Groundwater Management: Quantity and Quality (Proceedings of the Benidorm Symposium, October 1989). IAHS Publ. no. 188,1989.

The role of groundwater in Israel's integrated water system

D. HAMBERG TAHAL - Water Planning for Israel, P.O.B. 11170, Tel-Aviv 61111, Israel

Abstract Israel's national water system integrates water pumped from Lake Kinneret with two groundwater sources - the Karstic Aquifer and the Coastal Aquifer - supplying drinking water and irrigation water. In the rainy winter season irrigation demand drops and the surplus system capacity is used for aquifer recharge. Various multi-annual simulation techniques were used to plan an operating policy that would guarantee high reliability. The operating policy derived was based on decreasing the quantities supplied below the average potential of the sources, thus creating reserve storage in the aquifers. The Coastal Aquifer is today overexploited, which has led to seawater intrusion, salination of the fresh water wells and development of large craters in the water table. A rehabilitation programme was planned by a schematic model that represents the aquifer as one "black box". The model was checked and detailed by a finite differences model of the aquifer. These tools are part of a hierarchic set of models and techniques used to build a new master plan for the water system of Israel.

Le rôle des eaux souterraines, en Israël, dans le système hydraulique intégré

Résumé En Israël, le système hydraulique national intègre les eaux extraites du Lac Kinneret à celles de deux sources d'eaux souterraines - l'aquifère karstique et l'aquifère côtier - qui approvisionnent en eau potable et en eau d'irrigation. Pendant la pluvieuse saison d'hiver, la demande d'eau pour l'irrigation s'arrête et le volume excédentaire du système est utilisé pour la recharge artificielle de l'aquifère. Diverses techniques de simulation pluriannuelle ont été utilisées pour établir des modalités d'exploitation qui peuvent garantir une haute fiabilité de l'approvisionnement en eau. Cette fiabilité peut être atteinte par l'intégration de toutes les ressources et la réduction du pompage en dessous du rendement maximal des aquifères, pour y accumuler des réserves. Aujourd'hui, l'aquifère côtier est surexploité, ce qui a provoqué la formation de "cratères", l'intrusion d'eau de mer et l'abandon de puits, devenus salins. Un plan a donc été élaboré afin de sauver l'aquifère. Le régime

501

D. Hamberg 502

d'exploitation a été établi à l'aide d'un modèle schématique, représentant l'aquifère comme une "boîte noire". Les résultats ont été détaillés au moyen d'un modèle de différences finies. Ces outils font partie d'une structure hiérarchique de modèles appliqués afin de mettre en place un nouveau plan de tout le système hydraulique d'Israël.

INTRODUCTION

Israel's highly integrated water system is based primarily on groundwater, with a major contribution also coming from a large surface source, namely Lake Kinneret (also known as Lake Tiberias, or Sea of Galilee). The water of Lake Kinneret is transferred from north to south along the length of the country through the National Carrier which supplies about a quarter of the country's municipal and irrigation water consumption. The system is schematically depicted in Fig. 1. The main aquifers are:

- the Karstic Aquifer, considered one vast reservoir - the Coastal Aquifer, considered as parallel east-west strips.

Practically all the fresh water sources in Israel are already fully exploited today. In order to meet the growing municipal demand, it will be necessary to reduce the quantity supplied for irrigation.

The system supplies irrigation water mainly in summer, winter being a rainy season. However, the pumping station at Lake Kinneret and the main conduit are operated year round. Regulation is provided by adjusting the groundwater pumpage and by aquifer recharge of the surplus water in winter. In the future the role of the aquifers as seasonal regulators will be underlined, and most of the winter pumpage from Lake Kinneret will be recharged into the aquifers, to be repumped in the summer.

The increasing weight of municipal demand (which it is the policy to meet fully without cuts) and the sophisticated nature of Israeli agriculture and its irrigation needs highlight the need for high reliability of supply. Although the large volume of the aquifers serves to assure supply even in a drought lasting several years, nevertheless the regulating volume is limited and its operation requires means for higher recharge of surplus water in rainy years, and means for higher groundwater pumpage in a drought. The reasonable degree of reliability to be provided, and the way to provide it, were analysed as part of a new master plan for Israel's water system, as described in the next sections.

