Ecohydrology of karst

poljes and their

vulnerability

Prof. emeritus O. Bonacci Faculty of Civil Engineering, Architecture and Geodesy, Split

University, 21000 Split, Matice hrvatske 15, Croatia

E-mail: obonacci@gradst.hr

Swallow-hole opened at

the bottom of the

Boljunčica reservoir

through sediment

deposit

Karst is estimated to cover about 25 % of the surface of all the

continents. It represents a type of landscape with many specific

surface and underground features, which facilitate and accelerate

the exchange of surface water and groundwater. By this way karst

strongly influenced on development of environment and its

ecosystem. The total area covered by karst poljes represents

approximately 2 % of the total karst area. Although they are

relatively small in size, they are extremely significant from an

ecological, social and economic standpoint.

The importance of karst poljes is that they are the larger, fertile, and

inhabited oases in karst, commonly providing the only conditions

favourable for human beings as well as for development of reach

but very vulnerable and mostly endemic karst flora and fauna. Karst

poljes’ ecological role is extremely important but till now not

enough recognized and investigated. Due to strong and

uncontrolled anthropogenic pressure during last hundred years

karst poljes belong between the most endangered environments on

the Earth.

KARST POLJE

Karst polje as the geomorphological term refers to large

closed depressions with conspicuously flat bottoms

developed on karst rocks.

Their areas vary from less than 0.5 km2 to more than 500

km2, and exhibit complex hydrological and

hydrogeological features and characteristics.

Geomorphological and hydrogeological features of

poljes, mainly flat surface covered by impermeable soil,

make possible formation of permanent as well as

temporary streams and lakes, which enable development

of reach biological forms in and around them. Poljes can

be divided into the following groups according to the

hydrological regime: (1) permanently flooded or lakes;

(2) periodically, partly, or completely flooded; and (3) dry

poljes.

As a consequence of intensive

tectonic activity the poljes in Dinaric

karst have been formed as terraces

from an altitude of more than 1000 m

above sea level to the sea level. They

represent more or less

interconnected subsystems within

the process of surface and

groundwater flow through the karst

spring catchment. From the

hydrologic–hydrogeologic

perspective, a polje is to be

considered as part of a wider

system. It cannot be treated as an

independent system.

System of 7 karst

poljes in the

Ljubljanica River

springs

catchment

Developing the research interface between hydrology

and ecology has been recognised as a research

frontier in geosciences. Despite a history of research

that integrates insight from the two scientific

disciplines, they still operate somewhat independently

with different philosophies, conceptual frameworks,

terminology and experimental approaches.

Harte (2002) seeks a synthesis of what he calls the

Newtonian and Darwinian approaches to science. He

believes that such a synthesis offers opportunities for

progress at the intersection of physics and ecology

where many critical issues in earth system science

reside.

ECOHYDROLOGY !?

The pattern and intensity of hydrological variability

significantly influences on biotic structure and activity.

On the other hand, biotic structures can regulate

abiotic ones. As a result of these interrelationships a

new concept called ecohydrology has emerged. Its

rapid development is a consequence of fact that

complex scientific questions as well as environmental

problems can be effectively solved only if several

scientific disciplines are considered jointly.

Ecohydrology was the coupling of landscape

processes with hydrobiology. Due to particularities of

water circulation in karst areas the coupling of surface

water - groundwater processes is the most important

prerequisite for understanding constraints on

sustainable development.

Karst ecosystem analysis should be focused on

the flow of energy and the cycling of nutrients

through biotic and abiotic components of the

system. The absence of light in the karst

underground, particularly in caves makes

photosynthesis impossible. Plants, which are the

major food source on the earth’s surface, are

absent. Karst underground animals generally

depend for food on what enters haphazardly

from outside. In caves, there are two sources of

organic matter: vertically percolating water from

the epikarst and from sinking streams.

A karst ecohydrological approach means integration

karst studies into a more general ecological,

biological, hydrological, hydrogeological,

geomorphological, and geochemical context.

The karst environment has very different characteristics than all other

environments. Subterranean karst ecosystems are sensitive to

environmental changes that occur on the surface. The importance of

maintaining biological diversity goes far beyond mere protection of

endangered species and beautiful landscape.

Examples of few karst poljes

Cres island AI = 404.3 km2

Lake catchment area AL = 25 km2

Lake water surface AW = 5 km2

Lake water volume VW = 200×106 m3

Permanently flooded or lake karst polje

Vrana Lake on Island of Cres

P = 0.2831 × t + 508.5

R2 = 0.001

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t (year)

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Hav = -0.0428 × t + 96.875

R2 = 0.4726

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Hav

(m a.s.l.)

