Calculations have shown that electrohydrodynamic instabilities can produce surface distortions that may locally enhance the electric field to the extent that field evaporation of ions can occur from a liquid metal surface. Linear analysis of the electrohydrodynamic equations of motion has yielded the regions of instability as well as the wavelengths and growth times of the dominant unstable mode as a function of applied electric field. In the calculation presented here, the liquid surface is followed well into the nonlinear regime using a surface integral method. The results agree with the linear theory where appropriate, and add further support to the feasibility of a large-area liquid-lithium ion source for inertial confinement fusion experiments on a light ion-beam driver.