South-eastern Dinaric karst: contrasts in water treasury

South-eastern Dinarides comprise territories of four countries of SE Europe—Croatia, Bosnia and Herzegovina, Montenegro and Albania and their parts belonging to the Adriatic basin. The region consists mainly of highly karstified carbonate rocks and is characterised by a high precipitation rate. As a result, karst aquifers can accumulate large amounts of groundwater issuing at numerous karst springs, some being the world’s largest in terms of maximal flow. Locally, the annual total rainfall rate sometimes reaches 5000 mm or even more, while an average specific groundwater yield of examined karst aquifers ranges from 40 to 50 l/s/km2. However, as discussed in this paper, the region also has many contrasts: abundant water reserves yet water shortage in certain parts, natural phenomena and wonders yet limited sources of life, good natural water quality marred by its vulnerability to pollution, feasible engineering solutions accompanied by their partly negative impact on environment, and water management at the national level contrasting with the transboundary character of most large aquifers. Doubts whether waters from this “treasury” are utilised in the best way and what the prospects are for their sustainable use and protection in the future are also analysed in the text.


Introduction
The year 2022 is important for hydrogeologists, especially those dealing with karst and its many secrets. The theme of the World Water Day (22 March) was "Groundwater: Making the Invisible Visible", while the years 2021 and 2022 were declared as the "International Year(s) of Caves and Karst" by the International Union of Speleology, supported by UNESCO and many other institutions of the world. Karst and its features are present on all the continents and in more than 150 countries (Stevanović 2019). The World Karst Aquifer Mapping project (WOKAM) found that karst covers 15.2% of the continental ice-free land (Goldscheider et al. 2020) while Europe, with 21.8%, is the continent with the greatest percentage of karst aquifers distribution (Chen et al. 2017). Karst aquifers and their waters are a globally important resource because they provide potable water and ensure health, sanitary conditions, food production and economic development for almost 1 billion people on our planet. Water from karst aquifers supplies approximately 9.2% of the world's population with potable water (Stevanović 2019), but the Mediterranean basin, specifically the Dinaric Mts. Chain (Fig. 1), is the area with the richest groundwater reserves in karst and the largest specific yield. The Dinaric Mts. are also the area where karstology and karst hydrogeology were born, thanks to the works of Jovan Cvijić (1893). Since then, many local terms such as dolina, polje, uvala, ponor as well as several others are actively used by karst researchers (Ford 2005).
The Dinarides are a geologically heterogeneous, south European orogenic belt of the Alpine mountain chain (Alpides). The system extends from the Carso or Kras area bordering Italy and Slovenia over the countries of former Yugoslavia (Croatia, Bosnia & Herzegovina, Montenegro, Serbia and North Macedonia) and ends in the Albanian Alps. Southeastern Dinarides, which are the focus of this paper, comprise territories of four countries and their parts belonging to the Adriatic basin-Croatia (CRO), Bosnia & Herzegovina (B&H), Montenegro (MNE) and Albania (ALB) (Fig. 1). According to Cvijić (1926), "there is no deeper and more thorough karst development than Herzegovina-Montenegro's karst located between the lower Neretva River, Lake Skadar and the Adriatic Sea" (Figs. 2,3).
The karst of this region was the focus of many projects, especially of Yugoslav specialists after the World War II that resulted in the implementation of large water management projects including the construction of big dams and reservoirs (Milanović 1981(Milanović , 2002. The disintegration of former Yugoslavia and the appearance of several new sovereign states established complex transboundary interlinkages that impact the use and sharing of water for domestic supply, power generation and agriculture. Between the years 2011 and 2014, the large GEF-funded project DIK-TAS (Protection and Sustainable Use of the Dinaric Karst Transboundary Aquifer System) was implemented by UNDP and UNESCO's IHP (Kukurić et al. 2011;Stevanović et al. 2016) to overcome potential conflicts and ensure sustainable water use and cooperation between the countries. Despite the intensive investigations that were conducted in the past, such a complex aquifer system still has many secrets and contrasts, and this "water treasury" requires continuous research and monitoring. Some of these aspects as well as proposals on how to use and protect this great water potential more effectively will be discussed in this paper.

