [28 instantaneous]. temperature can reach 37 °C. The hottest month is July, and thePrevious coldest analyses is January indicate [28]. that in the recent period, annual rainfall has declined, while air temperaturePrevious has analyses tended indicate to increase. that Ifin these the patternsrecent period, continue, annual water rainfall availability has declined, and the ecologicalwhile air temperatureregime in the has intermittent tended toCikolaˇ increase. River If these and its pattern catchments continue, may be water significantly availability endangered. and the ecological regime in the intermittent Čikola River and its catchment may be significantly endangered.

(a)

Figure 5. Cont.

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(b)

Figure 5. Time seriesseries ofofannual annual precipitation precipitation (a ()a and) and mean mean annual annual air air temperature temperature (b) ( measuredb) measured at the at themeteorological meteorological station station Drniš Drniš over theover period the period 1957–2018 1957–2018 (1991–1997 (1991–1997 missing missing data due data to war due activity). to war activity). 4. Cikolaˇ Karst River Hydrology 4. ČikolaAlong Karst the Cikola ˇRiver River’sHydrology relatively short watercourse (39 km), there are three hydrological stations (TableAlong1). Detailed the Čikola and River’s reliable relatively hydrological short analysis watercours is inhibitede (39 km), by there di fferent are three durations hydrological of their stationsdischarge (Table time series,1). Detailed as well and as byreliable gaps inhydrological their operation. analysis Hydrological is inhibited station by different Ružić,located durations 6.6 km of theirfrom discharge the river spring, time series, has had as well the longest as by gaps period in their of measurement operation. Hydrological (58 complete station years). Ruži The otherć, located two 6.6downstream km from stations,the river Drnišspring, and has Kljuˇcice,have had the longest had period 32 and of 28 measurement complete years, (58 respectively. complete years). The other two downstream stations, Drniš and Ključice, have had 32 and 28 complete years, respectively. Table 1. Main characteristics of the three analyzed hydrological stations along the Cikolaˇ River. Table 1. Main characteristics of the three analyzed hydrological stations along the Čikola River. Distance from the Station Name Elevation (m a.s.l.) Available Period Missing Data Spring—LDistance from (km) the Elevation Station Name Available Period Missing Data RužićSpring—L 6.6 (km) 273.781(m a.s.l.) 1947–2017 1981; 1991–2002 RužiDrnišć 19.06.6 260.600273.781 1947–2017 1979–2017 1981; 1991–1997 1991–2002 DrnišKljuˇcice 30.519.0 81.310260.600 1979–2017 1987–2017 1992–1994;1991–1997 2008 Ključice 30.5 81.310 1987–2017 1992–1994; 2008 3 Figure6a shows three time series of the mean annual discharges, Q mean (m /s), measured at Figure 6a shows three time series of the mean annual discharges, Qmean (m3/s), measured at the the three hydrological stations along the Cikolaˇ River course: (1) Ružić(1947–2017, missing 1981, three hydrological stations along the Čikola River course: (1) Ružić (1947–2017, missing 1981, 1991–2002), (2) Drniš (1979–2017, missing 1991–1997), (3) Kljuˇcice(1987–2017, missing 1992–1994 1991–2002), (2) Drniš (1979–2017, missing 1991–1997), (3) Ključice (1987–2017, missing 1992–1994 and and 2008). The figure shows similar hydrological behavior of mean annual discharges at the three 2008). The figure shows similar hydrological behavior of mean annual discharges at the three hydrological stations along the river course. Over a period of 15 years (2003–2017, missing 2008), hydrological stations along the river course. Over a period of 15 years (2003–2017, missing 2008), when all three hydrological stations were active, average mean annual discharges were the following: when all three hydrological3 stations were active, 3average mean annual discharges3 were the (1) QRužić-mean-av = 4.4 m /s, (2) QDrniš-mean-av = 5.0 m /s, (3) QKljuˇcice-mean-av = 4.9 m /s. In all cases, following: (1) QRužić-mean-av = 4.4 m3/s, (2) QDrniš-mean-av = 5.0 m3/s, (3) QKljučice-mean-av = 4.9 m3/s. In all cases, t-tests show no statistically significant differences between average mean annual discharges (see third t-tests show no statistically significant differences between average mean annual discharges (see column in Table2). third column in Table 2).