The Coastal Aquifer was in the past intentionally overexploited, thus mining a large volume of water. This has led to the development of large "craters" in groundwater levels, accompanied by seawater intrusion and the salination of many wells. The time has thus come to decrease pumpage. from this aquifer.

503 Role of groundwater in Israel

Fig. 1 The Israel national water system.

Agriculture in the south depends on importing water from the north. The Coastal Aquifer flows in an east-west direction, but the Karstic Aquifer can serve as an underground north-south conduit, with limitations defined by losses to the sea occurring in the northern part of the aquifer, and salination hazards in the south.

Other aspects of the combination of groundwater and the National Carrier are related to water quality. The water of Lake Kinneret is considered inadequate for drinking purposes, according to new standards recently introduced, unless it passes through the groundwater by recharge, or unless it is treated. On the other hand the nitrate content of the Coastal Aquifer exceeds permissible values in an increasing number of wells.

D. Hamberg 504

TOOLS OF ANALYSIS

The analysis was performed using a combination of tools, most of which have for some time been in wide use in Israel as well as elsewhere. The interest lies in the iterative way these tools were used and the way the results were integrated into a plan for the whole system.

Some of the tools used and the analyses performed are: - simulation of the operation of the sources over time (series of 1000

years); - groundwater hydrological analyses that provide data for the various

models and test the consequences of the suggested operation plans; - use of schematic hydrological models of groundwater behaviour, in order

to choose overall policies; - simulation of the operation over time (in seasonal terms) and over space

(the National Carrier and the two aquifers, each divided into three parts); - manipulation of a large data base of all wells and consumer connections; - water balances of numerous regions, aimed at equitably sharing out the

limited amount of water available and calculating the water transfers needed, physical facilities required, etc.;

- calculation of water costs, considering various alternatives of cost alloca­tion, as a means of comparing variants as well as a basis for a pricing policy that would serve as an indirect tool for the management of the system;

- consideration of the actual installations and preliminary design of chan­ges needed.

The scope of this presentation permits us to concentrate on only two of the items above, namely: a) the multi-annual simulation of the sources (Lake Kinneret and the two main aquifers), aimed at gaming high reliability with only a minimal reduction of the irrigation supply level; b) determination of a rehabilitation policy of the Coastal Aquifer, as analysed by a schematic model.

The two types of analysis complement each other in that the first deals with an overall policy for the system considering the stochastic nature of the inflows, while the second deals with a deterministic policy for one segment of the system, interrelated with the first analysis and "riding" on it, opening a window on one part of it and dealing with it in more detail.

OPERATION POLICY ALTERNATIVES

The analysis of the system assumes a given series of withdrawals from Lake Kinneret (the output of an independent simulation model of the operation of the lake) and natural inflow series to each of the two aquifers (based on rain series).

505 Role of groundwater in Israel

The interconnections that exist between the National Carrier and the consumers supplied by both aquifers are not enough to create full integration. At one extreme, a simple operation policy would be to regard each aquifer and its consumers as an independent unit. This would not take advantage of the many interconnections that do exist. At the other extreme, complete integration could be assumed. Various operation policies were checked considering: (i) both aquifers and the National Carrier as one hydrological unit; (ii) the National Carrier and the Karstic Aquifer as one unit (they are indeed interconnected to a high degree) and the Coastal Aquifer as an independent unit; and (iii) the Coastal Aquifer as a separate unit that backs up the rest of the system in droughts. Policy iii looks very similar to (ii) in definition, but is much like (i) in nature. It differs from (i) only by treating the two aquifers separately, thus giving a clear picture of the operation of the problematic Coastal Aquifer and its projected storage levels.

The Karstic Aquifer has a limited storage capacity. Its base capacity of 550 Mm, which is the subject of the present analysis, is about 2/3 of the combined yearly potential of the Karstic Aquifer and Lake Kinneret. The capacity is limited, on the one hand, by a red line below which irreversible sea water intrusion may occur and, on the other hand, by a maximum level above which the water will flow out at springs.

The Coastal Aquifer's storage capacity is virtually infinite. Its levels are at present very low, and the aim is gradually to refill the aquifer to a target of 700 Mm above its present storage. This will stop further sea water intrusion and stabilize the interface at the desired distance inland from the shore. It may, as a result of prolonged drought, temporarily advance further inland. The whole strip where the sea water interface moves back and forth in a very slow multi-annual movement will have to be considered permanently saline.

