SURQ at watershed level

[Bassomrt]

I am looking for the amount of surface runoff at watershed level. 

What I'm looking for is the amount of runoff that is generated in the watershed. 

The model provide the variable SURQ in the std output file. 

The values present for the variable results equal or even smaller than the SURQ_GEN for the HRU and the SURQ of each subbasin. 

My doubt is the following: is the SURQ variable at the watershed level an average of the HRUs? If not, can somebody explain me why is not bigger?

Thanks,

Marta