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| Titel |
Freshwater mass balance and exchange of water masses with the open sea: the
Mljet Lakes (Croatia, Adriatic Sea) |
| VerfasserIn |
Urska Martincic, Nejc Bezak, Dusan Zagar, Tihomir Makovec, Davor Lucic, Vladimir Onofri, Vlado Malacic |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250125397
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| Publikation (Nr.) |
 EGU/EGU2016-4973.pdf |
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| Zusammenfassung |
| Two karstic seawater lakes (Veliko – Big and Malo – Small Lake) located in the National
Park Mljet on the Mljet Island in Croatia were investigated in this study. The Small and the
Big Lake cover 0.25 and 1.45 km2, respectively. The two lakes are connected to each other
and to the sea by narrow channels. The connecting channel between the Big Lake and the sea
is 12 m wide and 3 m deep. The connection to the Small Lake leads through another
artificial channel (2.7 m wide and 0.8 m deep). The average salinity of the Big and the
Small lake is 37.75 and 36.9, respectively, and the average salinity of the open
sea is 38.5. While previous studies have been conducted due to the lakes’ unique
ecosystem and the karstic characteristics of the area, the main aim of this study was to
determine the freshwater mass balance and exchange of water masses with the nearby
sea.
Several measurement campaigns were performed between 2008 and 2015 when
meteorological parameters as well as salinity, water temperature and water velocities in
both lakes and the channels were observed. A perpetual year was determined using
available meteorological data. The contribution of the surface runoff to both lakes was
modelled using the hydrological rainfall-runoff HEC-HMS model. Curve number
parameter was estimated using the CLC Corine Land cover and geomorphological maps.
Evaporation from the lake was calculated using the Verburg, Kondo and Coare
equations. We found that the annual evaporation approximately equals the annual
rainfall to the lake surface (cca. 550-600 mm). From the hydrological model and the
difference between precipitation and evaporation from the lake surface we calculated
the annual net excess of freshwater between 0.5 106 and 0.7 106 m3. The average
salinity in both lakes is lower than the salinity in the sea; therefore, we hypothesize
that the excess water should be discharged either through the channel between
the Big Lake and the open sea or through underwater karstic sink holes. In order
to determine the dynamics of discharge through the channel, we calculated the
volume of incoming and outgoing water masses using measured velocity fields
(ADCP) and sea surface elevations measured during the spring 2015 campaign. We
calculated monthly net inflow exceeding 1.5 106 m3. A comparison with previous
measurements (October 2013) revealed similar water flow pattern. Moreover, during
the spring 2015 measurements, when evaporation was relatively low, about one
third of annual precipitation occurred. At the calculated inflow and precipitation
rates salinity in the lakes would not increase significantly (<0.1 psu). However, the
calculated inflow rate suggests that the water elevation increase in the lakes would
exceed 1 m during the one month period. These results indicate strong subsurface
connections between the lakes and the surrounding sea to regulate the water level in both
lakes.
This hypothesis has to be tested using additional measurements in order to correctly
identify possible outflows from the lakes, either through the complex subsurface karstic
system or through the channel connecting the Lakes and the sea. |
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