The past available data of rainfall or runoff in the catchment is, therefore, used to work out the optimum value of the catchment yield.

Say, for example, in the past available records of say, 35 year, the minimum yield from the catchment in the worst rainfall year may be as low as say, 100 M.ha-m; whereas the maximum yield in the best rainfall year may be as high as say, 200 M.ha-m.

The questions then arises, whether the reservoir capacity should correspond to 100 or 200 M.ha-m yield. If the reservoir capacity is provided corresponding to 100 M.ha-m yield, then eventually the reservoir will be filled up every year with a dependability of 100%; but if the capacity is provided corresponding to 200 M.ha-m yield, then eventually the reservoir will be filled up only in the best rainfall year (i.e. once in 35 years) with a dependability of about 1/35 × 100 ≈ 3%.
In order to obtain a reasonable agreement, an intermediate dependability percentage value, such as 50 to 75%, may be used to compute the dependable yield or the design yield and for this dependability the value of design yield comes about 1.20 times higher than the firm yield or safe yield. 
Note : 
The yield, which corresponds to the worst or the most critical year on record, however is called the firm yield or the safe yield.