Table 2. Results of t-tests for average mean and maximum annual discharges between the three pairs of hydrological stations during common periods of their operation.

Pair of Stations Common Period of Work Mean Discharges Maximum Discharges Ružić–Drniš 26 years (1979–2017, missing 1981 and 1991–2002) p-value = 0.330 p-value < 0.01

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Table 2. Results of t-tests for average mean and maximum annual discharges between the three pairs of hydrological stations during common periods of their operation.

Pair of Stations Common Period of Work Mean Discharges Maximum Discharges

WaterRužić–Drniš 2019, 11, 2415 26 years (1979–2017, missing 1981 and 1991–2002) p-value = 0.330 p-value < 0.019 of 18 Ružić–Kljuˇcice 18 years (1987–2017, missing 1991–2002 and 2008) p-value = 0.599 p-value < 0.01 Drniš–Kljuˇcice 23 years (1987–2017, missing 1991–1997 and 2008) p-value = 0.728 p-value = 0.222 Ružić–Ključice 18 years (1987–2017, missing 1991–2002 and 2008) p-value = 0.599 p-value < 0.01 Drniš–Ključice 23 years (1987–2017, missing 1991–1997 and 2008) p-value = 0.728 p-value = 0.222 The situation is different in the case of maximum annual discharges at the three hydrological The situation is different in the case of maximum annual discharges at the three3 hydrological stations. Figure6b shows three time series of the maximum annual discharges, Q max (m /s), measured 3 atstations. the three Figure hydrological 6b shows stations three time along seri thees ofCikolaˇ the maximum River course. annual Over discharges, a period of Q 15max years(m /s), (2003–2017, measured missingat the three 2008), hydrological when all three stations hydrological along stationsthe Čikola were River active, course. average Over annual a period discharges of 15 were years the (2003–2017, missing 2008), when all3 three hydrological stations3 were active, average annual3 following: (1) QRužić-max-av = 40.2 m /s, (2) QDrniš-max-av = 49.4 m /s, (3) QKljuˇcice-max-av = 53.2 m /s. 3 3 Indischarges two cases, weret-tests the show following: no statistically (1) QRuži significantć-max-av = 40.2 diff merences/s, (2) between QDrniš-max-av average = 49.4 mean m /s, annual (3) QKlju discharges.čice-max-av = 3 The53.2 fourthm /s. In column two cases, of Table t-tests2 presents show no the statistically results of t -testssignific forant average differences maximum between annual average discharges mean betweenannual discharges. the three pairsThe fourth of hydrological column of Table stations 2 presents along the theCikolaˇ results River of t-tests over for a periodaverage of maximum 15 years (2003–2017,annual discharges missing between 2008), when the three all three pair hydrologicals of hydrological stations stations were active.along the The Č resultsikola River of the overt-tests a showperiod that of 15 Ružićdi years ff(2003–2017,ered significantly missing (p 2008),< 0.01) when from all both three downstream hydrological stations stations Drniš were and active. Kljuˇcicein The averageresults of maximum the t-tests annual show that discharges. Ružić differed The di ffsignificantlyerence between (p < average0.01) from maximum both downstream annual discharges stations betweenDrniš and Drniš Klju andčice Kljuˇciceis in average not maximum statistically annual significant. discharges. The difference between average maximum annual discharges between Drniš and Ključice is not statistically significant.

(a)

Figure 6. Cont.

