drainage auto-sizing-levelling and simulation toolbar,
click the 
Greenfield runoff button.
Calculate the greenfield runoff rate of a proposed site based on the soil type and hydrological region. This provides an estimation of Qbar calculated using the IH124 method. The Qbar values represents the peak flow rate of rainfall over an area of land for a 1 year return period storm.
On the drainage auto-sizing-levelling and simulation toolbar,
click the Greenfield runoff button.
After clicking on the Greenfield runoff button you will see the following window appear:
If you know the plan surface area of your site, you can type in the value in the Total Area box. If you don't know the area, you can calculate it by clicking the Pick... button and digitise a site boundary line. After you have drawn the site boundary line, right click and choose Finish to return back to the Quick Storage Estimate window.
These values are determined based on the site coordinates, clicking a point on the UK map will update these values based on the new location. Or you can manually specify them by clicking in the boxes and changing the values.
The Hydrological region index is from FSSR 14 and is in the range [1 to 13]. The Soil Type is the WRAP map of SOIL type from the Wallingford Procedure and is in the range [1 to 5]. The Standard Annual Average Rainfall (SAAR) is the average annual rainfall in mm, usually obtained from Flood Studies Report.
This value is automatically calculated based on the site details and hydrological region and soil type values.
If you manually click a new location on plan, the Reset Position to Site Extent button will appear at the bottom. If clicked, this will reset the position on the map to the current site position.