For each taxable year through 2020, the authority may levy a tax of not to exceed one mill annually on each dollar of taxable valuation within the boundaries of the authority for the payment of administrative expenses of the authority, including per diem, mileage, and other expenses of directors, expenses of operating the office, engineering, surveying, investigations, legal, administrative, clerical, and other related expenses of the authority. All moneys collected pursuant to the levy must be deposited to the credit of the authority and may be disbursed only as herein provided. The board may invest any funds on hand, not needed for immediate disbursement or which are held in reserve for future payments, in bonds of the United States, certificates of deposit guaranteed or insured by the United States or an instrumentality or agency thereof, and bonds or certificates of indebtedness of the state of North Dakota or any of its political subdivisions. During the period of time in which the authority may levy one mill annually as provided herein, any joint water resource board created pursuant to section 61-16.1-11, by or among one or more of the water resource districts in the counties which are included in the authority, must be limited to one mill under the authority of section 61-16.1-11.