(b) By any organization, whether incorporated or unincorporated, heretofore organized or which shall hereafter be organized, for the operation, maintenance, or management of an irrigation project or irrigation works or system and for the purpose of furnishing water to landowners, members or shareholders, the control of which is actually vested in those entitled to the use of the water from such irrigation works or system for the irrigation of lands to which the water from such irrigation works or system is appurtenant, including all title and interest in such property as owner, lessee, or otherwise. Provided, that if any portion of such property is used for commercial purposes by others than its landowners, members or shareholders, the assessor shall determine the entire value of such portion of the property so used and assess the proportionate part of the property that is used for commercial purposes.