Low Flow Estimation in the United Kingdom (1992)(en)(292s) by Gustard A., Bullock A.

By Gustard A., Bullock A.

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By Gustard A., Bullock A.

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The sensitivity of the derived BFI to this simulated error was then examined. The main results were that the percentage error in BFI was less than a given percentage error in flow data and the error was largest for catchments with low BFI values. 9) catchments. A similar sensitivity analysis was carried out to evaluate the impact of missing data on 24 calculated values of BFI. The value of BFI was shown to be sensitive to missing data primarily because several days of data are omitted from the base flow separation as a result of only 1 missing day of gauged data.

Losses are estimated from PE by Losses = r * PE where the adjustment factor r is a function of catchment rainfall. 1 presents values of r for given values of SAAR from the Low Flow Studies Report (Institute of Hydrology, 1980), which were derived from stations with more than ten years of gauged data and by consideration of the evaporation process. 0 for SAAR = > 1100 mm The value of r increases with SAAR and hence increasing water availability. For catchments with average annual rainfall in excess of 110Omm it is assumed that actual evaporation is equal to potential as a result of relatively short periods when evaporation will be limited by soil moisture deficit.

Calculation of the bias, B, for the revised water balance methodology, established that the underestimation of mean flow for low SAAR values, and overestimation in the range of SAAR between 800mm - I1OOmm, has been reduced in comparison with the 1980 methodology, although not eliminated. 2 compares the factorial standard error for subsets of area and rainfall between the two calibrations of the catchment water balance method, and illustrates that in general errors are higher for small, dry catchments but these errors are reduced by the new calibration.

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