The main target of the analysis was to find an acceptable compromise between two goals - maximum supply of water and maximum reliability of that supply, within the constraint of the aquifer's red line. In other words, the question was how much the steady supply should be set below the multi-annual mean potential in order to assure the desired degree of reliability. In practice, the supply would be gradually decreased as the red lines are approached, and increased when the levels are very high. However, a policy of constant steady supply was analysed as a good approximation.

PLANNING FOR RESERVES AND SYSTEM RELIABILITY

The whole system as one unit

The system's inflows are depicted in Fig. 2, for a historical period of about 50 years. A series of 1000 years was created out of the 50 historical values by a Monte Carlo method.

D. Hamberg 506

Figure 3 shows a simulation of storage of both aquifers together, taking into account inflows, withdrawals and losses to the sea. The present (initial) storage level is taken as zero, and the associated losses are also taken nominally as zero, while the losses for other storage levels are taken as 5% of the change in storage. Figure 3 represents the simulation when withdrawals equal the mean net inflow. Figure 4 shows the same simulation but with annual withdrawals lowered by 30 Mm per year; this reduction in withdrawals raises the storage till a new balance is reached - at a level of 600 Mm above the initial level - at which point the increased losses (5% of 600 Mm3) offset the reduction in withdrawals. The frequency with which the

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storage now dips below the zero line is about once every ten years on average (instead of every two years as in the previous case).

The coming 20 years will be a transition period during which the storage level of the Coastal Aquifer is planned to be raised by 700 Mm . To consider the behaviour and storage level probabilities of the system as a whole over that transitional period, simulations were performed with the 1000 years divided into 50 series of 20 years each. Separate simulations were performed for each aquifer, to gain more insight and to detail the policy planned for the transitional period, as described below.

507 Role of groundwater in Israel

200 400 600 800 1000

Years Years

Fig, 3 Storage in aquifers during 1000 years Withdrawals = mean inflow?;. inflows.

2 0 0 400 600

Years 8 0 0 1000

Fig. 4 Storage in aquifers during 1000 years with a reduction in withdrawals of 30 Mm /year. Withdrawals = mean inflows minus 30 Mm3/year

D. Hamberg 508

The sub-system of the Karstic Aquifer and Lake Kinneret

A simulation of the sub-system of the Karstic Aquifer and pumpage from Lake Kinneret to the National Carrier considers a constant withdrawal from the aquifer as long as the levels are above its red line. When the level drops to the red line the supply is cut so as not to cross it under any circumstances. Any net inflow to the aquifer above its storage capacity was assumed to overflow immediately (during the same year). It should be noted that the present storage level is very close to the red line (adopted as the "zero" level).

Figures 5 and 6 depict the probability that the supply from the sub-system will exceed different selected levels, after three and 20 years, respectively, under three withdrawal policies: (a) 823 Mm /year (withdrawal equals mean net inflow); (b) 803 Hm /year (a permanent reduction of 20 Mm3/year); (c) 773 Mm fyear (a permanent reduction of 50 Mm3/year.) The main results are summarised in Table 1.

Table 1 - Sub-system of karstic aquifer and Lake Kinneret supply reliability under three policies

Policy Time elapsed (years) Steady supply Reliability of steady supply Mean supply (M.S.) Loss of supply ("823 -M.S.)

Mnffyear

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3 20

823

80% 89% 796 812 27 11

b

3 20

803

85% 95% 784 796 39 27

c

3 20

773

88% 96% 763 771 60 52

Policy a will result in the highest mean supply, but also the highest unplanned cuts necessitated by low levels in the aquifer. A permanent reduction of the planned steady supply decreases both values, i.e. - increases the supply reliability while decreasing the supply itself. The reliability of Policy c is only slightly better than that of Policy b.

It should be noted that the damage caused by a cut in supply is much higher if the cut is unexpected rather than the result of a planned reduction in the steady supply (comparing two cases of the same multi-annual average supply volume). The economic value of the damage of an unplanned cut depends on its depth and duration and is not known. A sensitivity analysis of various damage functions led to the choice of Policy b, which provides almost the same reliability as Policy c with a significantly higher mean supply. It provides a significantly higher reliability than Policy a while losing only slightly in mean supply relative to this same Policy a.

