Assessments made in order to carry out the purposes of any general improvement district or of any divisional district, authorized in this chapter, shall be made annually on an ad valorem basis against the lands and improvements thereon, included within the operation of any such district; PROVIDED, That in assessing lands having and using a water right independent of the district system, the value of such water right shall be deducted from the assessable value of said lands.
RCW 89.30.571