THE EFFECT OF KARST ON THE REGULARITIES OF GROUNDWATER FLOW FORMATION

Professors B.I. KOUDELIN and V.P. KARPOVA Moscow University, USSR

Groundwater flow into streams is formed principally under the influence of three factors: climatic, topographic and hydrogeologic structure. Climate gives groundwater flow distribution features of pronounced latitudinal zonation. The groundwater flow moduli (1) in the territory of the European part of the USSR decrease uniformly from north and north-west to south and south-east from 6 to 4 l/sec/km2to fractions of a liter per second per km?.There is also a decrease in annual precipitation from 700 - 600 mm to 400 - 300 mm and increase in evapotrans- piration in the same direction. Topography,closely connected with the structure of the earth crust, gives ground- water flow features of vertical zonation which in turn are connected with distribution of precipitation which depends on the elevation. On plateaux, the largest values of groundwater flow are recorded in the uplands;within mountain-foldstructures, ground- water flow moduli generally increase with land elevation (to certain limits) along with an increase in precipitation and increase in erosion depth and the length of erosion cutting per unit area. The hydrogeological factor disturbs the smooth zonal character of groundwater flow distribution over the territory. It is most pronounced in areas of fissure-karstic water occurrence. As a rule, karst sharply increases the values of groundwater flow moduli as compared with their regional values .(?) The Urals are a typical example of karst effect on groundwater flow formation. The considerable water supplies of the territory and high long-term moduli of groundwater flow of up to 10-12l/sec/km2 in the basins of the rivers Shchugor,Vishchera, Kosva, etc. are due to calcareous karst development in the Paleozoic rocks of the western slope of the Urals. Here, rivers in their middle courses are incised in strata of karstic rocks and drain these aquifers well. At the same time, in basins situated in the vicinity, where karst is absent, the groundwater flow moduli do not exceed 3-4l/sec/km2. On the eastern slope of the Urals, karstic waters are observed occasionally in the basins of the rivers Sosva and Vagran and are characterized by long-term groundwater flow moduli of 3-3.5 l/sec/km2,while in basins devoid of karst they decrease to 2 I/sec/km2and less. The surface of the Plateau is composed of Lower Permian strongly karsted calcareous and gypsiferous rocks, which easily absorb precipitation.Hence the increase of groundwater flow modulis up to 4 l/sec/km2in the basins of the Ufa River and its tributaries Aya, , etc., though in the territory adjacent to the plateau the groundwater flow modulus is equal to 2-3 l/sec/km2. Strong effects of karst on groundwater flow formation are observed in the Crimea. Jn the mountainous part of the Crimea under common climatic conditions, ground- water flow forms only in the area of Jurassic karsted calcareous sediments rich in water, where flow moduli are 5-15 I/sec/km2 and more against the background of Triassic, Jurassic, Cretaceous impervious rocks practically devoid of water. (1) The groundwater flow modulus is defined as a volume of groundwater discharge in a unit period of time from a unit of underground drainage area. (2) The data on the characteristics of groundwater flow presented below are bor- rowed from a set of groundwater flow maps of the USSR compiled by a group of hydrogeologists and hydrologists under the scientific leadership of Professor B.1.Kou- delin, the hydrological work was performed under O.V. Popov.

208 The effect of karst on groundwater flow is also observed in other regions. For example,there are regions with anomalous values of flow moduli in karstic areas devel- oped among the mari-chalk rocks in the middle course of the River, namely in the basins of the rivers Serezha, Tesha and Pyana. In the north, a small area of intensively karsted limestones and dolomites of Car- boniferous age, belonging to the Onega-North Dvina interfluve, occurs. Here, in the basins of the rivers Emtsa and Vaimuga, the average annual value of groundwater flow reaches 3.0-5.0l/sec/km2, the background modulus value being 2.0-2.5l/sec/kmz. In the karsted basin of the Kuloi River, composed of Carboniferous limestones and dol- omites and Lower Permian gypsiferous sediments, the groundwater flow moduli are equal to 5.0-6.0l/sec/km2. In theTiman Ridge,an increase in flow moduli from the edge to the centre amounting to 4 l/sec/km2is observed.This may be due to development of karstic and fissure-karstic waters in Middle and Upper Paleozoic carbonaceous rocks. Similar examples of increase in groundwater flow modulis in karstic areas also can be found in the Black Sea zone of the southern slope of the Caucasus (Sochi- Sukhumi), where flow moduli reach 35-40 l/sec/km2. In karstic areas, a pronounced -increase in both average annual and minimum groundwater flow moduli is observed. Thus, during the period of steady low water, the long-termflow modulus for the Vishera River is 4.5 l/sec/km2,and for the karsted basins of the rivers on the eastern slope of the Urals it is equal to 1.5 I/sec/km2.On the Ufa and Onega-North Dvina plateaux the flow moduli are 3-3.5 l/sec/km2,and on the Kuloi Plateau up to 4.0l/sec/km2 and more, while for non-karsted river basins of these regions minimum groundwater flow moduli range from 1 to 2 l/sec/km2.Sharp increases in minimum groundwater flow moduli as compared with their zonal values are observed in karstic areas of the southern slopes of the Caucasus,in the Crimea and in the middle part of the Volga basin, etc. The controlling effect of karst on streamflow is thus mani- fested, which was established by many hydrologists and hydrogeologists working in karstic regions.

