Sizing of Rainwater Cisterns for Domestic Water Supply in the Grenadines

Everson J. Peters

Introduction

The Grenadines stretch from Ronde Island in the north of Grenada to Bequia to the south of St Vincent, with the largest being Carriacou with an area of 13 square miles, while others are just big enough to accommodate a single resort. The economy of the islands is supported by tourism, shipping, fishing, trading, livestock rearing with remittances from Europe and North America being the largest contributor. Generally, Grenadinians enjoy a higher standard of living on average than larger islands' mainlanders (Poverty Study 2000 and CDB report on SVG).

In the dry climate of the Grenadines an adequate water supply is highly dependent on the timely arrival of the rainy season. The steep hills and small surface area of the islands result in the absence of any perennial streams. With an an average annual rainfall of about 1,000 mm the islands depend almost exclusively on private and public rooftop rainwater catchment systems (RRCS) and concrete communial rainwater catchment systems (CRCS) as the primary source of domestic water. Small public and private earthen dams used to store rainwater harvested from runoff are primarily for livestock use. There are also several dug wells and bore holes that provide limited ground water supply.

Growing demands for water in the Grenadines and the increasing costs of water supply with the use of desalination, results in a need for these islands to maximize the use of the traditional and existing water supply techniques. There has been a great deal of interest in the development of guidelines for construction of primary supply and standby cisterns in small islands for example US Virgin Islands, Hawaii and Micronesia (Heitz et al., 1997 and Smith et al., 1999) The storage facilities are sized according to the rainfall in the area, the roof construction material and size, the expected water demand in the house the cistern serves, the cost of constructing the cistern and the degree of reliability the owner desires. Although some research on rainwater harvesting has been carried out in the region (Hadwen, 1987), no criteria for sizing are available for the Grenadines. There is some concern expressed as to whether the sizing requirements provide for too much storage. While a large storage system might be pleasant to boast about, the security it might seem to provide could be elusive. Further, if the system is oversized it might never become filled and/or it might never be emptied. In both instances there is excessive wasted capacity and investment of resources that might have been better used elsewhere (Smith et al., 1999).

Literature review

Rainwater harvesting technology

Rainwater harvesting has been practiced for more than 4,000 years, and, in most developing countries, is becoming essential owing to the temporal and spatial variability of rainfall. For more than three centuries, rooftop catchments and cistern storage have been the basis of domestic water supply on many small islands in the Caribbean. During World War II, several airfields were also turned into catchments. Although the use of rooftop rainwater catchment systems has declined in some countries, it is estimated that more than 500,000 people in the Caribbean islands depend at least in part on such supplies. Further, large areas of some countries in Central and South America, such as Honduras, Brazil, and Paraguay, use rainwater harvesting as an important source of water supply for domestic purposes, especially in rural areas.

Very frequently most of the rain falls during a few months of the year, with little or no precipitation during the remaining months. There are countries in which the annual and regional distribution of rainfall also differ significantly. Rainwater harvesting is necessary in areas having significant rainfall but lacking any kind of conventional, centralized government supply system, and also in areas where good quality fresh surface water or groundwater is lacking. Serious supply problems occur when the components of the RRCS are not sized appropriately. Therefore, there is a need for a criteria for properly sizing the components of a new RRCS or upgrading existing systems. There is also a need for suggestions on how to manage these systems.

Unlike the US Virgin Islands, RRCS are not required by law in the Grenadines. However, the increasing dependence on tourism of these islands would soon force at least public facilities to establish quantity and quality standards. With annual rainfall averaging about 1000 mm per year, daily per capita demand estimated at a maximum of 136 litres for local population plus livestock demand, can reach close to developed courntries daily per capita of 227 litres.

Storage capacity

The literature contains discussion of several approaches that may be considered in sizing rainwater stores. These approaches evolve from methods developed for the design of reservoirs being filled from streamflow or groundwater pumping. McMahon and Mein (1978) identified three general types of reservoir sizing model; critical period, Moran and behavioural. Critical periods use sequence of flows where demand exceeds supply to estimate storage capacity, Moran related methods are a development of Moran's (1959) theory of storage and behavioural methods to simulate the operation of the reservoir with respect to time (Jenkins et al., 1978, Latham 1983 and Fewkes and Butler, 1999). Almost all the different methods assume that water consumption is at a constant daily rate throughout the year, and they require data inputs such as: daily rate, details of roof plan area, rainwater catchment efficiency and rainfall distribution (Thomas et al., 1997). The outputs of these methods are recommended store sizes for one or more probabilities of storage failure (i.e. tank runs dry). Ntale (1996) reviewed some of these methods under Ugandan conditions and showed that in a not untypical location in Uganda, the crudest method (Mean Dry-season Deficit) gives a storage size very much less than given by more elaborate and accurate methods. Other variations of the dry season deficit are also suggested providing a cistern size based on a rainfall pattern, a roof plan area and a household demand. Design criterion based on drought concepts are the oldest forms of designing reservoirs supplied from stream flow. Similar methodologies are applicable in RWCS systems. Heitz et al. (1999) developed a rooftop rainwater catchment system design criteria for Micronesia using daily rainfall, family size, roof catchment and consumption rates in a program called ROOFRAIN (Heitz et al., 1998). Heitz et al. considered a minimum size that would provide reasonable level of protection against shortages of consumable water during droughts. Further, a Drought Duration Depth Frequency (DDDF) drought criteria proposed by Heggen (1999) requires only a record of historic rainfall and can be used to address system behaviour for duration and risks appropriate to RWCS need.

