Diurnal variation of the electrostatic potential of the rings of Saturn is considered by examining a wie range of possible combinations of plasma sources and sinks. It is shown that because of the presence of the ring absorption effect, the ambient thermal plasma density is kept very low (nˆ≲10-2 cm-3). If no trapping or thermalization of the photoelectrons, ionospheric plasma, and impact plasma generated by meteoroid impact occurs, the ring surface potential could vary between 4-8 V (dayside and nightside) and -7 V (shadow cone). The operation of trapping the field-aligned plasma flow changes the time variation of the ring surface potential only slightly. It is the variations of the peak ionospheric plasma flux and of the impact plasma which have more significant effect. The impact plasma, in particular, can change the ring surface potential from a value of -7.3 V to a small but positive value if its flux reaches ~106 ions cm-2 s-1.