Hi,
I think for -10% you can mutiply the rainfall values by 0.90 and for
+10% multiply them by 0.110.
You can change the weather files first through spreadsheets then copy
them in the weather directory or you can also use the Environment option
under the Treatment or even use the Sensitivity analysis option.If
you are using seasonal analysis for kind of mulitple run of the model I
go for the Tret./Environment option.
Regards
Mohammad Bannayan
Nottingham
UK