The Treaty rate for Real Property is 30%.
You should be filing a US tax return each year - 1040NR
Income from Real Property
1. Income derived by a resident of a Contracting State from real property (including income from agriculture, forestry or other natural resources) situated in the other Contracting State may be taxed in that other State.
2. For the purposes of this Convention, the term "real property" shall have the meaning which it has under the taxation laws of the Contracting State in which the property in question is situated and shall include any option or similar right in respect thereof. The term shall in any case include usufruct of real property, rights to explore for or to exploit mineral deposits, sources and other natural resources and rights to amounts computed by reference to the amount or value of production from such resources; ships and aircraft shall not be regarded as real property.
3. The provisions of paragraph 1 shall apply to income derived from the direct use, letting or use in any other form of real property and to income from the alienation of such property.