Water 2019, 11, 2415 10 of 18 Water 2019, 11, 2415 10 of 18

(b)

Figure 6.6. TimeTime series series of theof the mean mean annual annual discharges discharges (a) and (a maximum) and maximum annual dischargesannual discharges (b) measured (b) measuredat the three at hydrological the three stations hydrol alongogical the stationsCikolaˇ Riveralong course: the (1)Čikola Ružić(1941–2017—missing River course: (1) Ruži 1981,ć 1991–2002),(1941–2017—missing (2) Drniš (1970–2017—missing 1981, 1991–2002), 1991–1997),(2) Drniš (3)(1970–2017—missing Kljuˇcice (1987–2017—missing 1991–1997), 1992–1994, (3) Klju 2008).čice (1987–2017—missing 1992–1994, 2008). Although intensive precipitation events are frequent in the Cikolaˇ catchment, maximum discharges are notAlthough adequately intensive high, which precipitation can be explained events are by thefrequent influence in ofthe karst Čikola features catchment, and limited maximum outflow dischargescapacity of karstare not springs adequately [47]. high, which can be explained by the influence of karst features and 3 limitedFigure outflow7 shows capacity average of karst monthly springs discharges, [47]. Q month-av (m /s), measured at the three hydrological stationsFigure along 7 shows the Cikolaˇ average River monthly course duringdischarges, 18 years Qmonth-av in the (m period3/s), measured 1987–2017 at (missing the three 1991–2002 hydrological and stations2008). Maximum along the values Čikola of River average course mean during monthly 18 years discharges in the occur period in February,1987–2017 and (missing minimum 1991–2002 values andin August. 2008). Maximum It is interesting values to of notice average that mean during monthly January discharges and February, occur in the February, average meanand minimum monthly valuesdischarges in August. in Kljuˇciceare It is interesting higher than to atnotice the twothat upstream during January stations, and Ružićand February, Drniš, the indicatingaverage mean that whenmonthly groundwater discharges levels in Klju arečice high, are the higher karstic than catchment at the oftwo the upstream canyon drains stations, groundwater Ružić and directly Drniš, indicatinginto the stream. that when In March, groundwater April, and levels December, are hi averagegh, the meankarstic monthly catchment discharges of the canyon at Kljuˇciceare drains groundwaterhigher than at directly Ružić,but into lower the thanstream. at Drniš. In March, In every April, other and month December, from May average to November, mean monthly average dischargesmean monthly at Klju dischargesčice are higher at Kljuˇciceare than at lowerRužić, thanbut lower at the than two upstreamat Drniš. In stations—river every other month water sinksfrom Mayinto theto November, karst rock mass, average mostly mean reappearing monthly discharges at the Torak at Klju springčice in are its lower estuary. than at the two upstream stations—riverAt Ružićhydrological water sinks stationinto the (1947–2017, karst rock missingmass, mostly 1981, 1991–2002),reappearing the at numberthe Torak of dryspring days in perits year,estuary. N, varied over a broad range between N = 0 (1947, 1948, 1951, 1957, 1959, 1963, 1966, 1968, 1969, 1972,At 1978, Ruži 1979,ć hydrological 1980, 1982, station 1984) (1947–2017, and N = 184 missing (2003), 1981, with an1991–2002), average valuethe number of N = of56.3. dry Atdays Drniš per year,hydrological N, varied station over (1979–2017,a broad range missing between 1991–1997), N = 0 (1947, the number1948, 1951, of dry 1957, days 1959, per 1963, year, 1966, N, varied 1968, over 1969, a 1972,broad 1978, range 1979, between 1980, N 1982,= 0 (1983, 1984) 1984, and 1986,N = 184 1989, (2003), 1999) with and Nan= average178 (2006), value with of anN average= 56.3. At value Drniš of hydrologicalN = 75.9. At Kljuˇcicehydrologicalstation (1979–2017, missing station 1991–1997), (1987–2017, the missing number 1992–1994 of dry anddays 2008), per year, the numberN, varied of overdry days a broad per range year, N,between varied N over = 0 a(1983, broad 1984, range 1986, between 1989, N1999)= 93 and (2014) N = and178 N(2006),= 317 with (1989), an average with an valueaverage of valueN = 75.9. of N =At211.9 Klju [č28ice]. hydrological Figure8 shows station average (1987–2017, number of missing dry days, 1992–1994 N month-av and(day), 2008), during the numberthe months of dry monitored days per at year, the three N, varied hydrological over a br stationsoad range along between the Cikolaˇ N = River93 (2014) course and over N = 317 18 years (1989), in withthe period an average 1987–2017 value (missing of N = 211.9 1991–2002, [28]. Figure 2008). 8 Atshows Ružićstation, average number drying of never dry days, occurred Nmonth-av in January, (day), duringFebruary, the or months March. monitored At Drniš station,at the three drying hydrological never occurred stations in along March, the April, Čikola or May.River Atcourse the mostover 18downstream years in the station, period Kljuˇcice,flow 1987–2017 (missing cessation 1991–2002, occurred 2008). every month.At Ružić station, drying never occurred in January, February, or March. At Drniš station, drying never occurred in March, April, or May. At the most downstream station, Ključice, flow cessation occurred every month.