Methodology
As regards the overall character of this paper, most analyses were based on existing data; however, their validation and evaluation was also an important step towards the creation of an adequate platform for the authors' proposals for more efficient and sustainable water use.
To understand the hydrogeology of the complex systems of Dinaric karst aquifers, the first step requires the presentation and explanation of the geological development and settings. The creation of a conceptual model for karst aquifer functionality, recharge and discharge components served as a base for the estimation of water budget elements and water resources distribution. For this purpose, we collected and evaluated historical data from Hydrometeorological Surveys of four countries, Water Master Plans and studies, as well as data from numerous implemented projects. To obtain as  (Stevanović et al. 2016). The study area is squared. The original file can be found at http:// diktas. iwlea rn. org/ im/ hydro geolo gical-map-of-the-dinar ic-karst many as possible information that is relevant for water distribution, availability and its temporal and spatial variability, we particularly studied certain test sites such as Mt. Orjen, shared by three countries (Figs. 3,4).
Comparing specific groundwater yields of some selected aquifers of the Dinaric region with the values of this parameter obtained from other karst and non-karst aquifers in other countries and regions has been found to be the best way to validate the presumption of water richness of the South-eastern Dinarides. Another method was to compare water reserves with the actual demand and consumption in concerned countries and put collected data in the frame of "water stress" classification, as introduced in water practice by the specialised agencies of the United Nations.
For analysing the transboundary character of aquifer systems, but also the complexity of water distribution, we found the Trebišnjica River basin (Fig. 3) where a large hydropower system is actually used by Croatia and Bosna & Herzegovina-to be the best example.
Despite rich water resources, the Dinaric karst is characterised by many contrasts. Among others, the authors identified several that are the most important and further developed and discussed them in this paper: abundant water reserves yet water shortage in certain parts, natural phenomena and wonders yet limited sources of life, good natural water quality marred by its vulnerability to pollution, feasible engineering solutions accompanied by their partly negative impact on environment, and water management at the national level contrasting with the transboundary character of most large aquifers.
Finally, taking into account the above contrasts, further analyses concentrated on experiences from implemented and possible future projects and their sustainability.

Geological history and settings of the Dinarides
The main sedimentation cycle in the Mediterranean geosyncline (Tethys) started in the Permian and lasted through the Upper Cretaceous. Intensive uplifting and folding, during which most carbonate and flysch rocks were tectonised (Herak 1972), took place at the end of the Upper Cretaceous and during the Paleocene. After the Laramian tectonic phase, next intensive movements occurred in the Helvetian phase (Eocene/Oligocene) and all the main nappes along the Adriatic/Ionian Sea coastline can be related to this stage (Meçe and Aliaj 2000). Since the Oligocene, the Dinaric region has been continuously exposed to weathering, which provided favourable conditions for intensive subsurface water circulation and the development of karst features. Top: Wide plain with cultivated land in the delta of the Neretva River, bordered by karst hills (northernmost part of the study area). Bottom: Rijeka Crnojevića River with its submerged confluence to the Skadar Lake (southern part of the study area), Both locations can be seen at Fig. 6 at most Westerner and Sothern parts, respectively One of the main geologically critical periods for the entire Mediterranean basin occurred between 6 and 5.3 million years ago (Ma). The Messinian salinity crisis (MSC) was the period when the previously large Tethys Ocean became considerably reduced and decomposed, and the connection-via the Gibraltar strait-between the Mediterranean Sea and the Atlantic Ocean was broken (Roveri et al. 2014). Isolated from other oceans, the Mediterranean basin functioned as a large salty lake. The MSC was an ecological crisis caused by a powerful combination of geodynamic and climatic drivers. The fact that the water level was reduced by more than 1500 m in about 100,000 years had a great impact on the subsequent geological history and the development of a surface water network, especially of an underground system of karst conduits at great depth and below the present sea level (Bakalowicz 2014).
During the MSC, most of today's islands in the Adriatic basin (Croatia) were connected to the continent, while mountains built dominantly from carbonate rocks were exposed to intensive karstification and deepening of the groundwater table. The lowering of the erosional base level simultaneously caused the lowering of springs and conduit systems. Many deep canyons were incised, transferring river waters to a "new" and much deeper erosional base (Stevanović 2020).
The Pleistocene started not only with climatic changes (glacial process, lowering sea level) but also brought a new Fig. 3 The Dinaric karst between Neretva River (North) and Skadar Lake (South): hydrographic network, main water points and proven groundwater paths. Legend: 1. elevation of karst polje, 2. town, 3. spring, 4. ponor, 5. temporary river, 6. perennial river, 7. proven underground connection, 8. tunnel or pipeline, 9. dam, 10 structural and morphologic evolution. In the Adriatic basin, during the period of maximal glaciations the seashore line was moved by one to tens of kilometres to the west. Figure 3 shows a topographic-nautical map of the area of the Bay of Boka Kotorska. The isobath line of 100 m and measured depths of the seafloor indicate that Boka Bay was in fact a dry depression without any sea water during all main four glacial periods. The tracers of glaciers transported the material, and moraines are evidenced at Mt. Orjen high above the Bay. Marković (1973) stated that glaciers were not descended below the altitude of 600 m a.s.l. and indicated the route of Bokeljska Reka as one of the main post-glacial paleo streams. The direction of the groundwater flow was similar or keeping the general course of these glaciers or paleo streams. The springs, where most karst aquifers had drained during the interglacial stadiums are now at the sea level or just a few metres above (Gurdić, Škurda, Tabačina near Kotor, Ljuta Orahovačka, Risanska spila, Morinj) ( Fig. 4), or are functioning mostly as submarine springs (Sopot).
The specific tectonic pattern is a consequence of the collision between the Adriatic microplate and the Dinaric plate. The zone of high destruction can be followed along and under the Adriatic Sea. This collision resulted in rocks compression, folding and faulting above the Moho discontinuity. This very active seismic zone caused several catastrophic earthquakes of magnitudes ranging from 6.5 to 7.0 on the Richter scale. The high destruction formed blocks and ; dash lines show the reconstructed riverbed of paleoriver Boka and its tributaries based on actual deepest depths to the seafloor (according to Stevanović 2020) accompanying faults of different levels of crushing, which enabled the movement of water through the soluble rock mass and empowered the karstification process deep below the current sea level. As such, incised paleo riverbed of Neretva, the largest river in the SE Dinarides, can be followed for tens of kilometres at the sea floor.

