1. Yield of the Reservoir.

  • It is the amount of water that can be supplied by the reservoir in a specified interval of time.
  • The specified time interval may vary from a day for a small distribution reservoir to a month or year for large conservation reservoirs.
  • If we say that three million cubic metres of water can be supplied from a reservoir in a year then its yield is 3000000 m3/year.
  • The yield of the reservoir is dependent upon the inflow and thus varies from time to time.

2. 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.

3. 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.
  • The value of design yield for a reservoir to be used for water supply is taken less than the safe yield.
  • In the case of reservoirs used for irrigation purpose the design yield may be taken slightly more than the safe yield as crops can tolerate some deficiency of water during exceptionally dry season.

4. A 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.

5. Average Yield.

  • The arithmetic average of the safe yield and the secondary yield considered for a number of years is known as average yield.
  • The storage capacity of the reservoir and its yield are very much interdependent.
  • The water is stored in the reservoir to fulfill the safe yield requirements.
  • If capacity of the reservoir is more it can certainly provide more water and hence yield is more.
  • The reservoirs are designed to meet a specific demand of water.
  • The capacity of the reservoir and the yield are governed by the following storage equation:

Inflow – outflow = Increase in storage.