Karst Hydrology

Karst represents a specific area consisting of surface relief and a surface-underground hydrographic network resulting from the water circulation and its aggressive chemical and physical action in joints, fractures and cracks along the layers of soluble rocks, such as limestone, chalk and dolomite as well as gypsum and salt. Karst is characterized by soluble rocks located near or at the surface. The karstification process results from the physical and chemical water action on the solution and transportation of elements from the rocks. The power of water solubility in contact with rocks depends on the water temperature and its chemical composition, with the dominant component being CO₂. The coarseness of grain size is an important factor in limestone dissolution. It effects the chemical quality of karst waters (Sweeting 1973). The fineness of grain affects the solubility of the rock. Sweeting (1973) has concluded, after laboratory experiments on dolomites from Ontario, that the finer grained dolomites are about twice as soluble as the coarser grained beds. Owing to specific geologic and geomorphologic, and particularly hydraulic characteristics, karst areas exhibit a specific water circulation which should be studied by appropriate methods. Karst hydrology is a relatively new scientific discipline if it is considered as an independent scientific branch. It has been so far included in the investigations on the process of water circulation in karst as a part of geology and hydrogeology, but it is developing as an independent discipline.

The primary objective of this book is to study the hydrologic aspect of the problems of water circulation in the karst. Therefore, karst phenomena, and types and characteristics of karst will not be dealt with from the geologic and geomorphologic point of view. The following books or papers refer to the above problems: Zötl (1974), Milanovic (1981), Bögli (1980), LeGrand and Stringfield (1973), Sweeting (1973) and LeGrand (1983). Figure 2.1 is taken from the latter two papers and represents six general topographic categories which can be developed in the karst regions. The authors believe that these typical karst forms include all phenomena and are valid for the whole world. This division, as any other, has its advantages and disadvantages, but they are not important from the point of view of hydrology. The well-developed fissures of the karst areas, and consequently a very fast and great water sinking strongly affect the distinctive hydrologie behaviour of water in the karst terrains to be distinguished from the circulation in porous media. The possibility of surface flow is either unlikely or completely eliminated. In these considerations we should take into account the fact that apart from the bare karst (appearing in southern Yugoslavia, Dalmatia and Herzegovina) there are areas of covered karst (Slovenia and Gorski Kotar) with rich vegetation and a layer of covering soil up to 1m deep, in some places even several meters. In such situations the sinking is significantly slower, whereas all other processes of water circulation are identical to those in the bare karst.

The circulation of water through karstified rocks is similar to water circulation through non-karst terrains under normal hydrologic conditions apart from some specific features. The lateral circulation in karst is effected by the following three concentrating mechanisms (Gunn 1983): (1) overland flow; (2) throughflow, i.e. flow through a layer of soil above limestone; (3) subcutaneous flow. Each of the above flows differs from one situation to another, depending upon the soil cover and vegetation of the catchment, and on the development of the karst processes. The lateral water flow precedes the infiltration and percolation phases, i.e. the vertical sinking of water through the vadose zone down to the groundwater level.

Karst springs represent a natural exit for the groundwater to the surface of the lithosphere through the hydrologically active fissures of the karst mass. The springs in karst appear most frequently in the places of contact between the carbonate masses and the impermeable layers (e.g. flysch). The water flows to the surface through the permeable rocks, which are practically insoluble, and sometimes non-karstified (Bögli 1980). Lehman (1932) mentions the karst-hydrologic contrast expressed by the presence of numerous places through which the water sinks into the karstified mass, whereas there are relatively few karst springs. In the early phase of karstification fewer parts of the aquifer are oriented towards one spring, i. e. in this phase there are a great number of springs with a small catchment area. As the hydrologic activity increases the respective catchment area of a spring becomes larger and deeper. Consequently, certain springs stop functioning, and the remaining active springs become larger and have a greater capacity (Bögli 1980). It is already evident in this phase that the catchment area of karst springs changes in time depending upon the water quantity in the aquifer. This variability can be greater or smaller depending upon the local and geologic conditions. This statement can be confirmed by the fact that the water which is underground in one place often emerges in numerous springs distantly located one from another, and its appearance depends on the water quantity in the catchment expressed by the groundwater level.

Ponors are fissures in the karst mass through which the water sinks underground. They play an important role, from the hydrologic-hydrogeologic standpoint, in the water circulation in karst. According to their hydrologic function, ponors can swallow water permanently or can function partly as ponors and partly, i.e. temporarily, as an estavelle. From the morphologic standpoint (Milanović 1981) ponors can include: (1) large pits and caves; (2) large fissures and caverns; (3) systerns of narrow fissures; (4) alluvial ponors. All underground karst phenomena (jamas, channels and caves) can take over the function of ponors. Jamas most frequently function as ponors and present paths for the direct contact of the surface water with the underground water in the karst mass. Figure 5.1 shows a cross-section of the Tučić Ponor located in the Gračačko Polje (Yugoslavia). It is evidently a vertical jama with a speleologically investigated part 145 m deep, although there are much deeper sections. Figure 5.2 shows a photograph of a ponor in the Trebišnjica Catchment. It is located 30 m above the riverbed of the Trebišnjica and illustrates the size of the flood in the Popovo Polje. The ponor is lined with rock and once it was used as a mill for grinding corn. As a large storage basin was constructed upstream, the polje is not flooded to a high level and, consequently, the ponor and the mill are no longer used. Figure 5.3 shows the entrance to a Jelar Ponor in the Gračačko Polje. The entrance is closed by a grate to prevent its being blocked by logs, plastic material, etc., and to make clearing it possible.

