In this type of sources, the surface runoff is available for water supply schemes. The usual forms of the surface are as follows
- Lakes and streams
- Storage reservoirs
Lakes and streams
A natural lake represents a large body of water within land with the impervious bed. Hence, it may be used as a source of water supply scheme for nearby localities, lake water is available only on those towns and cities which are situated near them such as Nainital.
In the mountainous region, streams are formed by the surface runoff. It is found that the flow of water in streams is quite ample in rainy seasons. But it becomes less and in hot seasons and sometimes the stream may even become absolutely dry. The catchment area of lakes and stream is very small and hence, the quantity of water available from them is also very low. Hence the lakes and streams are not considered as principal sources of water supply schemes for large cities.
The water is available from lakes and stream is generally free from undesirable impurities and can, therefore, be safely used for drinking purpose.
A pond is a man-made body of standing water smaller than a lake. Thus the ponds are formed due to the excessive digging of ground for the construction of roads, house etc and they are filled up with water in rainy seasons. The quantity of water in the pond is very small and contains many impurities.
Rivers are born in the hill. When the discharge of a large number of springs and stream combine together. The river is the only surface source of water which has maximum quantity of water which can be-be easily taken, therefore at the very ancient times, the town and cities started developing along the banks of rives.
River water has self-purification action, due to which it automatically become clean in some distance travel from the point of disposal of sewage. In summer the quantity of river water is better than in monsoon because in rainy seasons the run-off water also carries with it clay, silt sand etc, which make the water turbid. River water should always be used after necessary treatment. Some rivers are snowed and perennial and have water throughout the year; therefore they do not require any arrangement to hold the water but some rivers dry up wholly or partly in summer, therefore they require special arrangement to meet the water demand during hot weather.
An artificial lake formed by the construction of a dam across in a valley is termed as a storage reservoir. Whatever may be the size or use of a reservoir, the main object or function of the reservoir is to store water and thus it stabilizes the flow of water.
At present, this is rather the chief source as water supply schemes for very big cities the multi-purpose reservoirs also make provisions for uses in addition to a water supply such as irrigation and power generation.