Water Resources of the Velebit Nature Park
D. Perković*, D. Mayer*, I. Dragičević*
*Faculty of Mining, Geology and Petroleum Engineering, University of Zagreb, Pierottijeva 6, 10000 Zagreb, Croatia
Keywords: water balance; catchment areas; water discharge
Introduction
It is well known that water balance of one area means mutual relationship between precipitation, sum of runoff (surface and subsurface runoff) and evapotranspiration during specific time. Catchment area is what we typically use for our analysis. In the case of Nature Park Velebit that is not possible because terrain comprises of many catchment areas. Additional complexity arises because of fact that the park area is one extremely karst area with significant topographic and climatic distinctions. Particularly, due to elevation in parts of the Park with higher altitude, surface runoff is increasing and sinking is decreasing. Also, evapotranspiration is decreasing due to lower air temperatures at higher altitudes. In addition, climatic characteristics inside the Park are changing from mediterranean to continental-mountainous. Regardless of complexity of that area, determination of water balance for Velebit Nature Park could allow more efficient use of water for water supply, sport, recreation and tourism still retaining maximum water protection.
Following its 1978 inclusion into the network of international biosphere reserves (UNESCO program Man and the Biosphere) the Croatian Parliament proclaimed the entire Velebit a nature park in 1981. (Velebit Nature Park, 2001).
Nature Park Velebit encompasses almost all Velebit – the most significant Croatian mountain based to its relief, geology, hydrogeology, hydrology, speleology and vegetation. Although entire Velebit presents exceptional nature resource value, two areas inside its territory are especially highlighted because of their characteristics. Those are National Park “North Velebit” and National Park “Paklenica”. (Figure 1.1).
Figure 1.1 Map of Nature Park Figure 1.2 Catchment areas inside the park
Climate
Two types of climate prevail in the park - the Mediterranean climate in the narrow coastal strip and the Alpine-Dinaric in the rest of the Park. The most specific characteristic of the entire region is a well-known north-easterly fierce wind called bura that blows from Velebit toward the Adriatic Sea. Annual average temperature is above 16°C in coastal parts and only about 3°C at higher mountainous altitudes. Also, great differences are present in annual average precipitation amounts. They vary from 1500 to very high 2500 mm/year. Snow is very common as precipitation in that region and some park areas have the highest maximum snow depth in Croatia (over 200 cm). Of course, in coastal part of Park, snow cover occurs very rarely, it’s very low and rarely exceeds 10 cm. In highest park areas snow cover can last more than 100 days annually. Number of days with thicker snow cover is negligible in coastal and south part of park. (Meteorological and Hydrological Service of Croatia & Croatian Meteorological Society, 2003; Meteorological and Hydrological Service of Croatia, 2004).
Geology and hydrogeology
Park covers area of two main catchments areas – Lika and Velebit catchment area and river Zrmanja catchment area. (Figure 1.2). Complex hydrological and hydrogeological conditions are result of geological structure and tectonics. Park area in its entirety belongs to carbonate platform of Dinarides, which enables good identification of continuity of sedimentation and research of paleogeographical conditions. Quaternary sediments are without greater impact on groundwater dynamics in that region. Older, carbonate sediments, as well as numerous tectonic blocks have primary significance for distribution and motion of the karst groundwater. Neotectonic faults are also especially important because of their influence on deep karstification and development of underground channels with water flows towards to sea. Very specific karst development and water dynamics produced karst phenomenons which are present in Lika and Velebit catchment area. Groundwater and surface water divide between the catchment areas of the Black Sea and Adriatic Sea is also very significant (Figure 1.2). Considering the permeability in this area, there are four groups of rocks:
- rocks with good permeability: limestones, limestones intercalated with dolomites, limestone breccias
- partially impermeable rocks: Jelar Beds and partially permeable dolomites;
- impermeable rocks: shales and sandstones, tuffs and tuffits, basalts;
- rocks with extremely variable permeability: Quaternary sediments with different lithologic composition
Few important facts have become known based on hydrogeological characteristics of rocks, tectonics, spatial and hypsometric position of geological bodies and finally morphology. Hydrogeological functions of certain regions are defined by their permeability (complete and incomplete barriers and permeable areas). The complex hydrogeological barrier of Velebit concurs with its tectonic structure. Barrier is defined by position of impermeable volcanogenic-sedimentary deposits, Palaeozoic clastics, “upper bed” Upper Triassic dolomites and also clastics along the west wing of anticline. Position of above mentioned deposits defines Velebit hydrogeological divide is defined by. Other, uncompleted hydro geological barriers do not prevent water flow from toward sea, they locally slow down or direct water flow to specific springs and vruljas. Permeable area is mostly built on well permeable carbonate deposits. Karstification in hinterland of Velebit barrier is relatively shallow, in karst poljes is shallower than 100 m. There is no surface water flows or springs in mountainous area outside the Velebit barrier. Local intermittent flows are not connected with groundwater. Lost rivers flow through this permeable area towards springs and vruljas. So far there are registered 14 swallow holes and 86 springs spaced on subcatchment areas. (Figure 1.3)
Figure 1.3 Subcatchment areas with springs and geomorphological features
Water rising on constant and periodic springs in mentioned catchment areas form 20 perennial and intermittent streams. River Zrmanja, with its confluents Krupa and Dobarnica, is the only significant surface water in the park. Those rivers are very significant for activities of Velebit nature park institution and it is essential that their water is used rationally. River Zrmanja, in its greatest part, represents base for groundwater discharge from the catchment area.