509 Role of groundwater in Israel

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D. Hamberg 510

The Coastal Aquifer

The operation of the Coastal Aquifer was simulated as a backup for the whole system. It was assumed that whenever withdrawals from the Karstic Aquifer are cut, the Coastal Aquifer will "contribute" water to the National Carrier to make up the deficit. Supply cuts to consumers will occur only when the levels of the Coastal Aquifer also reach its red line.

Three policies were examined for the Coastal Aquifer: (b-I) no permanent reduction in steady withdrawal from the Coastal Aquifer (withdrawal equals mean net inflow); (b-II) a permanent reduction of 25 Mm3/year; (b-III) a permanent reduction of 50 Mm /year. In all three cases Policy b was assumed for the sub-system of the Karstic Aquifer plus Lake Kinneret, i.e., withdrawals are 20 Hm /year below the potential of the sub-system. The main results are summarized in Table 2.

Table 2 - Supply reliability of the Coastal Aquifer under three policies assuming Policy b *for the Karstic Aquifer

Policy Time elapsed (years) Steady supply Reliability of steady supply Mean supply (M.S.) Loss of supply (295 - M.S.)

b-I 3 20

295

62% 84% 261 284 34 11

b-II 3 20

270

81% 95% 263 796 32 27

b-III 3 20

245

88% 96% 230 244 65 51

*This necessitates a contribution of the Coastal Aquifer to the integrated system of 7 Mmilyear after 20years and 19 Mm /year after three years

Policy b-II provides a lower mean supply than Policy b-I but with a higher reliability of the steady supply. Its reliability in the long run is almost the same as that of Policy b-III which provides significantly less water. After three years, however, the reliability of the steady supply of Policy b-II is still relatively low - an unplanned cut about every five years.

An economic sensitivity test showed that Policy b-II would be better than Policy b-I provided that the ratio between the damage of an unplanned cut and a planned permanent reduction is greater than 2. This ratio has to be higher than 6 to justify Policy b-III.

Considering these results, as well as analyses of the depth of the cuts, it was concluded that in the long run Policy b-II was the best, i.e., a steady supply of 45 Mm /year less than the potential of the system (mean inflow losses to the sea at the minimum level). The value of 45 Mm /year is the sum of a reduction of 20 Mm /year for theKarstic Aquifer and 25 Mm /year for the Coastal Aquifer. In the first eight years, however, Policy b-III (a permanent reduction of 70 Mm /year) is more appropriate.

511 Role of groundwater in Israel

The next section describes the rehabilitation of the Coastal Aquifer, to be achieved by a temporary reduction of withdrawals, according to a deterministic rehabilitation schedule based on the assumption that all years equal the climatic average year. The storage level of the rehabilitated aquifer at the end of the process will be regarded as the desired minimum level. In the course of the transition period to full rehabilitation the storage will increase along a gradually rising path, which will be treated as a sliding interim niinimum line below which the storage volume should not be allowed to fall.

REHABILITATION OF THE COASTAL AQUIFER

Detailed model of the Coastal Aquifer

A detailed finite differences model of the operation of the Coastal Aquifer has been in use for some time. The model represents the 120 km long aquifer as about 400 squares of area 2x2 km each. Although such an areal representation ignores some important features of the aquifer, which are treated in separate models, this FD model has proved to represent accurately the behaviour of the aquifer as regards water quantities, levels and sea water interface movement. The output of the model contains predictions (as a result of simulation) of the aforementioned variables for each square over the planning period (in our case about 28 years).

Schematic model of the Coastal Aquifer

The use of the detailed model as a tool for the planning of an operation policy is cumbersome. It involves an input of pumping and recharge quantities for each square for each year, and the output includes a large volume of data which is difficult to analyse. Furthermore, each run of the model is only one step in a procedure of trial and error aimed at deciding on the best operation policy. The approach adopted, therefore, was to use a simpler schematic model for deciding on the overall policy, and then use the detailed model to fix the operation policy for each square..

