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Rainwater Harvesting And Utilisation

An Environmentally Sound Approach for Sustainable
Urban Water Management: An Introductory Guide for Decision-Makers

How Can Rainwater Harvesting and Utilisation Contribute to a Sustainable Water Strategy?

Self-Sufficiency in Water Supply, Without Being Dependent on Remote Water Sources
Many cities around the world obtain their water from great distances - often over 100km away. But this practice of increasing dependence on the upper streams of the water resource supply area is not sustainable. Building dams in the upper watershed often means submerging houses, fields and wooded areas. It can also cause significant socio-economic and cultural impacts in the affected communities. In addition, some existing dams have been gradually filling with silt. If not properly maintained by removing these sediments, the quantity of water collected may be significantly reduced.

Decentralised “Life-Points”, Versus the Conventional “Life-Line” Approach
When the city increases the degree of its dependence on a remote water resource, and there is a long period without rainfall in the upstream dam sites, the ability of the city to function effectively is seriously compromised. The same can be said about a city’s reliance on a pipeline for drawing water from a water resource area to the city. A city which is totally reliant on a large, centralised water supply pipeline (or “life-line”) is vulnerable in the face of a large-scale natural disaster. A shift from “life-line” to decentralised “life-points” should be encouraged. Numerous scattered water resource “life-points” within a city are more resilient and can draw on rainwater and groundwater, providing the city with greater flexibility in the face of water shortages and earthquakes.

Restoring the Hydrological Cycle
Due to the rapid pace of urbanisation, many of the world’s large cities are facing problems with urban floods. The natural hydrological cycle manifests itself at different scales, depending upon climatic, geographic and biological factors. As rain falls over time and seeps underground to become groundwater, it feeds submerged springs and rivers. The concrete and asphalt structures of cities have tended to disrupt the natural hydrological cycle, and reduce the amount of rainwater permeating underground. A decrease in the area where water can penetrate speeds up the surface flow of rainwater, causing water to accumulate in drains and streams within a short time. Every time there is concentrated heavy rain, there is an overflow of water from drains, and small and medium sized rivers and streams repeatedly flood. These conditions can often lead to an outpouring of sewage into rivers and streams from sewer outlets and sewer pumping stations, thus contaminating the quality of urban streams and rivers.

Diagram of the Hydrological Cycle ( detail )
Concrete and asphalt have a profound impact on the ecology of the city. These include:
Drying of the city – This happens as rivers and watercourses are covered, natural springs dry up, and greenery is cut down.
Heat pollution – In some cities during the hot summer, an asphalt road at midday can reach temperatures of over 60°C. The heat expelled from air conditioners can further aggravate this.

This dramatically alters the city’s natural hydrological cycle and ecological environment.
Urban flood in Tokyo, Japan

In order to achieve a comprehensive solution to this problem, new approaches to urban development are required emphasising sustainability and the restoration of the urban hydrological cycle. Traditionally, storm sewer facilities have been developed based on the assumption that the amount of rainwater drained away will have to be increased. From the standpoint of preserving or restoring the natural water cycle, it is important to retain rainwater and to facilitate its permeation by preserving natural groundcover and greenery.

Introducing the Concept of “Cycle Capacity”
In thinking about sustainable development, one must view environmental capacity from a dynamic perspective and consider the time required for the restoration of the hydrological cycle. “Cycle capacity” refers to the time that nature needs revive the hydrological cycle. The use of groundwater should be considered from the point of view of cycle capacity. Rain seeps underground and over time becomes shallow stratum groundwater. Then, over a very long period of time, it becomes deep stratum groundwater. For sustainable use of groundwater, it is necessary to consider the storage capacity for groundwater over time. If this is neglected and groundwater is extracted too quickly, it will disappear within a short time.

Demand Side Management of Water Supply
In establishing their water supply plans, cities have usually assumed that the future demand for water will continue to increase. Typically, city waterworks departments have made excessive estimates of the demand for water and have built waterworks infrastructure based on the assumption of continued development of water resources and strategies to enlarge the area of water supply. The cost of development is usually recovered through water rates, and when there is plenty of water in the resource area, conservation of the resource is not promoted. This tends to create a conflict when drought occurs, due to the lack of policies and programmes to encourage water conservation. It has even been suggested that the lack of promotion of water conservation and rainwater harvesting is due to the need to recover infrastructure development costs through sales of piped water. The exaggerated projection of water demand leads to the over-development of water resources, which in turn encourages denser population and more consumption of water.

Sustainability of urban water supply requires a change from coping with water supply without controlling demand, to coping with supply by controlling demand. The introduction of demand side management encourages all citizens to adopt a water conservation approaches, including the use of freely available, locally supplied rainwater.

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