Water may be removed from anyone of a number of points in its movement through the hydrological cycle. The safe yield of the source must be sufficient to serve the population expected at the end of a design period which may be 10 to 50 years.
The safe yield is generally defined as the yield that is adequate for 95 per cent of the year. The selection of a source requires professional advice. In general, water sources must conform to two criteria;
The quantity must be sufficient to meet present and future requirement