Karst geomorphology of the region
South-eastern Dinarides are characterised by extremely karstified carbonate rock masses, extreme precipitation, dominant underground runoff, more than 30 occasionally flooded karst fields, and two crypto depressions-Hutovo Blato (area near Čapljina) and Skadar Lake (Fig. 3).
According to Milanović (2000), there are approximately 130 poljes in the Dinaric karst region. Karst polje surface water drains through ponors located both along the polje perimeter and at the polje floor within unconsolidated sediments or exposed carbonates. Ponors are frequently located in the polje areas nearest to the prevailing erosion base. Approximately 880 ponors and estavelles were identified in Nikšićko Polje (MNE, Fig. 3), 851 of which along its southern perimeter. If the capacity of ponors is not sufficient to absorb runoff water, the flooding of the polje is imminent.
The Adriatic Sea is the regional erosional base towards which a cascade system of poljes is oriented (Fig. 5). In the Adriatic zone, the most specific hydrogeological features are vruljas (submarine springs) (Milanović 2002). They are formed by the sinking of the floor of the northern Adriatic Sea and rising of the sea level, causing karstified erosional base to become submerged under the sea. According to Marković (1973), in the last 25,000 years the Boka Kotorska Bay (MNE) has descended for more than 600 m in comparison with Mt. Orjen (Fig. 4), which indicates high intensity of differential neotectonic movement.
According to Herak (1972), more than 12,000 caves have been explored in former Yugoslavia. More than 300 vertical shafts were registered (Milanović 2005) at the Kameno more (Stone Sea) and Mt. Orjen above Boka Kotorska Bay (MNE, Fig. 4) within an area of only 8 km 2 . Some of those shafts were speleologically investigated to depths of 200-350 m.

Specific climate
The Mediterranean climate prevails along the Adriatic coast, but high mountains that stretch alongside it cause a very high precipitation rate. Annual precipitation sums vary between 1500 mm or more inland to over 2000 mm alongside and closer to the coast. In the Boka Kotorska Bay, average annual precipitation, from sea level up to the elevation of 1800 m, varies from 2000 mm to more than 5000 mm (Mt. Orjen). The meteorological station Crkvice recorded an annual sum of 8063 mm in 1938, which is the highest ever recorded annual rainfall in Europe (Fig. 6).