Water 2019, 11, 2415 11 of 18 Water 2019, 11, 2415 11 of 18

Figure 7. Average monthly discharges measured at thethe three hydrological stations along the CikolaČˇ ikola River course over the period 1987–2017 (missing 1991–2002, 2008).2008).

Figure 8. Average dry days, N, in the months monitored at the three hydrological stations along the CikolaČˇ ikola River course over the period 1987–2017 (missing 1991–2002,1991–2002, 2008).2008).

5. Relationship between Groundwater and Surface Water The entrance into the CikolaČˇ ikola spring cave isis situatedsituated atat ca.ca. 279 m a.s.l. When groundwater starts to appear at the surface, it slowly fillsfills the depressiondepression with estavelles (Figure 33),), whichwhich startstart toto actact asas springs asas well.well. AtAt thethe end end of of this this spring spring zone zone is ais weira weir (low (low dam), dam), and and when when water water starts starts to spill to overspill itover significantly, it significantly, the river the starts river tostarts flow. to This flow. happens This happens at the elevation at the elevation of 285 m of a.s.l. 285 After m a.s.l. that, After the small that, increasethe small in increase groundwater in groundwater level in the Cikolaˇlevel in spring the Č resultsikola spring in significant results increase in significant of discharge increase at theof Ružićhydrologicaldischarge at the Ruži stationć hydrological (Figure9 ),station and after (Figure that, 9), at and Drniš after and that, Kljuˇcicestations at Drniš and Klju as well.čice stations After the as pointwell. After when the the pointCikolaˇ when spring the zone Čikola starts spring to spring zone starts over theto spring weir,all over water the quantitiesweir, all water from quantities Vrba and otherfrom Vrba smaller and springs other smaller in the area springs become in the negligible. area become negligible. Over the period 3 FebruaryFebruary 2009–8 NovemberNovember 2010 (645 days),days), continuouscontinuous measurement of groundwater levellevel (GWL) (GWL) was was performed performed at one at ofon thee extractionof the extraction wells drilled wells in drilled the Cikolaˇ in springthe Č/ikolacave (Figurespring/cave4). During (Figure that 4). period,During thethat minimum period, the GWL minimum was 266.91 GWL mwas a.s.l. 266.91 and m the a.s.l. maximum and the valuemaximum was 290.38value was m a.s.l. 290.38 The m relationship a.s.l. The relationship between the betw meaneen daily the groundwater mean daily level,groundwater GWL (m level, a.s.l.), GWL and the(m meana.s.l.), dailyand the discharge, mean daily Q (m discharge,3/s), measured Q (m3/s), at Ružić(dark measured at blue Ruži color),ć (dark Drniš blue (redcolor), color), Drniš and (red Kljuˇcice color), (greenand Klju color)čice hydrological(green color) stations hydrological during thatstations period during is presented that period in Figure is 9presented. Emergence in ofFigure surface 9. flowEmergence at Ružićhydrological of surface flow station at Ruži occursć hydrological when GWL station at the extractionoccurs when site GWL exceeds at ~274the extraction m a.s.l. At site the twoexceeds downstream ~274 m a.s.l. hydrological At the two stations, downstream Drniš hydrological and Kljuˇcice,surface stations, flowDrniš occurs and Klju whenčice, GWL surface exceeds flow ~278.5occurs andwhen ~284 GWL m exceeds a.s.l., respectively ~278.5 and (Table~284 m1). a.s.l., Groundwater respectively pumping (Table 1). quantity Groundwater is similar pumping every dayquantity (working is similar only every a few day hours (working during theonly night a few with hours quantities during the below night 180 with L/s), quantities and its influence below 180 on L/s), and its influence on groundwater levels proves to be very low, especially with minimal GWL, and therefore can be neglected. Although the Čikola spring starts to discharge groundwater from the

