Annual Water Yield Model - watershed

For the Annual Water Yield model, does the Watershed vector input have to be delineated from a DEM? Is a DEM needed for the model input? I assumed the watershed data input was a polygon?

I have spent a lot of time clipping, reclassifying and processing my land cover maps to a watershed boundary (polygon) I downloaded from the Environment Agency (UK). I only discovered that I may need a DEM by going through this forum. Unfortunately it did not say anywhere in the AWY model guidance that a DEM was needed (ctrl+F returned no mention of a DEM).