# Naturalising the gauged FDC's

The naturalisation of the gauged FDC data set is achieved by simply applying the following equation to each of the 12 months (m) 101 percentile values (x) of the gauged FDCs:

*Where* the following represents the total impact within the catchment, nett of any impact located above any impounding reservoirs :
- SWABS = surface water abstractions (this will be a negative value).
- GWABS = stream depletion for groundwater abstractions (this will be a negative value).
- DIS = discharges (this will be a positive value).
- ComInf = sum of complex influence monthly set of 1212 ComInf_{(x,m)} values, which have been interpolated within Qube from 26 annual IDC values (this may be a combination positive and/or negative values).

* And where* ImpInf = impact at the most downstream impounding reservoir(s) (this may be a positive or negative value).

The nett impact at the dam of an impounding reservoir is calculated as:

*Where:*

- Implnf_{(x,m)} = the nett impact of the reservoir at the dam location (this will generally be negative although may be positive at low flows if a regulating reservoir or if the compensation flows are higher than the natural flows).
- Q_{NatImp_{(x,m)}} = natural flow estimated by Qube for the reservoired catchment.
- RDC_{(x,m)} = release duration curve for the reservoir.

This yields a set of naturalised monthly flow duration curves and influence components. The naturalised annual FDC and influence components are estimated in the same way as for the artificially influenced annual FDC.