Water 2019, 11, 2415 12 of 18

Water 2019, 11, 2415 12 of 18 groundwater levels proves to be very low, especially with minimal GWL, and therefore can be neglected. Althoughcave at some the Cikolaˇ 279 m spring a.s.l., the starts real to discharge discharge starts groundwater when spring-zone from the water cave atexceeds some 279the top m a.s.l., of a small the real dischargeweir—ca. starts 285 m when a.s.l. River spring-zone flow measured water exceedsat all three the stations top of below a small that weir—ca. level at spring 285 mis quite a.s.l. low River ﬂow(Figure measured 9), especially at all three at stationsthe Ključ belowice station. that levelSince at the spring other is two quite stations low (Figure (Ruži9ć), and especially Drniš) atare the Kljuˇcicestation.situated outside Since of the the karstic other areas, two stations in Petrovo (Ruži´cand polje Quaternary Drniš) are sediments situated outside that overlay of the impervious karstic areas, inrock Petrovo masses, polje it Quaternary can be presumed sediments that that water overlay in the impervious Čikola River rock during masses, such it can periods be presumed (levels at that waterextraction in the Cikolasiteˇ between River 274 during and such285 m periods a.s.l.) is (levels connected at extraction with smaller site betweentributaries, 274 such and as 285 Vrba, m a.s.l.) and is connectedgroundwater with from smaller the tributaries,alluvial sediment such asof the Vrba, polje and itself. groundwater As the measured from the discharge alluvial in sediment the riverbed of the in such terms rarely exceeds a few cubic meters per second, the data are probably associated with polje itself. As the measured discharge in the riverbed in such terms rarely exceeds a few cubic meters heavy rains in the polje and/or rapid drops in water level during the beginning of the recession per second, the data are probably associated with heavy rains in the polje and/or rapid drops in water period. level during the beginning of the recession period.

FigureFigure 9. 9.Relationship Relationship between between the the mean mean daily daily groundwater groundwater level, level, GWL GWL (m (m a.s.l.), a.s.l.), and and the the mean mean daily 3 discharges,daily discharges, Q (m / s),Q measured(m3/s), measured at Ruži´c,Drniš, at Ružić, Drniš, and Kljuˇcicehydrological and Ključice hydrological stations stations over over the period the 3 Februaryperiod 3 February 2009–8 November 2009–8 November 2010. 2010.

ThreeThree time time series series of turbidity of turbidity S (NTU), S (NTU), groundwater groundwater temperature temperature T (◦C), andT (°C), electrical and conductivityelectrical ECconductivity (µS/cm) were EC measured (μS/cm) were using measured a datalogger using (ﬂuorimeter) a datalogger GGUN-FL (fluorimeter) during GGUN-FL one of the during two pumpingone of teststhe (the two second pumping pumping tests (the test second was shorterpumping and test is wa nots presentedshorter and here) is not that presented were done here) on that the wereCikolaˇ springdone (extractionon the Čikola wells) spring (Figure (extraction 10). Turbidity wells) (Figure ranged 10). from Turbidity Smin = ranged4.53 NTU from to Smin Smax = 4.53= 80.97 NTU NTU, to electricalSmax = 80.97 conductivity NTU, electrical from EC minconductivity= 391 µS/ cmfrom to ECECmaxmin = 391397 µμSS/cm/cm, andto EC groundwatermax = 397 μS/cm, temperature and fromgroundwater Tmin = 11.01 temperature◦C to Tmax =from12.49 Tmin◦C. = The 11.01 changes °C to in T ECmax and= 12.49 groundwater °C. The temperaturechanges in duringEC and the pumpinggroundwater test were temperature rather low, during and the values pumping are typical test were for rather karst groundwaterlow, and values in are that typical area. for Variations karst ingroundwater turbidity were in associatedthat area. Variations with the turbulentin turbidity start were of theassociated pumping with test, the but turbulent after less start than of athe day, theypumping became test, very but low after (up less to than six NTU) a day, and they steady. became very low (up to six NTU) and steady.