The hydrological regime of open streamflows in karst depends mostly upon the interaction between the groundwater and surface water. This problem has been dealt with in the previous chapters, particularly with reference to the catchment areas in karst. It has been noted that the groundwater levels in karst vary widely depending upon the effective porosity. Evidently, these variations in the groundwater levels greatly influence the hydrologic regime of the open streamflows. The regulatory influence of karst on the surface circulation depends directly upon the size of the fissures, i.e. the effective porosity, but also on the elevation of the groundwater levels and their change in time. If the catchment area is small and the precipitation intensive, the influence of karst on the surface circulation will not be significant. This fact can be explained since, in that case, there is no adequate volume in the karst mass for the sinking of storm runoff and surface water. The influence of karst differs from one streamflow to another and, therefore, general conclusions should be carefully drawn. Although some open streamflows flow through non-karst terrain, their springs are located in the well-developed karst hinterland. On the other hand, there are streamflows which are formed in a non-karst area, but flow through well-developed karst. The interaction between groundwater and surface water is different, in the two previously mentioned cases as well as in other numerous combinations. The regulatory influence of karst, i.e. the decrease in the high waters and the increase in the low waters, is felt only in the former case.

The poljes in karst have already been dealt with in Chapter 2 as a morphologic and surface phenomenon. As previously stressed, from the hydrologic standpoint, poljes represent a subsystem linked to the surrounding karst mass, other poljes or river valleys on higher horizons and to those poljes or river valleys on lower horizons. The preceding chapters discussed a few examples of the influence of the surface waters and groundwaters of higher and lower horizons under hydrologie conditions in the analyzed polje. When defining the water budget in the poljes difficulties arise in the flooding periods. In such situations it is very difficult, even impossible, to control the water inflow. Furthermore, the water then flows into the polje through a series of temporary springs and the estavelle function as springs. Figure 7.1 shows a probability curve for the dates of the occurrence of the maximum flood level in the Konavosko Polje near Dubrovnik (Yugoslavia). In addition to this curve, it is necessary to define the probability curves for the beginning and ending dates of the flood, as well as for the flood duration in the poljes. Flood analysis should be performed according to a schematic representation of the inflow and outflow hydrographs, i.e. according to the analysis of floods in the pre-ponor retention. Figure 7.2 A gives a graphic presentation of the inflow and outflow discharge hydrographs. It is the simplest case discussed by Ristić (1976). Figure 7.2B presents the water level hydrograph in polje H = f(t) and the curve of the water volume in the polje retention V = f(t), whereas Figure 7.2 C gives the mass curves of the inflow and outflow volumes.

Measurements of water temperature in karst make it possible to answer numerous questions related to water circulation, its origin, retention time underground, etc. In order to determine thermal changes such measurements should be carried out with sensitive equipment, since these variations have very small amplitudes, i.e. very often, a few tenths of 1 °C. According to Komatina (1984), favourable conditions exist in karst for the application of geothermal investigations while detecting and monitoring the spatial position of the groundwater flows; therefore, both the karst mass and the groundwater flow cause temperature anomalies measurable even at the surface of the terrain. Komatina (1984) reported that the anomalies depend upon the following factors: (1) temperature of the groundwater; (2) depth, settlement and size of the groundwater flow; (3) degree of karstification of the medium between the flow and the terrain surface.

Living conditions are not favourable for man in karst regions. The main reason for this is the unfavourable water regime. In autumn and winter excessive precipitation often results in the flooding of fertile poljes and river valleys, which frequently last a long time. The water is retained in these fertile areas, more than 3 months on the average; the maximum retention lasting 6 months. Immediately after the floods end, long dry periods begin. Both phenomena adversely affect the agricultural production of the area. Since the soil is fertile and temperatures exceptionally favourable (particularly in the Mediterranean regions), the agriculture could be significantly improved by the regulation of the water regime. The previously mentioned facts have since ancient times forced man, living in these areas, to build more or less significant hydrotechnical structures. All of these structures were built with the same objective, i.e. to improve the water regime, and hence, the living conditions in the region. Today, intensive works are being carried out related to the regulation of the water regime in karst. Past experience has shown that many of these works have been suboptimal. The benefit resulting from these works in one area was frequently smaller than the damage caused in another area. This frequently occurred when the system was not thoroughly studied from hydrologic and hydrogeologic standpoints. Damage occurred most frequently in the lower karst horizons, but negative effects were also seen in the higher terrains. This damage was primarily caused by the water regime and floods.