SAAR stands for Standard Annual Average Rainfall and describes the total annual rainfall which can be expected at a particular location in the UK. It was first developed as part of the Flood Studies Report and an updated Wallingford map showing values for any location in the UK was included in the Wallingford Procedure Volume 4. The full version of this map can be seen in the Wallingford Procedure and is included in the CivilWeb Rainfall & Runuoff Ultimate Guide which is provided free with the CivilWeb Rainfall & Runoff Calculation Spreadsheet.

Applications of SAAR

The SAAR value is used in several different runoff calculation methods including IH 124, ADAS 345 and the Wallingford Procedure as it gives an estimation of the general wetness of the catchment.

In the Wallingford Procedure the SAAR value is used to calculate the Urban Catchment Wetness Index (UCWI). This corrects the runoff coefficient calculation to allow for the likelihood of the catchment being wet already from previous rainfall.

In the related IH 124 and ADAS 345 calculation methods the SAAR value is used directly in the runoff coefficient calculations to account for the likelihood of a storm event occurring when the soil is already wet.

SAAR in the CivilWeb Spreadsheets

The CivilWeb Rainfall & Runoff spreadsheet includes SAAR information for 250 towns and cities across the UK. Alternatively, the SAAR value for any location in the UK can be obtained from the Wallingford Procedure maps. This map is reproduced in the CivilWeb Ultimate Guide.