Hydrogeological settings
The SE Dinarides include the river basins of the Neretva, Trebišnjica, Zeta, Morača, Buna and Drini rivers, as well as catchments of the Boka Kotorska Bay and Lake Skadar (Fig. 3).
The Neretva River, with the drainage area of approximately 7950 km 2 , is the largest basin. It is shared between B&H and CRO. A significant influence of the sea and intense mixing of saltwater and freshwater is evident in the delta of Lower Neretva. The Trebišnjica River (B&H) is the largest European sinking stream. Until the construction of a large hydro-energy system, Trebišnjica regularly flooded Popovo Polje. More than 500 ponors, estavellas and intermittent springs have been registered along the Popovo Polje (Milanović 2005), while the total sinking capacity has been estimated at 300 m 3 /s. The length of Trebišnjica is 90 km, from the springs near Bileća (now submerged) to the Ponikva Ponor in Popovo Polje (Fig. 3). The total area of the Adriatic watershed in Montenegro is 6267 km 2 . River Morača (Fig. 3) and its tributaries Zeta, Cijevna, Rijeka Crnojevića discharge into Lake Skadar; from there, Bojana/ Buna River flows towards the Adriatic Sea, crossing the Albanian territory first and making the border between the two countries further downstream (Hrvačević 2004). Lake Skadar (Shkodër, Skutari) is the largest lake on the Balkan Peninsula. It was granted the status of a National Park in 1983 and was included in the Ramsar List of internationally protected wetlands in 1995. It represents a true winter safe haven for European birds. Around 60% of the lake is located in Montenegro, while 40% is in Albania. The largest rivers in Albania that belong to SE Dinarides-Cemi (Cijevna), Buna (Bojana) and Drini (Drim)-are all transboundary.
Instead of being strictly defined, watersheds are usually commonly delineated in a zonary fashion, mostly underground. These dividing zones are variable in time and space and have frequent bifurcation areas whose position and characteristics depend on the current state of the groundwater elevation. The differences are often tens or even hundreds of square kilometres. Depending on the regime of underground and surface water in certain basins, which are separated from each other during the dry period, in the period of heavy rainfall, when water elevation rises, they become interconnected (the Trebišnjica basin with the Neretva basin, or the Ombla basin, Fig. 3).
The underground water network of the SE Dinarides is very complicated (Fig. 3). According to Milanović (2021), more than several hundred tracing tests have been performed in the Dinaric karst to date, 77 in the Lake Skadar basin alone (Radulović, 2000). Milanović (1981) noted that based on experiments conducted in eastern Herzegovina, the average flow velocity is estimated to range between 0.002 and 55.2 cm/s. Such extreme values are rare, whereas the average velocity is about 5 cm/s. Based on 380 conducted experiments, Komatina (1983) concluded that the frequency of groundwater velocities in Dinaric karst is as follows: in 70% of the cases-from 0 to 5 cm/s; in 20% of the cases-5 to 10 cm/s; and in 10% of the cases-more than 10 cm/s. The water marked with a dye takes two to five times less time to travel the same distance during a season of high hydrologic  Milanović 2005). Right: Mt. Orjen landscape and mean monthly sums of precipitation activity. Milanović (2000) presented an example of this: to cover the distance (34 km) from Gatačko Polje to the Trebišnjica Spring (Herzegovina, Fig. 3), the underground flow takes 35 days when the water table is low and inflow is small. During high water levels and large inflow, it takes water only 5 days to cover the same distance.
The karst aquifer recharges from precipitation and waters percolating from numerous sinking rivers. Depending on the locality, morphology and karstification properties, the average recharge rate is between 50 and 80% of the precipitation. In the Trebišnjica River catchment area, this percentage is even higher and reaches 90%. Effective infiltration in the Albanian karst is on average made up of 40-55% rainfall (Eftimi 2010). Although some swallow holes may have the absorption capacity of more than 100 m 3 /s of water, enormous maximal river flows have causes many karst poljes to be converted into lakes, as ponors cannot swallow all water inflow. Before the construction of the large hydro system, Trebišnjica River, the largest European sinking stream, used to flood Popovo polje (Fig. 7) 253 days out of a year on average, despite some 500 registered ponors along its riverbed.
High precipitation rates and the high effective recharge result in abundant but variable water reserves. More than 70% of the precipitation occurs during the wet season (October-March). For instance, enormous amounts of rainfall sink and are lost uncontrollably towards the Adriatic Sea. For the Boka Kotorska Bay alone, these quantities are estimated to be 1.5 billion cubic metres of the highest quality groundwater per year. Unfortunately, this happens over a couple of winter months. And then the karst swallows each raindrop, leaving a drought on the surface. Milanović (2005) noted that "through just three very large springs along the Neretva Valley and the Adriatic coast (Buna, Bunica and Ombla, Fig. 3) and a few spring zones in the Bay of Kotor, more than 150 m 3 /s of water is discharged annually into the Adriatic Sea, directly or indirectly, through River Neretva". The region is so rich in large karst springs that the WOKAM project applied more severe criteria for the purpose of showing these springs on the produced map (Goldscheider et al. 2020). For instance, in Bosnia & Herzegovina, there are 8 springs on 1000 km 2 that regularly discharge more than 2000 l/s. In the smaller country of Montenegro, there are five such springs. Buna spring near Blagaj (Bosnia & Herzegovina) is one the world's largest, with maximum discharge of 380 m 3 /s (Fig. 8). However, some other springs are also crucial for the local economy and tourism. The Ombla spring supplies the entire municipality of Dubrovnik with potable water (Fig. 8), while the intake project involving the spring Bolje Sestre on the shoreline of Lake Skadar is called "the project that changed Montenegro" because it managed, after many years, to solve the problem of water shortage that was present along the entire Montenegrin Coast, enabling intensive development of tourism and national economy (Stevanović 2010). Table 1 shows the characteristics of some of the most important springs in SE Dinaric karst, while their location can be seen in Fig. 3.
Most of these springs have a siphonal character. Karstified channels at Ombla were thus drilled at the depth of 160 m, while speleo divers reached the depth of -63 m. The discharge points of springs Buna, Bunica, Oko, Duboka ljuta and all the springs located around the Boka Kotorska Bay and Lake Skadar have a much lower position of paleo