The controlling rôle of karst is manifested not only in annual redistribution of streamflow,i.e. a decrease in spring floods and increase in stream low-waterdischarges, but also in the ratio of groundwater Row to surface runoff. The portion of groundwater Row in the total annual streamflow, or the so-called coefticient of base flow, invariably increases in karstic areas. Over most of the European territory of the USSR,the groundwater flow constitutes 20-30% of the total streamflow;in the south however it is less and in the north-east of the country in the perma-frost area it is less than 10%. In the areas where karst is developed, in the Urals, the Ufa and Onega-North Dvina plateaus and the Central Devonian field, the coefficient of base flow increases to 40-50%; on the Kuloi Plateau, in the area of the Caucasus and in the Crimea it is more than 50%. The coefticient of groundwaterflow,i.e. the ratio of groundwater flow to precipitation, also sharply increases in karstic regions. In karsted areas of the Kuloi Plateau, the western slope of the Urals and the Caucasus, it is equal to 30-40% and more (from long- term data), while in non-karstic areas it amounts to 10-20%. In the Crimea Yaiila, the coefficient of groundwater flow reaches 70%.

In the Silurian Plateau on the coast of the Gulf of Finland,over an area of 3000 km2, where recent intensive calcareous karst is developed (revealing itself on the surface as single or multiple sink holes), surface runoff is absent altogether. All the precipitation, except that lost by evaporation and transpiration, is absorbed by karstic rocks and recharge the karstic waters to form groundwater flow. According to V. K.Kolotilshchi- kov’s water balance investigations during the period of 1948-1954,evapotranspiration

209 losses constituted 41 to.61% of the total precipitation,the remaining 39 to 59% becoming inflow and percolation into the limestone strata. For these seven years, an average of 38.7% of the total precipitation amount went to spring flow, and 2.5-23.2%went under- ground to change groundwater resources and form deeper groundwater flow. The phenomenon of intensive absorption of precipitation and surface runoff by karstic rocks, exposed on the land surface, leading to elimination or sharp diminishing of runoff is well known and observed in individuai areas of the Crimea Yaiila, the Ufa Plateau and other regions of the USSR. G.A.Maksimovich and D.S.Sokolov give examples of this phenomenon in many karstic regions of Italy, Morocco, Australia, France,the Balkan Peninsula and other countries. The “modulus of absorption” of the surface runoff in the Southern Urals in the basins of the rivers Kamenka, Blinovka, Termenevkd, Pokrovka, etc., according to P.V.Molitvin‘s observations, amounted to 4-5 l/sec/km2in 1953 and 6-7l/sec/kmz in 1954;in the Uluir River basin it reached 4-6l/sec/km2. For the karsted basins of the rivers of the Onega-North Dvina divide and for the Pyarga River basin in particular the “modulus of absorption” was 6.5 l/sec/km2in 1949, for the Sukhaya Sheksna River basin about 7.5 I/sec/km2,according to the same data.

The rivers mentioned above have a steady flow only at their mouths as a result of karstic water discharge. Along all the river course steady flow is observed only in the periods of spring floods, a significant amount of streamflow being lost in karstic rocks at the bottoms of river channels and valleys. However, losses of surface runoff and precipitation are largely of local nature and lead to redistribution of groundwater flow in relatively small areas. Highly indicative of this are the results of P.V.Molitvin’s investigations in the karstic regions of the Northern and Southern Urals and the Onega-North Dvina interfluve. The estimation of the water balance of the rivers Vagran and Sosva carried out by P.V.Molitvin in 1947 showed that practically all the waters absorbed in the upper parts of the basins of these rivers returned to rivers “at the moment of the rivers emergence from the limestone band into the zone of non-karstic rocks, not influencing the water volume at the final gauge station”. This phenomenon was also established by the same investigator in the Southern Urals. The waters lost by surface runoff and channel flow in the Kamenka River return underground to the Aii River, which is the main drainage artery of the region .Withinthe area of the Aii River basin, 5384 kin2,the average annual streamflow modulus becomes stable. The channel flow of the Emtsa River, the main stream of the Onega-North Dvina interfluve,has also undergone great changes along its course. These changes are due to the development of varied forms of calcareous karst inits basin. However,with the basin area of 1467 km2the streamflow modulus improves and approaches the “climatic standard”. The flow which is lost “in the basins of the rivers Pyarga and Sykhaya Sheleksna returns underground to the Emtsa River forming large spring openings in its valley”. The phenomenon mentioned above is of great importance to-the study’of ground- water flow in karstic regions. In regional estimation of the groundwater flow values and natural groundwater resources in karsted territories using the method of river hydro- graph separation,it is necessary to take sufficiently large basins, which according to the classification of P.V. Molitvin, G.A.Maksimovich and L.A.Vladimirov are the so- called “neutral river basins”, within which the exchange between surface and ground- waters is of “closed” nature, i.e. absorption of the surface runoff and streamflow in some areas of a basin is compensated by strong groundwater efflux in other areas. Other types of rivers, namely,rivers “with predominance of flow losses in karstic rocks” or “with partial flow loss into karst” cannot be used for estimatim and mapping of groundwater flow and naturalgroundwater resources, as in this case:underestirnated values of groundwater ,flowmoduli will be obtained. Conversely, karstic rivers “with

