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Rain water is the only source of water in Sri Lanka and is used mainly for domestic purposes and crops and animal production. The average annual rainfall in Sri Lanka is around 1,700 mm. In the wet zone this is around 2,425 mm and 1,470 mm in the dry zone.

Some parts of the country (arid zone) receives only around 1,000 mm of rain annually. Even that is received only in some months of the year and there is hardly any rain during the other months.

A part of the rainwater infiltrates (enters) the soil. Depending on the nature of the soil surface and its depth, a portion of the infiltered water in the soil is retained and the remaining percolates down into the ground water. The portion of water that does not infilter runs off and enters water bodies such as lakes, rivers, reservoirs etc.

When the soil is not exposed water cannot enter into the soil and hence simply runs over the ground and enters into drains. Thus, most of the rainfall in urban areas does not enter the soil and hence runs off and finally end up in the sea.

The total amount of water the country receives in the form of rain is around 100 billion cubic meters. (The total area of the country is 6 million ha. 1 ha is 1,000 sq m. Hence, the total land area is 60,000 million sq.m or 60 billion sq m. The average rainfall is 1,700 mm i.e. 1.7 m. Therefore the total volume of water that falls on the country is 60 x 1.7 billion cubic meters i.e. App. 100 billion cubic meters.)

On an average only about 60% of this water is harvested and about 40% runs-off. That is around 40 billion cubic meters i.e. 40,000 billion liters (note. 1 cubic meter is equal to 1,000 liters). A portion of the run-off water is collected in reservoirs and lakes and it is estimated that around 20 billion cubic meters (20,000 billion liters) of water escapes to the sea.

Rainfall in urban areas:

Although actual data is not available, it can be assumed that 60% (this can be more) of the rainfall that falls in the urban areas escapes into the sea, which is considerably more than the average for the country (40%). This is likely because most of the soil surface in urban areas is not exposed for rainwater to enter the soil. It is covered by buildings, tarred roads etc.

If Colombo Municipality which has an area of 37 sq km. i.e. 37 million sq. m (1 sq km is equal to 1 million sq. meters) and an average rainfall of 2000 mm and 60% run off is considered, the total run off would be around 44 million cubic meters. i.e. 44 billion liters (1 cubic meter is equal to 1,000 liters) or 8.8 billion gallons.

The value of this run off water is Rs. 1,980 million (The present water charges is Rs. 45 per 1 cubic meter for very high level of water consumers.) The approximate amounts of water that go waste in some of the urban areas in the country and their values are indicated in Table 1. Therefore, collection of rainwater in urban areas is extremely important.

Note: It is assumed that 60% of the rainwater escapes as run-off-water. The values in parenthesis indicates the value of water in Rs. Million, assuming rs. 45 per 1 cubic meter.

The rainwater that falls on the roofs of extensive buildings such as hospitals, schools, housing complexes etc. can be collected in tanks in the premises itself. Water that falls on a roof of 1,000 sq m in Colombo (average rainfall is 2,000 mm) during a period of one year would be around 2,000 cubic meters (i.e. 2 million liters or app. 400,000 gallons).

The actual cost of this amount of water would be around Rs. 90,000. Water thus collected could be used for numerous domestic purposes. This will reduce water bills, save purified water, which can be used for drinking purposes. Once the collection system is installed there is no additional cost involved except on pumping of collected water to the main water supply system.

Using a roof of 1,000 sq. m

Cost of gutter pipes and down pipes for a 1,000 sq.m roof - Rs. 1,000

Cost of a 5 cubic meter tank. - Rs. 15,000

Value of water saved per year @ Rs 45. per cubic meter

(assuming the water collected is 2,000 cubic meters) - Rs. 90,000

Thus the cost involved will be covered within first year itself.

Advantages of rainwater harvesting

Rainwater is one of the purest sources of water available as it does not come into contact with many of the pollutants that are often discharged into local surface waters. It comes free and can be used to supply potable (drinkable) water and non-potable water. If collected properly, it can be used for all domestic purposes including drinking.

Some factors such as pollution by extraneous matter (eg. bird faecal matter etc. on roof surface may affect the quality of rainwater collected). However, even tap water tend to have such material and hence, comparatively roof-rain water is satisfactory.

Rainfall in areas where heavy industry or crop dusting is prevalent may not have the same purity as rain falling in other areas. Rainwater is soft. It can significantly lower the quantity of detergents and soaps needed for cleaning.

Soap scum and hardness deposits do not occur. There is no need for a water softener as there often is with well water. Water heaters and pipes are free of the deposits caused by hard water and should last longer.

For centuries the world has relied upon rainwater harvesting to supply water for household, landscape and agricultural uses. Before city water systems were developed rainwater was collected (mostly from roofs) and stored in cisterns or storage tanks. Today, many parts of the world, including Hawaii and the entire continence of Australia, promote rainwater as the principal means of supplying household water.

On many Caribbean islands where rainwater is the most viable water supply option, public buildings, homes, and resorts all collect rainwater to supply their needs. In Hong Kong, rainwater is collected from skyscrapers to supply water needs. Elsewhere, countries like Germany, Japan, United States and Singapore are also adopting rainwater harvesting.

(This article is based on a paper presented by Prof. C. S. Weeraratna, at the 7th Symposium of Lanka Rainwater Harvesting Forum on Rainwater Harvesting in Urban Areas)

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