(2) As used in this section, "purpose" means the type of service to be performed by the district, or subdistrict, as set forth in ORS 553.020 (1) and (2). When the construction of an improvement serves more than one purpose, the cost of construction or the cost of maintenance shall be allocated between the two or more purposes on the basis of engineering studies. [1965 c.623 §16; 1969 c.691 §9]