(occupational tax enacted via Nebraska Law Bill 701 constitutional; tax constitutes an excise tax levied on irrigation activities rather than a property tax; legislation underlying tax arose out of Republican River Basis compact with states of Kansas and Colorado in attempt to provide cash fund to help state comply with compact, and initial version of tax enacted in 2007 as a property tax was declared unconstitutional; while case on appeal, state Natural Resource Districts adopted occupation tax under different provision of same legislation to provide funds to comply with compact; occupation tax provision determined to be excise tax on irrigation activity and applied to small portion of land that had been subject to property tax that had been declared unconstitutional and was not tied to land's value and could be avoided by landowner certifying that land not irrigated).