Water supply continues to be one of the most pressing problems in the growth and development of Bangalore metropolis. Apart from the unpredictable monsoons, the reasons for this deplorable situation are the un planned & depletion of water resources of Bangalore in recent years. The demand for water by various social and economic sectors of metropolitan activity is ever increasing while water availability is declining gradually. If development of water resources has to be sustainable, equitable and community- based, traditional systems based on harnessing nature have to be rejuvenated and developed. Mahatma Gandhi said " We do not need mass production ,we need production by masses". Indeed, these are golden words today and the economic growth and sustainability of Bangalore depends on water. In that context, the preservation and harvesting runoff of large quantity of water has become a vital issue and has to be a part of overall growth and development of Bangalore.
Urban centers like Bangalore had sprawling historical tanks in the past. Over the years, these water bodies have vanished due to man's greed for land and development needs of urban expansion. One of the biggest environmental challenges that Bangalore faces in the coming decades is that of balancing its increasing demand with the diminishing availability of water resources. The future scenario can be characterized as over exploitation of water resources, decreased accessibility to clean water, increased competition and potential for conflict over water resources. Bangalore certainly requires an effective and consistent action plan to save its precious resources of water.
A survey of developments, within India and abroad, in the past decade shows that numerous projects have been undertaken to promote local water harvesting, both in urban and rural areas. There have been some outstanding efforts to promote water harvesting in countries of south Asia, Israel and industrialized countries like Japan and Germany. Water harvesting can no longer be labeled a technology of the past and fit for only poor and rural areas. Water harvesting is as relevant today for the rich, as it is for the poor and it is as relevant for urban areas, as it is for rural areas. Equally, it is as relevant to all, with location-specific applications.
The Hydrological Cycle
The rainwater or precipitation falls on the land and is dispersed in several ways. A sizeable portion is intercepted by land cover with vegetation and is temporarily detained in surface depressions. Further, rain which enters at the interface of soil is referred to as infiltration that is the beginning of groundwater storage. The soil column gets saturated with its pores and capillaries, which is referred to as the "water holding capacity of the soil". This depends on the nature of clay, pore space and the organic matter content and finally the soil matrix. This water, which is held in the capillaries and other pores, is held against gravity and is referred to as "capillary water" or available water. The excess water after fulfilling the saturation capacity starts percolating down the soil columns and is referred to as "gravitational water". It is this water, which is available for groundwater recharge.
A part of the surface water continues to move laterally when the soil is saturated or when no more infiltration takes place. This is termed as overland flow. This overland flow reaches streams, channels and lakes and is termed as surface runoff, which together with other components of flow forms total runoff. The evaporation of surface water along with transpiration of vegetation cause the water to return to the atmosphere. This forms a cycle and all these are the components of the Hydrological cycle.
It is this surface water or runoff water that needs to be trapped or harvested in ponds, lakes, water harvesting structures, in recharge units of borewells, or in the sub soil by increasing downward percolation and decreasing the surface runoff using appropriate methods. This can be properly implemented by studying the regional geography, landform, climate, soils, the weathered mantle and the landuse of the area.
Rainwater harvesting model for urban areas
In urban context the applicable forms of rainwater harvesting includes
Rooftop rainwater harvesting
Rainwater harvesting in paved and unpaved areas (landscapes, parks, open fields, storm water drains, roads etc)
Rainwater harvesting in lakes and tanks
Different types of storage methods can be adopted for different levels of collection. These are:
Houses and apartments
Layout and city level
Institutional and industrial areas
Underground sumps, ground level tanks, overhead tanks, below ground in soil and aquifers
Tanks, lakes, reservoirs, aquifers
Sumps, tanks, ponds, aquifers