Between the first Monday in March and the first Monday in June of each year, the district assessor shall examine each tract of land or legal subdivision of land in the district, including entered and unentered public lands of the United States, subject to entry under any act of Congress, and all other lands publicly or privately owned. In determining irrigation or drainage benefits, the district assessor shall first ascertain the number of irrigable or drained acres [hectares] within each tract or subdivision, as determined by the board of directors, and this must form the primary basis for determining the benefits accruing due to construction, acquisition, or operation of irrigation or drainage works. In addition, the district assessor shall include such factors as methods of irrigation, power consumption, water conservation, and whether or not irrigable acres [hectares] are actually being irrigated. The amount of benefits apportioned or distributed to each tract of land as determined and equalized must be the basis for fixing the annual assessments levied during that year against the tracts or subdivisions in carrying out the provisions of this chapter.