I have two rows with monthly rain run off per watershed as in the picture below.

I'd like to obtain the average runoff between may and august for basin 218187 and 218265 (note that basin 218187 will be 0 but the attribute table contains more rows). To do that I've created a new field but I'm struggling to write the code in the field calculator.

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You have to select "Update existing field" > AverageRO and then type the following into the expression field:

("runoff_may" + "runoff_jun" + "runoff_jul" + "runoff_aug")/4

This will give you the arithmetic mean of the runoff values.

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