COMBINED USE OF SURFACE AND GROUND WATER IN AN URBAN AGGLOMERATION AS A MEANS OF REDUCING DEFICIT OF WATER RESOURCES

Velikanov Alexander L.

Professor, principal research investigator

Water Problems Institute, Russian Academy of Sciences,

Moscow,717735, ul. Gubkina,3,Russia

Tel.: (7/095) 1355415, E-mail: avel@aqua.laser.ru

Yu Fuliang,

Phd

China Institute of Water Resources and Hydropower Research

PO. Box 366,Beijing 100044,China

E-mail: yufl@ihw.com.cn 

Abstract: The paper analyses the combined use of surface and ground water resources, when ground waters serve as a reserve to compensate for deficiency in water resources caused by river runoff fluctuations. It also presents the results of calculations for a case study of the water supply to the Moscow City agglomeration. 

Keywords: dificit, surface and ground water resources, runoff fluctuations, water supply, combined use, reliability, aquifers, firm yield, environmental consequences, management rules

1   INTRODUCTION

Since the moment of its construction, the system of water supply to the Moscow City agglomeration has been destined to operate using mainly surface flow. As a result, a network of canals, pipelines, large reservoirs, pumping stations has been constructed, which has changed radically the natural hydrological regime in the basin. At the same time, intensive water withdrawal by numerous water intake facilities has largely disturbed the ground water flow regime. The Moscow water supply system, which has long been developing intensively, is now on the verge of its environmental exhaustion. Further operation and development of this system necessitates searching for ways to decrease the environmental danger. One of such ways is the combined use of surface and ground waters in a common system of water supply, in which ground water is not used as a permanent source of water, but as a reserve to compensate for water deficiency in dry periods.

2   WATER RESOURCES OF THE REGION AND THEIR USE

Surface water resources of the Moscow Region, used for its water supply, include the Volga River flow from its source to the Ivankovo Reservoir, the flow of the Volga's right-hand tributary- the Vazuza River, the flow of Moskva River and its tributaries- the Istra, Ruza and Ozerna to the Rublevo Reservoir. The total catchment area is about 56.000 km². The total amount of water is 340 m³/s or 10.7 km³ on the average. The alimentation of these rivers is mainly with melt water and summerfall precipitation. The role of groundwater alimentation is not significant. The flow regime is characterised by high spring floods and summer low-flow period. From 65 to 70% of the annual flow passes during the high-flow period (March-May). In the Rublevo Reservoir site, near the hydropower station, closing the Moskvoretskaya part of the Moscow water supply system (located on the Moskva River) the maximum mean-annual water discharge of 75.3 m³/s was registered in 1933-1934, the minimum one (20.8 m³/s)- in 1921-1922, the average long-term discharge of 544 m³/s was registered in April 1948-1949, the minimum (monthly) one- 6.6 m³/s -in August-September of 1938-1939. At present, the water resource system (WRS) of the Moscow Region includes 8 reservoirs in the neighbouring river basin, as well as river reaches, canals, pumping stations and other hydraulic facilities. The main parameters of the reservoirs, regulating the river flow, are given in Table 1. The flow regulation ensured the increase in the reliability of water supply in the region. For example, for a 95% reliability (according to the number of uninterrupted years) in the Ivankovo site of the Volga River the regulated flow is 3.0 times as large as the natural one, in the Rublevo site of the Moskva River- 4.0 times, in the Zubtsov site on the Vazuza River- 4.0 times as large as the natural one. The total safe water yield of the system grew 3.4 times. As it was mentioned above, the surface flow is used as the main source of water supply to Moscow. In the regional water balance, ground waters make only 10% of the surface ones. The main developed aquifers are jointing limestones and dolomites of the Upper, Middle and Lower Carboniferous. Aquifers occur as storeys at depths from 10-30 to 150-250 m and have a thickness of 15-20 to 70-80 m each. The aquifers are interbedded by clayey strata. In river valleys, hydraulic connection between surface and ground waters is frequently observed. By the beginning of the year of 1988, operational reserves of ground waters of Carboniferous aquifers are estimated at 83.8 m³/s in the whole Moscow Region. At the same time, calculations show that the aquifers in the vicinity of the Ivankovo Reservoir allow water withdrawal of up to 8 m³/s. This value is hereafter regarded as the maximum level of surface runoff deficit that can be compensated for under the combined management of surface and ground waters.

3    INCREASED DEMANDS TO THE RELIABILITY OF WATER SUPPLY

The estimation of the available water resources in the region shows that the further development of its water supply system can be implemented either by constructing and interbasinal water transfer, or by involving new ground water masses in the process of water supply, thus increasing the part of the ground water in the existing water supply balance. One way or another, these measures can have negative environmental consequences, and that is why, they require serious scientific substantiation. At the same time, lately, resource-saving technologies have been introduced all over the world. This refers, particularly, to water-saving technologies. In solving water supply problems, this is connected with considerable reduction in specific water consumption. Undoubtedly, a similar process will also occur in the Moscow Region. However, reduction in industrial water consumption will inevitably lead to increased demands to the reliability of uninterrupted water supply, because the less water is spent per unit product, the larger economical damages, caused by water deficit. This also can be referred to social and economical consequences of interruption in the water supply, since the more economically the water is used for municipal or environmental purposes, the larger the social or ecological damage from the violation of normal water supply. The reliability of water yield can be increased by two means: (1) increasing the regulatory volume of the reservoir; (2) involving additional water sources. The latter can cause new environmental problems. Our investigation have shown, that the risk of interruption in water supply can be decreased by applying a new scientific approach to the combined use of surface and groundwater flow.

