Depletion can be defined a couple of different ways. The 27.50% you refer to is Texas State Statute for the Trust Principal and Income Act. This is the portion of each royalty check assignable to the principal of a trust offsetting the decline in value of a depleting asset. Secondly, the IRS Tax Code also has a tax deduction called depletion tha can be in the form of percentage depletion. at 15% or cost depletion. When filing your 1040 this number will be illustrated on Schedule E. The concept is again due to a depleting asset.