21Q predominant groundwater drainage” will give over-estimated values of groundwater flow moduli due to groundwater movement from adjacent basins and due to lack of coincidence of areas of surface and subsurface catchments.These rivers may be used for the purposes mentioned above, ifthere is an opportunity to determine the areas of subsurface catchments sufficiently precisely. These circumstances were taken into con- sideration while compiling maps of groundwater flow of the USSR. The characteristics of groundwater flow for karstic regions were obtained by the method of separation of total streamflow hydrographs for sufficiently large basins of from 1000-3000to 10,000kmz, and rarely 30,000km2, which can be classed as “neutral river basins”. However, the position of gauges on rivers has never exactly coincided with the boundaries of karst development. In this sense, the data presented above are somewhat conditional, though they reflect the principal features of groundwater flow formation in karstic areas. If river gauges were located specifically, these features would be expressed more clearly.

On the basis of the available material, obtained at the present time as a result of many investigations,the following main conclusions can be made concerning the effect of karst on the regularities of groundwater flow formation: 1. Karst leads to an intensification of groundwater flow. The averagelong-term,annual maximum and minimum flow moduli (or inches of runoff), and the coefficients o groundwater flow and base flow in karstic regions have markedly greater values than the regional ones. 2. Karst interrupts the smooth zonal character of the distribution of groundwater, flow values over the territory, which ordinarily depend on the climatic latitudinaif zonation or vertical zoning in mountain-fold areas. 3. Within karstic regions, wide fluctuations of groundwater flow and surface runoff are observed: areas abundant in water are replaced by arid waterless areas. This is due to the nature of karst and hydrography of karstic regions. 4. Karst leads to redistribution of groundwater flow into rivers within relatively small areas. When studying the groundwater resources of karstic regions by the genetic sepa- ration of hydrographs of total streamflow, it is necessary to take sufficiently large “neutral basins”, within which a full cycle of interrelation (exchange) of surface and groundwaters takes place. 5. The large values of the moduli and coefficients of groundwater flow and base flow in karsted regions are due not only to highly favourable conditions for absorption of precipitation and surface runoff by karstic rocks but also due to the peculiar features of the karstic water regime noted for its vigorous, turbulent and rapid groundwater movement, relatively direct paths of water flow and well-developed recharge and dis- charge areas. The average coefficients of groundwater flow for karstic waters are gener- ally considerably higher than those for other types of groundwaters, excluding fissure waters in mountain-foldstructures. 6. The type of groundwater flow in karsted massifs in many cases (depending on the type of karst) roughly resembles a streamflow regime and has the same phases with the latter, but sometimes may lag behind the streamflow maxima and minima. 7. Karsted rock massifs are noted for relatively limited water storage as compared with loose sediments. The average volume of interstices of karst massifs for large rock blocks is typically a few per cent, whereas the porosity of loose sediments is equal to tens ofper cent.The values of groundwater flow moduli are conversely related.Accord- ingly the time of renewal of karstic waters is much briefer than that of waters in loose sediments. The peculiarities of groundwater flow formation in karstic areas described above are undoubtedly of a generalized nature. The regularities of the formation of groundwater flow into streams are greatly influenced, apart froin karst, by other physiogeographic factors-the presence of forests, lakes,marshes, the ruggedness of the territory, perma- frost in the north-eastof the USSR,the human factor, etc., which superimpose on the principal factors which have been discussed above.The factors do not act in isolation, but are interrelated. Only broad field experiments, which could be most easily achieved in the programme of the International Hydrological Decade,will help elucidate the true rôle of karst in development of groundwater flow processes. It should be pointed out that the typical features of the groundwater flow regime in karstic areas,as described above,are of more general nature.They quite well characterize groundwater flow in areas of fissure development in mountain-foldstructures,in coarse detrital rocks composing alluvial fans and in some other rocks, for example, young quaternary lavas in the Armenian Highlands.

212