Input Variables

Most of these values can be imported from data files obtained from the web service or FEH CD-ROM software package via the on-screen ‘Import’ Button.

However on small catchments it is particularly important to check them against a site survey to verify the data where possible.  


Area

This is the catchment drainage area.


Version

Select either 1999 or 2013.  With the 1999 option the six site constants can be typed in.  2013 data can only be loaded in from a valid .csv or .xml file and displays "Point" or "Catchment" based on the data in the file.  If a loaded file does not contain 2013 data then the option is disabled.


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.


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.


Baseflow

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.