Water 2019, 11, 2415 13 of 18 Water 2019, 11, 2415 13 of 18

Figure 10. Pumping test in 2009: Time series of turbidity S (NTU), groundwater temperature T (◦C), andFigure electrical 10. Pumping conductivity test in EC 2009: (µS /Timecm) measuredseries of turbid everyity 2 minS (NTU), during groundwater the period temperature 16 September T (°C), 2010 at 6 hand 23 minelectrical 49 s–25 conductivity September EC 2010 (μS/cm) at 6 hmeasured 52 min 24 every s (868 2 min data). during * Period the period when 16 datalogger September was 2010 out at of water6 h 23 due min to technical49 s–25 September problems. 2010 Groundwater at 6 h 52 min levels 24 s on(868 separate data). * axisPeriod (pumping when datalogger rate constant, was out 180 of L/ s), redwater arrow due shows to technical the end problems. of the pumping. Groundwater levels on separate axis (pumping rate constant, 180 L/s), red arrow shows the end of the pumping. The pumping test was conducted using maximal pumping quantity in all four wells, 180 L/s, withoutThe disruption pumping (excepttest was marked conducted period, using Figure maximal 10). pumping The test quantity started duringin all four very wells, low 180 GWL, L/s, ca. 267without m a.s.l., disruption at the end (except of a long marked dry period—the period, Figure riverbed 10). The was test completely started during dry. In very the firstlow GWL, few days, ca. 267 GWL declinedm a.s.l., with at the an end almost of a long linear dry trend, period—the but very riverbed slightly. was On completely 25 September, dry. In rain the startedfirst few in days, the spring’sGWL catchment,declined with and inan lessalmost than linear one trend, day, pumping but very becameslightly. negligible,On 25 September, GWL roserain significantlystarted in the (overspring’s 20 m catchment, and in less than one day, pumping became negligible, GWL rose significantly (over 20 m during one day), and the river started to flow. Of course, the pumping test was stopped. During during one day), and the river started to flow. Of course, the pumping test was stopped. During both both pumping tests, groundwater temperature was equivalent to the average air temperature in the pumping tests, groundwater temperature was equivalent to the average air temperature in the catchment area, ca. 11–12.5 C, while EC values approximately 395 µS/cm were typical for Dinaric karst catchment area, ca. 11–12.5◦ °C, while EC values approximately 395 μS/cm were typical for Dinaric groundwater. EC increases with increasing water hardness [32,48]. The karst groundwater conductivity karst groundwater. EC increases with increasing water hardness [32,48]. The karst groundwater µ measuredconductivity at several measured karst at several springs karst in France springs ranged in France between ranged 220between and 470220 andS/cm, 470 withμS/cm, a modewith a of 380modeµS/cm. of 380 During μS/cm. the During pumping the pumping test, total test, drawdown total drawdo waswn approximately was approximately 5 m, followed 5 m, followed by a slightby increasea slight in increase the EC value in the and EC T.value After and the T. first After heavy the rain first in heavy the catchment rain in the area, catchment the spring area, reacted the spring quickly. reactedOne ofquickly. the most challenging issues when researching karst systems on a regional scale is the positioningOne of waterthe most divides, challenging i.e., delineation issues when of catchmentresearching areas. karst Probablysystems on the a mostregional powerful scale is tool the for thispositioning delineation of iswater a groundwater divides, i.e., tracingdelineation experiment of catchment (see connectionsareas. Probably proven the most by tracingpowerful in tool Figure for2 ). Thethis catchment delineation delineation is a groundwater in such tracing karst systems experiment is especially (see connections difficult proven in intermittent by tracing systems, in Figure when 2). a springThe catchment or river delineation dries out. Thein such catchment karst systems area isis stronglyespecially associated difficult in withintermittent geological systems, construction, when anda spring the influence or river of dries topography out. The catchment is minor. Asarea mentioned, is strongly the associated hydrologically with geological determined construction, catchment areaand of the the influenceCikolaˇ River of topography at Ružićstation is minor. is approximately As mentioned, 300 the km hydrologically2, while, for thedetermined previously catchment described geologicalarea of the reasons, Čikola it River cannot at exceed Ružić 250station km 2is. Theapproximately absence of 300 hydrological km2, while, stations for the in previously higher karst areasdescribed of Svilaja geological Mtn. is reasons, probably it the cannot reason. exceed It is also250 km important2. The absence to point of out hydrological that in significant stations partsin ofhigher the Dinaric karst karst,areas dividesof Svilaja are Mtn. so-called is probably zonal the divides, reason. meaning It is also they important change to position point out according that in to hydrologicalsignificant conditions,parts of the groundwater Dinaric karst, levels. divides Strong are karstso-called springs zonal of neighboringdivides, meaning catchments they change preclude position according to hydrological conditions, groundwater levels. Strong karst springs of the possibility of increasing the Cikolaˇ spring catchment area; only common use of hydrological and neighboring catchments preclude the possibility of increasing the Čikola spring catchment area; only hydrogeological approaches can lead to increase in knowledge and improvement in the positioning of common use of hydrological and hydrogeological approaches can lead to increase in knowledge and groundwater divides (they cannot practically be connected with topography). The groundwater divide improvement in the positioning of groundwater divides (they cannot practically be connected with in the karst plateau between the Krka and Cikolaˇ rivers is probably closer to Cikola,ˇ but it changes

