Hi @Icey -
Yes, you may use PET data for Annual Water Yield, the model will calculate actual ET, using PET and the evapotranspiration coefficients you provide for each land cover type.
Using this kind of simple model, we do often see greater water yield in urban areas, mostly due to there being less plants to take up some of the water. So we advise caution when interpreting the results, since it can look like “if we want water, cut down the forest and build a city”, which obviously does not take into account the complexities of infiltration, dry season baseflow, etc, that are more likely to be provided by natural vegetation.
You can provide spatial data with different resolutions, and in the case of Annual Water Yield, the model will resample all of your layers to be the same resolution as your land cover map. So you do not need to make all of the data the same resolution, but personally I usually prefer to, so I know that the resampled rasters look correct.
~ Stacie