Water regime and its regulation
As seen in Table 1, the ratio of maximal and minimal discharges (Qmax: Qmin) varies from less than 1:2 for some small springs in Albania to even 1: 3660 for the ascending spring Ljuta in Boka Kotorska Bay. Such enormous disproportion in case of Ljuta is a result of a very deep and large storage of littoral karst aquifer. The divers had explored vertical siphon of this spring to the depth of -133 m, with total length of channel of 150 m (Milanović 2007). Such channel can drain more than 150 m 3 /s in periods of maximal floods.
Very dynamic regime can also be viewed by comparison of maximal and average discharges. Such data are available for four major springs (Table 1). The difference between maximal peaks and an average yield is ranking from 1:3.4 (Bregava springs) to 1:16 (Buna spring).
Vrelo Bune, the largest in the region and one the world's largest springs concerning its maximal discharge is also a typical illustration of very dynamic karst water regime in the SE Dinaric karst. Figure 9 shows correlative diagram of rainfalls and discharge for one high-water and one recession period in chosen typical hydrological 1970/71 year. While maximal yield reached extreme value of 380 m 3 /s the minimal discharge in July 1971 was 4 m 3 /s, or one hundred times lesser. The reaction of aquifer, which catchment area is estimated on 1100 km 2 (Milanović 2005(Milanović , 2021) on heavy rains is usually very quick, discharge may raises significantly within 24 h or even less. The very specific was winter period at the end of year 1970 and beginning 1971 when 10 days of continual and very intense rains took place. In only one day fallen 150 mm and then in less than 24 h an extreme discharge peak happened (Fig. 9). Quick response of aquifer on such input rain signal is usually sign of limited storage capacity of an aquifer, but this is not case here. In contrast, the Buna aquifer system has very good storage within its deeper sections, and described situation represents piston effect of fully saturated aquifer system. The main spring's siphon has been explored by divers to the depth of -72 m while the total length of is 520 m (Touloumdjian 2005). Good storage is evidenced at spring hydrograph ( Fig. 9): During period of extended drought of almost 90 days (May-July 1971) discharge of this large aquifer remains significant and not dropped below 4 m 3 /s.
In natural conditions, the water regime is therefore characterised by extreme hydrogeological parameters: in certain areas, the fluctuation of the groundwater elevation is up to 320 m (Nevesinjsko polje, B&H, Fig. 3) with the rate of water level increase of up to 90 m in less than 24 h; in an extreme case, the sudden increase of the spring discharge  The water source of the city of Trebinje. The original spring site is submerged by reservoir Gorica, while a new intake has been constructed inland to tap ground waters before they are reaching the reservoir. The pump is 25 m below an average reservoir level Ombla (Fig. 8 Fig. 3), but they can also run dry during most of the drought season. Such water regime variations reflect negatively on many aspects of human life, resulting in the instability of eco systems. This is why serious efforts were made in the past to regulate the water regime. The first project that took an idea all the way to a practical solution date back to the period 1888-1896, when the Klinje Dam (the Mušnica stream, B&H, Fig. 3) was built.
Today, many streams are dammed and their waters used by hydro-power plants (HPP). Large dams and reservoirs were built on Neretva, Trebišnjica, Zeta and Drini rivers. The Dinaric karst is thus becoming a reference area for the successful completion of dams in karst, which happens to be a very problematic media when it comes to water loss (Milanović 2000(Milanović , 2015. About 2/3 of the total existing hydro-power facilities of the four countries are located in the SE Dinaric karst area, and hydro-power generation plays a significant role in these countries' economies. The construction of the Multipurpose Hydrosystem Trebišnjica began in the late 1960s and has yet to be completed. However, the core structures of two consecutive dams on this river-the Grančarevo and Gorica dams (Fig. 3) Losses from the upstream Bileća Reservoir, dammed by Grančarevo, are negligible, while losses from the Gorica regulation pool range between 3 and 4.5 m 3 /s depending on the pool level (Milanović 2021). These losses appear mostly downstream from the Gorica Dam, producing guaranteed ecological flow to the Trebišnjica River through the Trebinje urban area. Downstream of Trebinje, its 65 km long riverbed flowing through Popovo Polje is blanketed by 5 cm-thick reinforced shotcrete (Fig. 7) (Milanović 2000).
The number of artificial reservoirs in Montenegro, built mainly in Nikšićko Polje (Krupac, Slano, Vrtac, Liverovići, Fig. 3) is small in comparison with the available hydropower potential. The total capacity of these reservoirs is cc. 1 × 10 9 m 3 of water. This amounts to about 7% of the estimated total amount of surface water formed in the territory of Montenegro (Hrvačević 2004). Water from the reservoirs is used by the Perućica HPP (307 MW). All the reservoirs in Nikšićko Polje have been built in highly karstified rocks. Slano and Vrtac required intensive and expensive anti-infiltration works from the very beginning. The grout curtain along the southern rim of Slano is one of the longest in the world. It has a length of 7011 m, a depth of 57 m, and a surface of 396,122 m 2 (Vlahović 2020). The current hydropower capacity of the main Albanian plants is 1750 MW, and most of the dams were built on the Drini catchment (Eftimi 2020).
The natural water quality of karst aquifers is generally good; water is of Ca-HCO 3 class, low mineralised but with often increased turbidity due to the washing of karst conduits during periods of flooding.

