Hi @fipsi2000 -
Is wyield_vol from the aggregated watershed results? Is the value of 0.02 m3 for the whole watershed, or a sub-watershed? Are your inputs in a projected coordinate system (with linear units of meters) or geographic coordinate system (units of degrees)? They should be in a projected coordinate system, or the calculations for volume will be weird (and probably very small) when trying to calculate volume over degrees instead of meters.
wyield_mn is the mean per-pixel value within the watershed/sub-watershed. When you say that it’s very different from wyield.tif, did you use Zonal Statistics to calculate the mean wyield.tif value within the watershed/sub-watersheds and found it to be different than what the model output?