FEH Unit Hydrograph

The following Input Variables are used in the FEH Unit Hydrograph.

Note: Most of these values can be imported from data files obtained from the web service or FEH CD-ROM software package via the Import button. However on small catchments it is particularly important to check them against a site survey to verify the data where possible.  

Note: This method may not be available for your region. For more information, refer to the Regionalization topic.

Version -  Only files containing 1999 data will be loaded and only 1999 can be selected.

Site Location - This descriptive field allows the site location to be recorded.  If the FEH details are imported from a .csv or .xml file then this field is filled in with the grid reference details.

Areal Reduction Factor - Reduces the rainfall as it calculated as point rainfall based on time and area. The value may be obtained from FEH Figure 4.3.4.

SAAR - The standard average annual rainfall (1961-1990). This can be determined by clicking on the 'Map' button and selecting the location.

CWI - The catchment wetness index.

SPRHOST - is calculated by the FEH CD program and written to the .CSV file. Where measured flow records are available the SPR may be calculated from the Base Flow Index (BFI) from research published in FSSR 16 and it is not covered here – see also FEH, Volume 4, Chapter 1.

DPSBAR - The mean catchment slope.  Mean of all inter-nodal slopes.

DPLBAR - Mean drainage path length.  This is the mean of the distances between each node (on a 50m grid) and the catchment outlet.

PROPWET - The proportion of time when the soil moisture deficit <= 6mm during 1961-1990 as defined by Met Office Rainfall and Evaporation Calculation System (MORECS).

URBEXT - The extent of urban and suburban cover.  Either select the 1990 or 2000 values if available or simply enter a user-defined value.  Please note that the year 2000 values are only available if using the FEH CD-ROM Version 2.  If values for 1990 or 2000 are zero, please check the values as they may not be available from the FEH data.

Base Flow Details

LAG - This value can be used in lieu of DPSBAR, DPLBAR, Urban and PROPWET to calculate the time to peak TP0.  It is defined as the time delay between the centroid of a hyetograph and the resulting peak flow at the catchment outfall.

Note: This may be appropriate if measured lag values are available or if the engineer estimates that the calculated time to peak is in error.  This may be the case, for example, if the ditches have been improved or piped.

Base Flow - This value replaces the calculated baseflow value to allow a user defined baseflow.  The baseflow calculation can be switched off by entering a value of "0 ", it should be left blank in order for the software to calculate it.