Discussion
The paradox in SE Dinarides is that water, its greatest wealth, is at the same time the root cause of numerous controversial events that occurred both in natural conditions and during the period of technical interventions that were undertaken to regulate the water regime. Even more contrasts and doubts are arising from the implementation of numerous measures to adjust the water regime aimed at helping the region's socio-economic development.

Landscape and natural wonders vs. limited source of life
The karst of SE Dinarides contains many natural wonders and in the last few decades it has become a very attractive tourist destination. Visitors are not only concentrated along the Adriatic Coast; they also visit deep canyons, waterfalls, springs and high mountains located inland. The region boasts several national parks and protected areas such as those under the Ramsar convention, as well as endemic Fig. 9 Correlative diagram of rainfall and discharge of Buna spring for period December 1970-July 1971(after Milanović 2021 species that inhabit the underground world of caves. Due to its historical importance for the development of karst science (for its exemplary karst development with numerous geo-heritage sites and abundant groundwater resources), an initiative has recently been taken to include the Dinaric region and its selected areas in the UNESCO list of World Heritage Sites.
In contrast, life in karst was never easy. It has been a long time since the first people inhabited this area, and it was nature that determined where they would build their houses, when they would sow and harvest, or-as is the case in Popovo polje-when there would be farmers and when fishermen. Such an uncomfortable life resulted in many migratory waves, especially at the end of the 19th and the beginning of the twentieth century.

Abundant groundwater reserves vs. water shortage
In Montenegro, Bosnia & Herzegovina and Albania, each citizen has more than 5000 m 3 of water available annually. This amount is ten times higher than the level indicated in certain UN documents as the limit for "water stressed countries". In an average hydrological year, each inhabitant of Montenegro has access to 21,395 m 3 of water, but uses just 1.18% of it. In Bosnia & Herzegovina, the utilisation rate is even lower, below 1%. Citizens of Croatia and Albania also use less than 5% of water that is available to them per capita (Stevanović 2021).
In contrast, there are areas where local populations and ecosystems often suffer from water shortages. In the mountains high above the sea level and the erosional base, the groundwater table is often very deep, sometimes several hundred metres, and there is no available surface water. The only way to provide water to local villagers and their livestock is to build cisterns and specific intakes for collecting rainwater (Fig. 10).
The shortage of water is locally evidenced even along the Adriatic Coast in summer and early autumn months, coinciding with increased demands during the tourist season.
In such circumstances, the principles of ecological flow towards downstream consumers have to be fully respected.

