A State tax imposed on the severing of natural resources from the land based on the value or quantity of production. These types of taxes are usually calculated either as a flat rate per unit of production (sometimes called a "unit" or "specific" severance tax), or as a percentage of the value of the resource produced (sometimes called an "ad valorem" or "percentage" severance tax). The tax base for an ad valorem severance tax is generally either the gross or net value of resources produced or sold.