Check your Acceptable Solutions and Verification Methods E1 Surface water document has the correct formula, as provided here and in the online document.
This information was confirmed as current in February 2016. It originally appeared in Codewords newsletters prior to January 2014.
Some printed versions of Acceptable Solutions and Verification Methods E1 Surface water have a printing error in the following formula, which appears in Paragraph 2.1.1 for calculating surface water run-off:
Unlike the correct version above, in some printed versions of the document the summation sign was inadvertently omitted.
If you have a hard copy of the document, check the formula and if necessary add the summation sign as given above.
This erratum was first notified in May 2009.