4    METHODS OF THE RESEARCH

The essence of the new approach should not be reduced to opposing surface water to ground water. On the contrary it should envisage the use of more sluggish resources of ground water during interruption in the water supply, caused by river flow fluctuations. The technique of the analysis of WRS operation, using the resources of both surface and ground waters, developed at the Water Problems Institute of the Russian Academy of Sciences, is as follows. For every WRS, under its fixed parameters, curves of the probability of water deficit are plotted as a dependence on the adopted firm water yield and its probability. At the same time, for an adopted hydrological series, chronological schedules of the interruptions in water supply are determined. It should be taken into account that the characteristics of interruptions, which appear when river runoff is used for water supply, are to a greater extent dependent on the reservoir management regulations. The higher the extent of runoff regulation, the better the possibilities of managing water resource deficiency, that is, its depth and duration. The development of WRS management regulations to allow the most efficient utilisation of water resources available in the region is among the most urgent problems in the implementation of combined use of these resources. The schedules characterise the depth and duration of interruptions. The obtained characteristics are used to calculate the regime of intensive ground water withdrawal during the interruptions for surface flow WRS. The dependence of ground water level fall on the time is plotted for various values of the firm water yield and its probability. The obtained dependences of ground water level fall during the operation of water intake facilities in variable regime are compared with the dependences, obtained under the constant regime of ground water withdrawal. From the environmental point of view, the regime of the combined work of two sources of water supply is optimal, when the ground water sources become practically renewable, i.e. when the system works with a minimum damage to the environment.

5    CALCULATIONS OF THE COMBINED USE OF SURFACE AND GROUND WATER FLOW

The above technique was verified in the calculations of the combined use of the regulated yield of the Upper Volga part of the Moscow water supply system together with ground water resources of the Dubna area. For the task under consideration, the most interesting are low-flow years and their groups, when there is a deficiency in surface waters. In order to reveal such years, water inflow to the reservoir, changing from month to month and from year to year, was compared with the firm water yield, assigned in variants. As a result of the calculations, water management indices were obtained, characterising possible water deficit - the volume of unsupplied water in each of the interrupted time intervals (month, year); concrete interrupted years in the investigated hydrological series and the number of deficient months in each of these years were also determined.

Several values of the total firm yield of the Verkhnevolzhskoe and Ivankovo reservoirs were considered in this study. The reliability in terms of the number of uninterrupted years, the duration of the uninterrupted period, and the volume of unsupplied water was determined for each water yield value. In particular, for the existing firm yield of these reservoirs of 78 m³/s, the reliability in terms of uninterrupted years amounts to 97%; for water yield of 82 m³/s, the reliability is 95%, and for 86 m³/s, it equals 91%. As can be seen from these data, a 10% increase in the firm yield (from 78 to 86 m³/s) notably reduces the reliability. The number of uninterrupted years in the available hydrological series decreases about threefold. This is absolutely inadmissible as far as the water supply to a city with a nine-million population is concerned. Moreover, the deficit in the interrupting years abruptly increases and in extremely low-water years can be as high as 20–30 m³/s, and can rise to 50 m³/s in some months in the end of the reservoir drawdown. On the other hand, as mentioned above, the maximum yield of subsurface water sources in this region that can be used to compensate for the surface water deficiency does not exceed 8 m³/s. Therefore, we have developed special management regulations of the Upper Volga reservoirs, which allow the deficit of water resources to be reduced at the expense of an increase in the duration of the deficiency period. Figure 1 gives an example of the rules developed for the Ivankovo Reservoir for the combined use of surface and ground water resources for increasing the reliability of water supply to Moscow. Similar to the dispatcher rules, the working storage of the reservoir was divided into three zones corresponding to three conventional states: wet, dry, and average. Each zone has a so-called line of desirable water reserve in the reservoir and the priority indices for meeting the requirements of different water users. The reservoir itself, depending on the zone in which its level lies, can be regarded either as a water user (reservoir filling period) or a water source (reservoir drawdown). The patterns of zones, the desired water resources, and the priority indices were determined using a simulation model so as to provide the minimum depth of interruption under low-water conditions. Calculations following the suggested rules showed that it is possible to control the river runoff to provide the specified firm water yield of 86 m³/s with meeting the condition that the deficit should not exceed 8 m³/s in the years lying within 97% probability. The occasional use of ground water allowed us to increase the firm water yield of the Upper Volga reservoirs from 78 to 86 m³/s with the same 97% reliability, thus meeting the existing requirements to the reliability of water supply to Moscow. Owing to the nonuniformity of water withdrawal, the groundwater level can be quickly restored, and ground waters, in fact, become renewable resources.

6   CONCLUSION

The developed scheme of the management of combined use of surface and ground waters allow for an appreciable increase in the WRS firm water yield and preserved the required reliability of water supply. Such management is optimum from the environmental viewpoint, since it does not require the construction of new reservoirs, whereas ground water sources act as renewable ones. It is only natural, that in addition to its ecological expediency, such water management should be evaluated specially for every specific case. The final decision should be made only basing on the results of integral technical and economic evaluation of the situation.

This study was supported by the Russian Foundation for Basic Research, proj. no. 00-05-81143.

Table 1  The main parameters of the reservoirs of WRS for water supply of Moscow

Fig. 1  Management rules for the Ivankovo Reservoir