(i) safe yield
- It is also known as firm yield.
- It is the maximum quantity of water that can be supplied from the reservoir with full guarantee during the worst dry period.
(ii) secondary yield
- The quantity of water available in excess of safe yield is known as secondary yield.
- This yield is available during period of high inflows.
- This secondary yield of the reservoir can be used either to generate extra hydroelectric power or for irrigation of extra lands.
(iii) design yield
- The critical period for a reservoir is generally considered, when natural flow in the reservoir is minimum.
- There is possibility that sometimes the minimum natural flow in the reservoir may even fall short of guaranteed yield.
- Hence a lower value than the guaranteed yield or safe yield may be taken for design purpose.
- This yield whose value is smaller than the safe or firm yield is known as design yield.
