Title 42, Article 5, of the Arizona Revised Statutes provides the method for valuing pipeline property. The statutes require the Department to determine the value of property owned by each pipeline operating within state borders. A.R.S. 42-14201 to -14204. The Department begins the process by mailing all pipeline companies an annual property tax reporting form. A.R.S. 42-14202. Each company must submit the completed form to the Department on or before April 1 of the valuation year. A.R.S. 42-14202(A). Pipeline companies must report the cost of all their property as well as income and other information. A.R.S. 42-14202, -14204. In A.R.S. 42-14204(F)(1), the legislature directs the Department to determine the base value of the pipeline, which is "the final full cash value of the system plant in service." A.R.S. 42-14204(H)(3). The Department must also compute the value change factor - "the average of the income change factor and the asset change factor" - then multiply that figure by the base value to obtain the "preliminary system value." A.R.S. 42-14204(F)(2), (3) and (H)(15). To complete the valuation, the Department adds to the preliminary system value the cost of materials and supplies, leased property, gas stored underground and the construction work in progress. A.R.S. 42-14204(F)(4).