Rainwater systems in the Grenadines

Rainwater storage systems in the Grenadines are underground and above ground conrete cisterns (average 30,000 litres), metal tanks with capacity of (760-1900 litres), plastic tanks (760-3000 litres), drums (170 litres), wood barrels (130-150 litres). There is a steady increase in the construction of household cisterns; 21% and 70% in 1981 and 1991 respectively. Communal cisterns can be found at public buildings, schools, hospital and medical clinics, churches, community centres, and administrative offices. In Carriacou and Petite Martinique there are 33 communal catchments and 78 public storages. Typical systems, as shown in the accompanying photograph, consist of a rooftop water harvesting surface, a conveyance system for the harvested water, a cistern for storage and a means of distributing the water either by gravity, drawing using bucket and rope or a pumping and plumbing system. The collection system are usually corrugated galvanized roofing material, concrete tiles, clay tiles, asphalt type shingles or wooden shingles. The runoff is collected by means of gutterings which are mainly plastic. Very seldom are devices used to divert the first flush of water from the roof to waste or are there treatment devices to enhance water quaility. The cisterns are usually sealed at the manhole and at the over flow with insect screens. Routine maintanance for gutterings and the tanks can be carried out over periods of 2 to 3 years.

In most cases in the Grenadines, although the cistern can account for 5% to 30% of a property, sizing is done arbitrarily and is often of a lower concern than bedroom sizing, roof design and the quality of interior finishing. This results in many cases where cisterns are oversized and unnecessary mortgage burdens, or undersized creating inadequate water supply during the dry season.

An evaluation of the homes in Carriacou and Petite Martinique shows that the average size house is 87 m² and costs about US$27,700.00 (see Figure 2 below). For homes valued over US$135,000.00 water consumption tended to be closer to that of the developed world, while for houses valued US$19,000.00 and less, per capita consumption was closer to that of water stressed developing countries.

Residential water needs vary depending on the type of dwelling, number of residents, and type of plumbing fixtures all of which are influenced by the economic status of the users. By far the largest percentage of indoor water use occurs in the bathroom, with 41 percent used for toilet flushing, 33 percent for bathing (USEPA, 1992), 20 percent for laundry and 5 percent for drinking and cooking. A range of 27 to 200 liters person per day is generally considered to be a necessary minimum to meet needs for drinking and sanitation, bathing and cooking (Gleick, 1996).

In practice, most households will use the water copiously during the rainy periods, especially if the tank is overflowing, while rationing during the dry periods. There are also three consumption rates observed in Carriacou in a household of average sized house and cistern; the dry season consumption rates (60% of mean per capita) during the period January to the beginning of the rainy season, when conservation and rationing are widely implemented the wet season consumption rate (125% of mean per capita) after the tank is filled when there is liberal use of water and the mean per capita sometime after the beginning of the rainy season and when the tank is filled.

The beginning of the dry season consumption period is also influenced on the level of the water in the tank. The level at which dry season consumption is applied is based purely on the experience of the household. Identifying a minimum tank level for initiating conservation or rationing would improve cistern operation. The increase in mean per capita is expected to increase in the future particularly with the increase in internal plumbing and the use of flush toilets and washing machines in average Grenadine homes. The use of internal plumbing with flush toilets increased from 9% and 22% in 1981 and 1991 respectively and estimated at about 37% in 2002.

In the Grenadines current per capita consumption is about 46 litres per person per day and would reach 136 litres per person per day (Procicaribe, undated). The number of residents in a typical Grenadine home is stable for most of the year except for the periods of Christmas, Carnival, Easter and the regular regattas. During these periods, each household could have between 2 to 6 additional overseas-based family members with water per capita consumption higher than that of the permanent residents. On average a typical home would have to accommodate annually 3 persons for 3 weeks at Christmas and Regatta and 2 persons for one week at Easter and Carnival seasons.

