Cal. Health & Safety Code § 13004

Every person is guilty of a misdemeanor who harvests grain or causes it to be harvested by means of a combined harvester, header, or stationary threshing machine, or who bales hay by means of a hay press, or harvests by means of a mechanical harvester other agricultural crops which are flammable at the time of harvest, unless he keeps at all times in a convenient place upon each machine or press, one backpack or pump-type water extinguisher of not less than four-gallon capacity fully equipped, filled with water and ready for immediate use.