Water 2019, 11, 2415 14 of 18 position significantly according to hydrological conditions (unfortunately, there are no observed GWL data in Svilaja Mtn., nor in mentioned karst plateau; no monitoring boreholes at all). All of this area is usually considered to be a catchment of the Torak spring, but during high GWL, it strongly influences Cikola,ˇ as is recorded at the Kljuˇcicehydrological station (Figure6). Krka canyon is deeper (up to 200 m) than the Cikolaˇ canyon (up to 130 m). Both canyons were cut in the same geological structure, mostly Eocene Promina carbonate clastites and Cretaceous carbonates, which are both intensively karstified. Krka’s stream is permanent and much stronger, due to stronger contributories and a much bigger catchment area, while Cikolaˇ flows at higher altitude and that is the main reason for more emphasized sinking within its own riverbed after entering karst terrain (canyon), except during periods of very high GWL. In karst terrains, groundwater and surface water constitute a single dynamic system. Determination of the catchment area and boundaries in karst environment presents an extremely difficult and complex task, which very often remains unsolved. A karst basin represents a complex system of water circulation governed by numerous extremely different surfaces and underground karst forms. Locations, dimensions, and spreading of underground karst phenomena are generally unknown. In such heterogeneous and anisotropic systems, water may circulate in unexpected directions, mainly depending on groundwater level. The interdependency between surface water and groundwater in karst occasionally or permanently changes in time and space. These changes are caused by natural processes, as well as anthropogenic interventions, which have been very intensive in the last hundred years or so and are generally uncontrolled. Due to previous briefly specified reasons, the discovery, explanation, and quantification of water circulation in karst media is not an easy task, and in some cases is even impossible.