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annual precipitation 1926-2012 mean annual water level 1929-2012

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1987-2012

Hav1 = 13.41 m a. s. l.

Hav2 = 10.91 m a. s. l.

H

(m a.s.l.)

t (year)

ΔHav = 2.350 m

ΔH(1982-1990) = 4.37 m

ΔHmax = 6.46 m

Hmax = 15.84 m a. s. l.

Hmin = 9.38 m a.s.l.

The Trebišnjica River traverses

through the Ombla Spring

catchment. The mean annual

discharge, after the canalization of

over 60 km of its watercourse with

spray concrete (from 1981), is 24.1

m3/s. In natural state the mean

annual discharge was 28.4 m3/s.

Ecological impact

on broader region

!!????

The Lika and Gacka Rivers (Dinaric karst of Croatia)

are typical sinking streamflows : At the same time the

Lika is losing river.

Vrulja

FLOOD IN KARST POLJE

Floods are one of the most dramatic interactions

between human beings and the environment.

People look at floods as a catastrophe but in

reality floods are integral part of nature, playing a

critical role in ecosystem function. At the same

time flooding brings many benefits particularly for

ecological variability and soil fertility. Flooding

promotes exchange of materials and organisms

between habitats and plays a key role in

determining the level of biological productivity and

diversity. Those processes are especially

important for karst environment.

Poljes are regularly flooded in the cold and wet

periods of the year. Because of this they can be

treated as wetlands. Flooding of the poljes in the

Dinaric karst in natural conditions lasts on average

from 3 to 7 months per year, mostly between

October and April, but there are cases when

flooding can even persist for 10 months. With the

objective of flood prevention in poljes, attempts

have been made to increase the capacity of ponors.

Such attempts have usually failed because the

capacity of ponors depends on the conduit system

to which they drain, and on the groundwater level.

The most effective measure to prevent the flooding

or to reduce duration of floods is the construction

of a tunnel. Ecological consequences ???

Tunnels for karst polje flood control:

An example of the Konavosko Polje

Catastrophic flood

on the Upper Dobra River 29 June 1999

Qmax = 254 m3/s

Maximum swallow capacity of the Đula Sink ≈ 80 m3/s.

80 m3/s << 254 m3/s

FLOOD IN THE VRGORSKO POLJE

The tunnel operation significantly

reduced the durations of flooding, but

did not eliminate them entirely.

Cyprinid fish

(Phoxinellus ghetaldii)

Hawes (1939) gives an example of possible underground

colonisation of the karst underground in the Popovo Polje.

Cyprinid fishes spend most of their time underground. Floods wash

them out in great quantities and regularly every year at the

beginning of wet and cold period. Breeding occurs at this time,

and the young fish are left to spend a year in the open, through

their parents are carried back into the karst underground. After the

next flood, the young fish in turn are swept into the underground.

The eyes of Phoxinellus ghetaldii are normal but the fish exhibits a

tendency to reduce scales, which is remarkably common among

cave fishes. Maybe in this case we are witnesses of the early

stages of colonisation of caves by an epigean fish.

Because of extreme groundwater oscillations in karst, different hydrological connections are active depending on the groundwater level. The methodology of water tracing was developed especially to determine water interconnections in karst. In addition to tracing with salt, dye or temperature, the use of rich endemic fauna, their dispersion and population structure is lately considered. In this paper, population structure of the endemic fish species is used to confirm hydrological data as well as suggest new underground water connections in part of central Dinaric karst.

Role of dry stonewalls

The influence of drystone walls on ecological and geomorphological

processes is important. Terracing is very effective measure against

erosion. Stonewalls are of fundamental importance as a habitat for a

very diverse Mediterranean flora and fauna.

Changes of the biotic and

abiotic factors above the

ground lead to changes

down through the massive

stone clearing to deep

karstic aquatic and

terrestrial habitats. Massive

stone clearing means

entirely disappearance of

stonewalls. This process

could be very dangerous

from ecological and

hydrogeological point of

view.

Canora, F., Fidelibus, M.D., Sciortino, A., Spilotro, G.:

Variation of infiltration rate through karstic surface due to

land use changes: A case study in Murgia (SE-Italy).

Engineering Geology 99 (3-4) (2008) 210-227

Massive stone clearing

The above scenario represents a worrying example of man-made evolution of the karst surface’s textural features. Current studies indicate that the agricultural activities in the Murgia have important consequences on groundwater quality. Direct observation of the stone cleared surfaces evidences a net loss of the fine soil component, so farmers are obliged to add new soil. A great part of the lost soil finally reaches the sea during the frequent floods. The soil loss is the closest precursor of the desertification of the concerned areas.

Island of Hvar

(Vira Bay)

28 August 2009

Thank you for

your kind

attention!

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