For the purpose of the care, operation, management, or improvement of such portions of the project as are in use, including salaries of officers and employees, and all other operating expenses, the board may in lieu (either in part or in whole) of levying assessments as in this chapter provided for, fix rates of tolls and charges for irrigation or available irrigation and other services rendered by the district, including the use of ground water, and collect the same from all persons receiving the benefit of the irrigation or other services. The tolls and charges shall be proportional as nearly as possible to the service rendered.