4.2　Computation of Runoff Regulation

4.2.1　In the computation of runoff regulation，the data that shall be gathered include long-series monthly（10-day）runoff and daily runoff in typical years，rating curve downstream of the hydropower station，evaporation at the reservoir surface and seepage in the reservoir，reservoir stage-capacity-area curve，water demand of authorities for multiple purposes，and the requirement for ecological water consumption，etc.

4.2.2　In the computation of runoff regulation，water balance shall be made according to regulating capability of the hydropower station and water demands of various authorities，to calculate firm power of the hydropower station，mean annual electricity production and typical water heads，and then to clarify operation characteristics and benefits of the hydropower station.

4.2.3　The design reliability of a hydropower station may be selected within 80%-90% according to the proportion of capacities of the hydropower station in the electric power system，the regulating performance and the installed capacity of the hydropower station，etc.

4.2.4　The chronological series method shall be applied in the computation of runoff regulation.For multiyear regulating reservoir and annual regulating reservoir，long series（not less than 30 years）shall be adopted for computation based on mean monthly（10-day）discharge；for non-regulating or daily regulating hydropower stations，the computation may be made by adopting the mean daily discharge in long series or the mean daily discharge in a typical year according to data availability.Three representative years with high flow，normal flow and low flow may be selected as the typical year.Two representative years with moderatelyhigher or moderately-lower than normal years may also be added.

4.2.5　If，upstream or downstream of the hydropower station in design，water and hydropower projects have been built or are planned to be built in the design level year，the computation of runoff regulation shall be made for cascade hydropower stations.

4.2.6　The firm output shall be defined based on the curve of output reliability plotted from the runoff regulation computation，and then determined by the selected design reliability.

4.2.7　The long-series annual electricity production or the average value in a typical year may be adopted as the mean annual electricity production of the hydropower station.