The intense electric field beneath a thunderstorm often produces corona discharge from bushes and trees. The average steady state corona current per unit horizontal area, Jc, is approximately proportional to the time rate of change of the electric field at the ground when the value of the field changes sign following a lightning flash. This paper derives this relationship and proposes that the contribution of corona discharge to the charge budget of a thunderstorm be evaluated from measurements of the electric field at many sites below a thunderstorm rather than from direct measurement of the corona current.