2.2　Runoff

2.2.1　Computation of runoff shall provide all or part of the following information at dam site:

1　Annual，monthly，10-day and daily runoff series，and mean annual runoff.

2　Annual and periodic runoff frequency.

3　Intra-annual distribution of runoff in typical design year，and duration curve of mean daily runoff，etc.

2.2.2　Design runoff shall be calculated by the following methods based on the data available:

1　If the consecutive data of runoff series（including extrapolation）is available for more than 30 years for the dam site，the design annual and periodic runoff shall be calculated directly by the method of frequency computation.

2　If runoff data for dam site is available for less than 30 years，but there is more than 30 years of runoff data（including extrapolation）for the upper，lower or neighboring basins，the design runoff of a reference station in the above-mentioned basins may be applied to the dam site after being revised based on the differences at catchment area，precipitation and underlying surface condition.

3　If there is no runoff data in the design river basin while the precipitation data is available，the runoff may be derived by analyzing the relationship between the precipitation and runoff at a hydrometrical gauging station in a similar river basin nearby.

4　If none of the above-mentioned data is available，the regional comprehensive method may be used for the computation of design runoff.

2.2.3　If the runoff is affected by human activities in the basin upstream of the cross-section of the design runoff，the extent of impact shall be investigated and analyzed，and restoration computation shall be done for the runoff data.If restored flow data is insufficient，the computation of design runoff may be done by analyzing the direct statistics of actually measured runoff series that have been impacted by human activities，or by the runoff computation method in case of data shortage.

If the actually measured runoff data is insufficient，the method such as correlation analysis may be used for runoff series extrapolation.

If correlation analysis is used for runoff series extrapolation，the correlation coefficient should be larger than 0.8.The years for extrapolation series should not exceed the years of series used in the correlation analysis.

2.2.4　The time period for runoff computation may be selected as annually or periodically（for non-flood season or dry season）according to the design requirements.In n consecutive runoff series，the mth empirical frequency in descending order，i.e.Pm，shall be calculated by the formula below:

The Pearson TypeⅢfrequency curve may be adopted，and the statistical parameters may be estimated initially by using the moment method and then adjusted and defined by the curve fitting method.

2.2.5　If the runoff computation is made by using regional comprehensive method，the regional precipitation runoff approved by the competent authorities，isoline map of statistical parameter or empirical formula for runoff computation shall be used.

2.2.6　With regard to the selected annual runoff series，their representativeness shall be analyzed by comparing the statistical parameters of long and short series on the basis of data gathered by regional hydrometric gauging stations and precipitation gauging stations.

2.2.7　With regard to the intra-annual distribution of runoff in the typical design year，the actual year with its empirical frequency of annual and periodic runoff similar with the design frequency may be selected as the typical year，and then amended and determined by using the design runoff.

If the actual measurement data is limited，the annual distribution of runoff in the typical design year may be derived by available regional integrated charts of runoff.

2.2.8　If a river where the hydropower station is located has special hydrological and geological conditions，their impact on the design runoff shall be analyzed and studied.

2.2.9　In calculation of mean daily discharge duration curve，the following methods may be used depending on data available:

1　Statistics by sorting the mean daily discharges in three typical years with high flow，normal flow and low flow.

2　Adjusted mean daily discharge duration curve at a reference station calculated based on catchment area and precipitation of the site of hydropower station.

2.2.10　The result of runoff analysis and computation shall be compared with the computation results of upstream，downstream，main stream and tributaries，and adjacent basins so as to analyze and check its rationality.