The schematic model consists of a "black box" containing the whole aquifer as one unit. Thus it involves'a simulation of one cell only (instead of 400). It behaves in accordance with simple functions, mainly: flow to the sea as a function of storage, and sea water interface movement as a function of the storage and the present location of the interface. The parameters of the functions were fixed by a regression analysis of the results of the detailed model. The regression lines were smooth, with, a correlation factor above 97%. This indicates that the schematic model satisfactorily represents the overall behaviour of the aquifer, even though its functions do not explicitly

D. Hamberg 512

take into account disturbances such as the large craters in the water that exist about 4 km inland from the shore.

Alternatives checked

Five main alternatives were analysed, with the results shown in Figs 7,8, and 9. The first four alternatives (A to D) relate to the deterministic rehabilitation of the Coastal Aquifer. Alternative E is an integrated policy for the whole system, related to the rehabilitation of the Coastal Aquifer together with the reserve considerations discussed earlier under the heading dealing with system reliability.

The following alternatives were examined:

A. Anetpumpageof350Mm /year. This will result in perpetuating (appro­ximately) the low levels of today, while allowing the sea water interface to continue to "race" inland.

B. Continuing the existing net pumpage of about 300 Mm /year. This will result in increasing the storage of the Coastal Aquifer by about 400 Mm3

above today's state. This is still substantially lower than the goal of 700 Mm , which is regarded as a rehabilitated aquifer. The interface will reach, at the end of the planning horizon of 28 years, only slightly further inland than the goal of 1.5 km from the shore, but will continue to move and will stabilize at 3.4 km from the shore.

C. A pumpage of about 250 Mm /year, which results approximately in reaching the prescribed goals.

D. A pumpage of 300 Mm /year for the first nine years, decreasing later to 230 Mm /year. This has a similar result to Alternative C, but with a higher present value since the decrease in pumpage is delayed.

E. Alternative D minus 70 Mm /year in the first eight years, and minus only 30 Mm /later. This is the policy suggested by the analysis of the system as a whole, presented in Table 2, which gives a "loss of supply" of 32 Mm for Policy b-II after 20 years and 65 Mm for Policy b-III after three years.

Recommended policy for the Coastal Aquifer rehabilitation.

A commonly accepted criterion for the extent of the rehabilitation of the Coastal Aquifer is the intrusion distance of the sea water interface. All policies (except Alternative A) slow down the rate at which the interface advances inland, and stabilize it at about the same position by the end of the planning horizon (differences of 100 m are not significant). In the case of Alternative E, after reaching the common position, the interface begins to recede towards the shore. This phenomenon, however, has no advantage

513 Role of groundwater in Israel

since the extent of the damage caused is presumably determined by the maximum intrusion of the interface.

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Pumpage (MCM/y):

350 - a 300 - + 2J0 - O 300-230 - x 230-200 - v

Fig. 7 - Volume in Coastal Aquifer under various pumping policies.

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Pumpage (MCM/y):

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Fig. 8 - Average intrusion of sea water interface for various pumping policies.

D. Hamberg 514

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According to the above deterministic analysis (i.e., assuming each year to have the same natural inflows), Alternative D seems to be the best. However, considering the stochastic nature of the inflows and the need for a reserve volume for the whole system, Alternative E is better, as already suggested in the discussion on system reliability. The higher average storage of the aquifer will guarantee the firm supply to the aquifer's local users as well as to users of the national system, at the prescribed reliability. It will ensure that even after a series of dry years the levels of the aquifer will not drop to dangerous values, and the interface intrusion will not exceed that planned to be reached a few years hence. The relatively fast rehabilitation will ensure the reliability of supply already in the near future.

CONCLUSIONS: OVERALL SYSTEM OPERATION PLAN

The operation policy of the system depends on backing by the aquifers for the users of the whole system. The reliability of the firm supply is guaranteed by a permanently lower value of this firm supply. The decrease in withdrawals results in a higher storage which, while causing higher losses to sea, serves as a reserve for dry years.

In this way unplanned cuts in supply to users (and/or too low levels of groundwater) will become very rare (at a desired probability). These rare cuts are spread over most of the country, thus moderating their effects. In practice, operation rules can be somewhat less rigid, with step-wise cuts when levels become extremely low, and vice-versa.