Good natural water quality vs. high vulnerability and many hazards
Unstable water quality and vulnerability to pollution are the typical characteristics of karst aquifers in the Dinaric karst. Karst aquifers formed in well-karstified carbonate rocks are recharged mainly from rainfall and sinking flows, mostly in sparsely populated catchments in mountainous areas, and the natural water quality is satisfactory. However, when pollutants are present, which is often the case due to intensive economic and tourist development of the region (e.g. mines, industrial and domestic waste waters, untreated sewage water, solid waste dumps, septic tanks, fertilisers), deterioration of water quality in unconfined and vulnerable karst aquifers is almost certain (Stevanović et al. 2016). The main threats to the overall groundwater quality in the region are solid waste and wastewater disposal. In the four project countries, there are hundreds of unregulated landfills and illegal dump sites. The number of wastewater treatment plants is insufficient, with about half of the population not connected to this service. For the vulnerable karst environment of the Dinaric region, which has a very limited auto-purification capacity, this is the most serious current as well as potential future problem. Karst groundwater resources in the region are also contaminated by agricultural and industrial activities, albeit to a lesser degree. Therefore, strict enforcement of already established sanitary protection zones and preventive measures is necessary to safeguard water quality. Dinaric karst aquifer system areas require specific solutions and compromises in land use planning, and the protection of nature and water resources.
Another problem of many aquifers along the seacoast is salt intrusion. The case of springs that supply the city of Kotor in Boka Kotorska Bay is highly typical, which is why the main water system for the Montenegro Coast had been constructed inland, with water running through a 40 km long pipeline before it reaches the coastline.

Engineering solutions vs. ecology and environmental impact
One of the most significant public controversies was related to the feasibility of constructing reservoirs in karst. After the first reservoirs built in USA, Spain and Iran could be filled only partially, or were left without a drop of water (Milanović 2000(Milanović , 2014(Milanović , 2015, some members of the expert public formed the opinion that the construction of dams in karst should be abandoned. For example, in the case of the large reservoir Bileća (Fig. 3), with a volume of 1.3 × 10 9 m 3 of water, the opinion of some eminent foreign experts was that it would never be filled. However, more than 50 years of its operation have proven that it does not lose water. When choosing a water-bearing storage area, the geological structure of the terrain and the reconstruction of the evolution of the karst outcrop play a key role.
The construction of large hydro systems is of great importance for regional development because the positive effects far outweigh the negative: there is flood control, water supply, irrigation (food production), development of vegetation cover, power production, improvement of the minimum flow of downstream sources, better infrastructure, as well as many other secondary effects.
However, there is almost no intervention to change the nature for the benefit of humans that has absolutely positive effects on an ecosystem and all its elements. As such, based on good knowledge of the nature of the SE Dinarides, it was clear that numerous problems could be expected as a result of the imposed changes. Some were predictable and measures were taken to eliminate or mitigate them. Others, such as flooding of cultivated fields, resettlement of small villages, drying of the soil along the canalled (or concreted) riverbeds, reduced wildlife and endemic fish, were, however, unavoidable. Therefore, experiences with the construction of such water management systems have shown us that, due to the unpredictability of karst, unexpected reactions of nature are always possible and can sometimes even annul the main engineering ideas (Roje-Bonacci and Bonacci 2013). If a water project is going to be implemented, the involvement and reactions of the "green sector" and non-governmental or international organisations such as the World Wildlife Fund (WWF) should be taken into account, and projects should be based on properly prepared environmental impact assessment studies valuing positive and negative effects of proposed engineering solutions and appropriate mitigation measures.

Water management vs. transboundary aquifers and impacts
Since SE Dinarides are divided by several states, entities and regional and municipal borders, it is clear that serious controversies should be expected in terms of water management. The DIKTAS project has been set up to improve the understanding of transboundary groundwater resources of the Dinaric region and facilitate their equitable and sustainable utilisation, including the protection of unique karst groundwater dependent ecosystems (Kukurić et al. 2011). The DIKTAS project has focused on several selected Transboundary Aquifers (TBAs), examining current and potential issues of concern. Based on five criteria (importance, representativeness, data availability, issues of concern, relevance), eight TBAs-five of them in the SE Dinarides-have been selected and a detailed analysis has been conducted to establish a platform for sustainable water management.
The DIKTAS Strategic Action Plan is focused on several priority actions: improving the quality of water, eliminating sources of pollution, improving minimal discharges, ensuring ecological flows, and establishing proper water monitoring systems (Stevanović et al. 2016). DIKTAS countries are committed to exchanging information amongst themselves at the regional or bilateral level and are preparing an instrument to serve as the legal basis (such as a memorandum of understanding or another agreement). DIKTAS countries will exchange collected data through national institutions responsible for managing the National Water Information System, under the supervision of the established Consultation and Information Exchange Body.

