ISSUE 6
OCT-DEC 2004

Cisterns

A traditional method for household water management is the use of cisterns that collected rainwater to be used later for various domestic uses. Today this method can be adapted to promote rational use of water resources in each household, by providing a means of storing water during the low-consumption times and alleviating pressure on the water network during peak times. The use of a cistern functioning as a water tank in each building to provide a buffering effect in the spatial and temporal distribution of the water supply could make a significant difference in the coverage of demand in areas facing problems of seasonal deficits and reduced flow at peak consumption times.

The proposed scenario assumed the existence of a state subsidy for the construction of household cisterns in areas with seasonal deficits. In order to enable modelling of this option in the DSS, it was assumed that the construction of cisterns was represented by the construction of a small storage reservoir set on the water network of each area selected for application. Each of these reservoirs was set with a capacity reflecting the total of the proposed cisterns, and the corresponding cost. The capacity of the reservoirs was estimated according to the permanent population of each area, assuming an initial penetration of the intervention at the range of 50% of households. A possibility for expanding this capacity by 20% after 5 years or more was also considered. Initial capacity and associated capital cost for each municipal department are presented in Table 1.

Table 1. Estimated capacity and cost for cisterns (Assuming an average cost of approximately 2,200 €/cistern)

Municipal Department	Permanent Population (2001)	Number of cisterns	Initial Capacity (m³)	Capital Cost (thousand €)
Paroikia	5812	937	20,150	2,063
Marpissa	984	159	4,000	350
Arhilohos	910	147	3,150	323
Naoussa	3027	488	10,500	1,075
Kostos	374	60	1,300	133
Total	11107	1791	39,100	3944

Option results

Figure 1 presents the total volume of water stored and used during the summer.This abstraction may vary according to the water availability for each year. Figure 2 presents abstractions from the cisterns in the Paroikia municipal department for the three scenarios in year 2009.

Figure 1. Cistern Abstractions under the BAU + Normal scenario

For an average year, cisterns are used during September and October, since available supply is adequate to meet the August peak. In cases of drought and limited available resources, abstraction takes place in August, while for wet years cisterns are not needed at all.

Figure 2. Abstractions from Paroikia cisterns under the three scenarios (2009)

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Effectiveness

Domestic deficit does not present any significant improvement; as demand escalates, the initial improvement of 10% falls to approximately 3% in 2030 (Figure 3 and Figure 4). The small increase in groundwater abstractions, used to fill up the cisterns, results in a similar deterioration of irrigation deficit and decreases in effectiveness (Figure 5 and Figure 6).

Figure 3. Percent demand coverage effectiveness of Cisterns to Domestic use

Figure 4. Percent Improvement of deficit in Domestic use with respect to the reference scenarios

Overall, the option can help to alleviate some pressure on available resources during the high demanding summer months while improving the reliability of domestic supply. Of course the small capacity cannot adequately meet the structural deficit appearing in some municipal departments, and an expansion of the application of the option in the hotel sector is considered unrealistic. Additionally, the measure is not effective in cases of drought and escalating demand.

Figure 5. Percent demand coverage effectiveness of Cisterns to Irrigation use

Figure 6. Percent Improvement of deficit in Irrigation use
with respect to the reference scenarios

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Direct and Environmental Costs

Figure 7 presents the direct cost (expressed as the present value over the period 2004-2030) between the three scenarios after cistern construction. In all three cases, direct costs increase about 15-16% due to the high subsidies for allowing the penetration of cisterns into the supply system.

Figure 7. Total direct cost difference of the Cisterns option
under the three scenarios (Present Value – Million €)

Environmental costs present a marginal increase, due to the augmentation of groundwater abstractions, that are used for filling up the cisterns during the low peak months (Figure 8).

Figure 8. Total environmental cost difference of the Cisterns option
under the three scenarios (Present Value – Million €)

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