Methodology

This research was carried out in two parts. First a survey of 200 homes in Carriacou and Petite Martinique was undertaken to establish the status of rainwater harvesting in the Grenadines. The survey investigated issues like storage facilities, water quality, maintenance issues and consumption patterns. Data from the certified property valuation list for Carriacou and Petite Martinique were used to develop the relationship between floor space, contributing roof area, residential construction costs roof area and current cistern size. Per capita consumption was estimated based on survey results and the limited unpublished data (Procicaribe, undated). The restriction on the design was that the cistern was allowed to empty once in 25 years and there was sufficient water to fill the tank from another source before commissioning a new cistern. Finally a visual basic program was developed to simulate cistern sizes from the available data including; available rainfall data, different consumption patterns, and residential occupancy.

Data

Monthly rainfall data for Carriacou, Union Island and Bequia for periods of 1927-1959 (GoG, 200), 1923-1981 and 1930-1981 (GoSVG, 2002) respectively were used in the simulations. Mean monthly rainfall for the 3 islands are in Table A-1 in the Appendix. Monthly variation in consumption pattern are shown in Table A-2. Typical roof contributing roof areas for both single and multi-story residents from a survey of existing houses in Carriacou and Petite Martinique are summarized in Figure 2.

Results

The results of simulations of the performance of cisterns are shown in the following figures. In Figure 3 it can be seen that there are two distinct regions for the size of a cistern. In region one the required size of the cistern for a given number of permanent residents, increases rapidly as the available contributing roof area decreases. In region two, as the contributing roof area increases, the decrease in cistern size decreases much slower. The line AB in Figure 3 represents an optimum cistern size.

Simulations for a household of six permanent residents were carried out for per capita consumption of 50, 75 and 100 litres per day. The results are shown in Figure 4. As the per capita consumption increases, the size of the cisterns that would be required to meet the demands so that the cistern is allowed to dry once in twenty five years increases non-linearly.

Figure 5 is a monograph of the cistern design criteria. For a known contributing roof area selected on the x-axis for example 220m2, with a permanent household population of 6, the size of cistern should be 3000 gallons in capacity. If the house contributing roof area was half the size of 110m² the required tank volume would be about 9000 gallons.

Discussion and conclusions

A number of factors enter into the estimation of the cistern. In reality, the issue of cistern size is reduced to the rate of emptying versus the rate of filling and how much is required as a buffer. This depends primarily on the expected length of dry spells which could be probabilistic, and how difficult it is to get water from another source if the cistern dries up. In this study the estimates of cistern sizes assumed that the risk acceptable was for a cistern emptying completely once in 25 years. If the acceptable risk decreases that is cisterns are allowed to empty completely more often, then the desirable cistern size would decrease.

The reliability of RWCS can be determined from the expected reliability of the rainfall amount obtained from frequency analysis of monthly and annual rainfall. In this study the historic monthly rainfall data was utilized. Although shorter recording intervals for example daily or hourly are more predictive in the performance of RWCS, their application is not very practical because rainfall occurrence probabilities are normally very low, especially in semi arid regions (Ngigi, 1997).

In the debate between rough and ready methods of sizing storage and more 'exact' methods it should be remembered that:

• Uninterrupted rainfall records are not available (or are not readily accessible) in most developing countries
• In the case of below average wet seasons leading to incomplete tank filling, householders are likely to reduce usage in the subsequent dry season

It is not clear that households will discipline themselves to keep to the water usage rates assumed in any storage sizing exercise; certainly household occupancy can fluctuate and unplanned activities consume unplanned quantities of water. In this study, it is observed that there is an optimum size of a cistern which can be obtained by balancing the contributing roof area and the cistern size. However in practice the roof area is usually fixed, hence the size of the cistern depends on the number of occupants in the household and the consumption patterns.

At a consumption pattern of 50 l/day per person, the required cistern is about 15% of the property value. To satisfy a 75 l/day per person consumption, the cost of the desired cistern would be about 40% of the property value. Unlimited sizing of cisterns to meet an increased usage of domestic water in the Grenadines is not a viable option. Increased availability of domestic water would be created by (a) designing roofs to obtain maximum roof area for a given floor space (b) maximizing the available roof area, that is utilizing as much of the roof space as possible and (c) reducing use by using low volume bathroom and kitchen facilities. In the Grenadines if rainwater is going to be the main source of domestic water the per capita consumption would have to remain around the 50 l per person per day.

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Appendix

© Everson Peters, 2003.

HTML last revised 25th November, 2003.

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