Possible solutions for more sustainable water use
To eliminate controversies in regional and transboundary water management of the very complex karst system of the SE Dinarides, it is crucial to establish a precise system for monitoring the water regime. Due to the fact that, in the period of saturation, karst outcrops have the characteristics of a hydraulic system under pressure, the changes and prolongations of peak floods are extremely fast, practically instantaneous. Consequently, measuring these changes at certain time intervals offers unreliable information. Only monitoring that includes continuous data collection in real time would not be subject to controversial interpretation. It would prevent incorrect conclusions and enable optimal controllability of such a system.
As explained above, karst aquifers of the SE Dinarides contain large groundwater reserves that are however variable in space and time. Milanović (2005) estimates that 70% of these reserves is not under control. It is only in the case of the transboundary Mt. Orjen aquifer, which mostly discharges into the Boka Kotorska Bay, that dynamic reserves are estimated at 21.2 m 3 /s. Analyses that were made within the DIKTAS project found that from none of studied transboundary aquifers more than 10% of its potential is utilised (Stevanović et al. 2016). Therefore, many aquifers could be better developed, and many new projects can be implemented. The complex hydrogeological survey, which inter alia should include remote sensing, modern geophysical techniques and speleology (including speleodiving), exploratory drilling and testing the boreholes, tracing tests and continued hydrological monitoring of all water points, should be applied during the first investigation phase, which together with the Environmental Impact Assessment Study should create a platform for deciding whether proposed karst aquifer regulation solutions are feasible or not (Stevanović and Milanović 2015). However, in some localities such as the Ombla karst spring or springs located along the Boka Kotorska Bay, preliminary investigations have already confirmed large potential and prospect for large projects, including hydropower generation. Milanović (2000Milanović ( , 2021 suggested the construction of an underground reservoir at Ombla spring, while for springs in Boka Kotorska he proposed the construction of a tapping gallery, which should enable direct capture of groundwater at higher elevations (200-330 m a.s.l.) before they reach the drainage zone on the coast.
The authors of this paper state that, in many cases, environmentally friendly engineering solutions could open a new perspective not only for better utilisation of the existing water potential for local purposes, but also for water transfer to other Mediterranean countries that suffer from water shortage, primarily those along the Mediterranean Coast in North Africa and Near East. This idea is not new and has been already included in the development plan of the Regional Waterworks for the Montenegro Coast. However, under the current circumstances and considering the way the tapped sources are maintained, it cannot be implemented.

Conclusions
The Dinaric karst provides essential and extremely valuable ecosystem services and supports the development of concerned countries' economies (drinking-water supply, tourism, hydropower production). Although generally rich in groundwater resources as almost no other European or Mediterranean region, water management here is not easy. There are two main factors that complicate the development of sustainable water resources and related technical solutions. The first is karst as the most complex aquifer system with great groundwater variability in terms of both quantity and quality, coupled with high vulnerability to pollution. The second is the transboundary character of the entire Dinaric system and its numerous karst aquifer basins, where water is shared by two, sometimes even three countries.
Extensive and complex hydrogeological investigations in the Dinaric karst region of former Yugoslavia were undertaken as part of large infrastructure projects that included the construction of large and medium dams, large intake structures for water supply and other engineering works. Successful technical solutions for water regulation in the karst of SE Dinarides have been globally recognised as examples of the possibility of managing water resources even in a complex environment such as karst. Despite having some negative impacts on the environment, which inevitably follow large construction operations, these projects reduced migratory waves from the region, opened new perspectives for the local populations, enabled tourist and economic development, and generated higher income. Consequently, people from this region recognise the importance of water. Regardless of their transboundary character, demand to better utilise some of the aquifers are often discussed among experts and decision-and policy-makers.
The hazards and concerns regarding water are not only environmental. Since the disintegration of former Yugoslavia they have also become economic and political. Such a situation confirms the premise that the definition of water resources development strategies in the Dinaric karst area based on sound governance principles is the key requirement for regional socio-economic development. Establishment of advanced monitoring systems, systematic data collection and exchange, and cooperation through joint water committees are preconditions for future sustainable use and protection of the karst groundwater of the SE Dinarides.
Author contributions ZS and PM wrote the main manuscript text jointly and equally. All sketch maps and photos are result of their original works. Both authors reviewed the manuscript.
Data availability Climate elements dataset (precipitation, air temperatures) can be found at web sites of meteorological services of the countries Croatia, Montenegro, Bosnia & Herzegovina, Albania. Most data on geology and hydrogeology is available in cited references (published books and articles), while some data are not publicly available as included in engineering projects and studies of employed companies.

Conflict of interest
The authors declare that they have no competing interests.