6. Conclusions In the case of an intermittent karst river, the specificity of the karst geological setting strongly influences complex interrelationships between the surface stream and groundwater. Detailed hydrogeological and hydrological monitoring and investigations can help elucidate and better explain these processes. The example of the Cikolaˇ River provides good evidence of this fact. An exclusively hydrogeological or exclusively hydrological approach would not be adequate for interpretation, as presented. Further improvements can be accomplished with more detailed monitoring of both river and groundwater, which would include continuous measurements of levels and physical–chemical characteristics, especially during storm events and the end of recession. Delineation of catchments and subcatchments can only be achieved with several iterations among hydrogeologists and hydrologists. Due to ongoing climate changes/variations, the monitoring must be more detailed, considering both spatial and temporal distribution, and must cover hydrological, hydrogeological, and ecological processes in the analyzed area. Previous analyses have revealed strong and dangerous increasing trends of air temperature and decreasing trends of precipitation over the last 20 years. This paper shows that only a multi-methodical approach is possible in terrains as such, where hydraulic modeling is not an option. Nevertheless, within further research, a thorough program of hydrochemical, isotopic, and geophysical methods should be included to achieve better understanding of the system. To better understand and manage IRES of crucial importance is to bring together hydrologists, hydrogeologists, biogeochemists, ecologists, modelers, environmental economists, social researchers, and stakeholders to form a research network for synthesizing the fragmented, recent knowledge on IRES, improving our understanding of IRES and translating this into science-based, sustainable management of river networks. Drying affects flow continuity through time and flow connectivity across longitudinal, lateral, and vertical dimensions of space. The conclusions of the presented Cikolaˇ River and Spring research are comprised of several points: (1) The spring (cave) has limited discharge capacity, and after it reaches its maximal rate, several supplemental springs occur in the vicinity; (2) the reaction of the spring on rainfall in hinterland is rapid and abundant; (3) the river flows in three different environments—through the karst polje, Water 2019, 11, 2415 15 of 18 through the canyon, and in estuary; (4) groundwater and surface water are interrelated; and (5) after appearing in polje, the discharges have to increase (Figure 10) to provide enough water for the river to flow through the canyon. There are multiple areas in which a lack of knowledge hampers the effective management of intermittent streams [49]: (1) Hydrologic processes, (2) the effects of flow stress on ecological processes, (3) restoring degraded IRES, and (4) assessment of the condition of IRES. For both hydrology and ecology, there is a paucity of detailed scientific data on IRES, especially these located in karst regions. Traditional flow-gauging systems have vastly underestimated the number of rivers and streams flowing intermittently [3]. Unfortunately, for more detailed and more reliable conclusions, the analyzed case requires measures from more hydrological stations and a dense network of piezometers drilled in karst aquifer (and Petrovo karst polje as well) along the Cikolaˇ River watercourse, as well as precipitation measuring gauges within the catchment areas. Despite this obvious shortage, the example discussed in this paper is very characteristic for practically all IRES in karst terrains and can usefully help other colleagues dealing with the analyses of karstic IRES. Larned et al. [4] and Boulton [50] suggested six primary objectives for IRES conservation: (1) Preservation or restoration of their aquatic–terrestrial habitat mosaics, (2) preservation or restoration of their natural intermittency, (3) identification of flow requirements for highly valued species and processes, (4) adoption of conservation measures that explicitly acknowledge the importance of the spatial arrangement of ephemeral streams to the entire drainage network, (5) protection of the contributions of ephemeral streams to local and regional aquifer recharge and water quality, and (6) recognition of the cumulative importance of the brief periods of flow to aquatic and terrestrial ecological processes. Climate change may exert a strong effect on any IRES, and particularly on the Cikolaˇ River. Intensification of global warming will cause earlier initiation or later completion of flow cessation. Additionally, water pumping can influence extension of drying duration. A definite conclusion is that water and ecological resources in this vulnerable karst catchment are under stress. These effects are presently difficult to predict. It is realistic to expect that the occurrence of spring and river drying in the nearby future will be much more frequent and severe. As a result, conflicts between water supplies and the environment will very likely be stronger and with more dangerous consequences. To help mitigate negative consequences, Dragoni et al. [51] insist on a better understanding of complex interrelations between surface water and groundwater, as well as the influence of future anthropogenic actions. We hope that this paper can help in achieving better results in understanding and management of the extraordinarily valuable and complex systems of karstic IRES.

Author Contributions: O.B. provided conceptualization, hydrological investigation, supervision, and validation, and wrote the original draft. J.T. provided data curation, hydrogeological analysis and conceptualization, and data interpretation, and wrote the original draft. T.R.-B. provided supervision and validation, and wrote the original draft. T.F. provided data curation and visualization, and wrote the original draft. Funding: The hydrological part of the research received no external funding. The hydrogeological part of the research was partially funded by the Croatian Geological Survey’s fundamental project, Basic Hydrogeological Map of Croatia, and partially by Croatian Waters who funded the pumping tests presented. Acknowledgments: The authors wish to express their gratitude to all authors of the numerous technical reports used for this paper. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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