Estimating desalination requirements in semi-arid climates: A Mediterranean case study
Introduction
The difficulty in meeting water requirements in semi-arid climates is increased by the large inter-annual variation in natural rain-fed water supply, a variability which increases with aridity [8]. This inter-annual variability in precipitation should be taken into account when planning desalination especially when inter-annual water storage capacity is limited and expensive as in the subject of this case study; the semi-arid region between the Mediterranean Sea and the Jordan River.
The large areal as well as temporal variations in water supply within this region led to the construction of Israel's National Water Carrier in 1964 which linked the wetter northern with the more arid southern areas and increased the national water storage capacity to 3300 Mm3 by adding Lake Kinneret to the Coastal and Mountain aquifers (Fig. 1). Until recently depletion of this storage capacity was the only option available to meet water requirements in years when demand exceeded the supply by natural recharge; however this strategy led to environmental damage during a succession of dry years and increasing water use. In the 1970's an additional source of water, treated domestic and industrial effluents, became available and by 2010 provided more than one third of the annual agricultural water consumption and one fifth of the total national water consumption. In the last decade large scale desalination of sea water added a new, substantial and growing source of potable water. Recognition of the need for a regional approach to water planning has been reinforced by agreements for cross-border water transfers (www.water.gov.il) and this approach has been followed in this case study of the need to incorporate the inter-annual variation in natural recharge in determining the need for desalination in the light of the supply and demand for water expected in the future.
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Measurements and methods
The following method was used to assess natural regional recharge, defined as the total volume of precipitation replenishing surface and sub-surface reservoirs. The sum of annual values of recharge to the ten major hydrological basins between the Mediterranean Sea and the Jordan River were taken from Table 6 of Hydro Report 11/1 [20] for the 1975 to 2008 period: these annual totals include the recharge of waters with chloride concentrations greater than 400 mg L− 1 which averaged 250 Mm3 out of the
Results
The relationship between annual regional values of recharge R, and those of rainfall P, shown in Fig. 2 for the 34 years of data available, was fitted by the following linear regression equation
The root mean square error of recharge estimated from rainfall with Eq. (1) was 169 Mm3 or 9.4% of the mean annual value.
Applying Eq. (1) to the annual values of rainfall over the 78 year period between 1931 and 2008 for which regional values of rainfall are available
Discussion
The high linear correlation between precipitation and recharge may partly be an artifact due to the fact that in some groundwater basins recharge is estimated from the use of a recharge coefficient with precipitation. Nevertheless this first order approximation enables the analysis of long term series of regional water resources and is supported by the fact that the mean annual recharge derived from the use of Eq. (1), i.e. 1792 Mm3, agrees closely with the mean value of 1846 Mm3 calculated for
Conclusions
The absence of evidence for any change in the natural rain-fed water recharge in the semi-arid region between the Mediterranean Sea and the Jordan River during the last 78 years allows its inter-annual variation in regional rainfall to be used to estimate the probabilities of natural recharge and projected additional water production falling below or exceeding the current net regional water consumption by a given volume and hence the need for desalination.
Analysis of trends in water consumption
Acknowledgments
The authors wish to thank Amos Porat of the Israel Meteorological Service for the rainfall map reproduced in Fig. 1. The study was supported by the Agricultural Research Organization, Israel.
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