Expenditures shall not be made for carrying out the purposes of the reclamation law except out of appropriations made annually by Congress therefor, and there shall annually, in the Budget, be submitted to Congress estimates of the amount of money necessary to be expended for carrying out any or all of the purposes authorized by the reclamation law, including the extension and completion of existing projects and units thereof and the construction of new projects. The annual appropriations made hereunder by Congress for such purposes shall be paid out of the reclamation fund provided for by the reclamation law.