A method and system for computing a stable index value of fresh water based on water quality category and/or grade from current values of root commodities (i.e. freely marketed products) that either have fresh water as a constituent and/or utilize fresh water in their production, usable to develop a stable investment instrument for investing in fresh water. A processor may process associations between a root commodity and a water quality category of fresh water. A display associated with a computer may display updateable current prices of fresh water of a particular water quality category and/or grade and updateable current prices of one or more root commodities. A water measurement device may measure at least one water source of fresh water with respect to at least one of TDS, carbonate content, pH, hardness, sodium, chloride, nitrate, toxins and sodium adsorption ratio and compare against standards to categorize the fresh water.