The Federal Water Law 9433 enacted in 1997 gave the legal frame relative of the water usage charge. The aim of this paper is to interpret legal terms through the emergy environmental accounting approach in order to establish the donor-side costs related to water usage under the user-pays and polluter-pays principles. The procedure was performed for agricultural activities at the Jundiai-Mirim Basin, located at São Paulo State, Brazil. The proposed procedure resulted in costs comparable to those already implemented at the watershed under study by using other economical procedures.