Turf Grass in Calculations

You are correct - turf grass is more water hungry than other types of vegetation, and it would be a good idea to account for this. I think that you can use still use the "mixed" vegetation by accounting for the turf in the Species Factor (Ks) you choose. See page 185 of the Reference Guide for the chart with Species Factor choices (Low, Average, or High). "Turf grass" independently has a higher species factor than "mixed", so if using "mixed" that includes turf be sure to choose either the Average or High coefficient for the Species Factor based on how much turf there is in comparison to other vegetation.