Question about ERA5 volumetric soil water data


I have a question about the volumetric soil water data from ERA5 reanalysis datasets . I wonder if someone could help answer whether the volumetric soil water parameter from ERA5 is calculated as water volume / total volume or water volume / (total volume * porosity). 

The reason I want to know is that I hope to use ERA5 volumetric soil water data to calculate the water table depth at each soil layer. I am not sure which function below should be the correct one if I use ERA5 volumetric soil water data: 

1) water_table_depth = soil_layer_depth * volumetric_soil_water 

2) water_table_depth = soil_layer_depth * volumetric_soil_water / porosity

Thank you!