Results and discussion
Nature Park Velebit covers area of 2247 km2. Annual average rainfall is estimated about 2000 mm. Assumed annual average evapotranspiration is 500 mm (80% of average value for Croatia which amounts 635 mm). Nature Park is not closed catchment area region, it encompasses more catchment areas and water dynamics is conditioned with complex geological and consequently hydrogeological and hydrological conditions (Mayer, D. et al, 2003).
It can be assumed that approximately 4.5 billions m3/year of water appear in the Park area in state of all types of rainfall. Out of that amount about 1.125 billion m3/year go back in atmosphere by evapotranspiration and about 3.4 billions m3/year runoff as surface flow or infiltrate in underground. Occasionally, in rainy period, surface runoff is more intensive in canyons in form of torrents. There are many dolines, sinkholes and swallow holes on smoother hill-sides with rich vegetation. Bottoms of dolines and sinkholes are covered with clastic deposits which enables retention of precipitation waters. On the other hand swallow holes “engulf” water resulting in no surface flow. Where they are and Quaternary clastic deposits are weakly permeable and thick, smaller plashes arise in specific morphological areas in terrain parts with thinner surface deposits, precipitation water relatively rapidly infiltrates in karst underground. That, together with water which came in the underground through swallow holes, makes Velebit extremely rich in groundwater. Groundwater dynamics is conditioned by very sophisticated geological and hydrogeological relationships resulting in springs. Springs form creeks, inflow into rivers Ričica or Lika, sink into one of numerous swallow holes and then flow by underground to exsurgence of Zrmanja river etc. All the waters go back the park underground either through swallow holes or through river Lika. Park underground water inflows in the catchment area of the Adriatic Sea. With this fact those waters do not present negative element in water balance. Based on that fact we can conclude that almost entire water accumulated in Park area, except direct surface runoff by torrents into the sea, outflow from the park on three ways. First way is discharge by river Zrmanja which is the main recipient in south part of park. Second way is through couple of high altitude periodic springs in hinterland of Velika and Mala Paklenica, thereafter outflow by periodic creeks in sea near Starigrad. Third way is efflux on currently registered coastal springs and vruljas between Senj on north and Rovanjska on south. If we try to make preliminary balance based on uncompleted and unreliable available data, we can assume the following:
Average annual discharge from the Velebit nature park by river Zrmanja is 0.98 billion m3. Average annual discharge into the sea by registered springs and vruljas in coastal area and springs and creeks in hinterland of Mala and Velika Paklenica amounts 35.6 millions m3. If we sum these quantities and compare with total annual precipitation amount decreased for predicted evapotranspiration, it comes out that about 2.5 billions m3 of water gets uncontrollable discharged from the park, annually. The discharge consists of torrential streams, unregistered vruljas and diffuse draining into the sea.
Conclusions
Nature Park Velebit area is one of the areas in Croatia with highest rainfall. However, owing to lithological structure and complex tectonical relationships, there is still shortage of large perennial streams. The fact is that waters originating in the north-northwest side of Velebit often sink after short surface flow and then discharge in the Adriatic Sea. Precipitation waters from west-southwest sides of Velebit become torrential intermittent streams which quickly drawn into Adriatic Sea. Smaller parts of those waters are infiltrating into karst underground and like waters from the north and north-east side of Velebit, are becoming coastal springs and/or vruljas, or they discharge diffusely into sea.
Estimated annual quantities that discharge into the sea in uncontrolled way are huge and unfortunately seasonally very unevenly distributed. Those water quantities can have great importance for water management of Croatia. Detail quantification of those amounts is one of the next tasks in investigation of this area. Field investigations should include:
- Survey and registration of currently unregistered springs and vruljas
- Survey of swallow holes which are at this moment placed on areas with mines
- Establishment of hydrometric measurements at some springs in River
Zrmanja catchment area.
Cabinet work could include processing of existing hydrological data for Zrmanja, Krupa and Dobarnica.
References
Mayer, D., Dragičević, I., Žugaj. R. & Božičević, S. (2003) Water Resources of the Velebit Nature Park, Report. Faculty of Mining, Geology and Petroleum Engineering, Zagreb
Meteorological and Hydrological Service of Croatia Croatian Meteorological Society (2003) Zavižan between snow, wind and sun. Monography. Zagreb.
Meteorological and Hydrological Service of Croatia (2004) National programme for combating desertification and drought and prevention of land degradation. Climate. Report for 2004
Velebit Nature Park (2001) http://www.velebit.hr