For the purpose of providing for United States reclamation projects a feasible and comprehensive plan for an economical and equitable treatment of repayment problems and for variable payments of construction charges which can be met regularly and fully from year to year during periods of decline in agricultural income and unsatisfactory conditions of agriculture as well as during periods of prosperity and good prices for agricultural products, and which will protect adequately the financial interest of the United States in said projects, obligations to pay construction charges may be revised or undertaken pursuant to the provisions of this subchapter.