The pilot of the tailwheel equipped airplane reported that, while in cruse flight, about 1,200 to 1,500 ft above ground level (agl), near the Pacific coast, in visible moisture, the engine began to run rough. He applied full throttle and full carburetor heat. The engine initially regained power but again began to lose power at a progressively worsening rate. Concerned that he could not continue to the nearest airport he elected to land on a field. During the landing roll, the main landing gear impacted an irrigation line and the airplane nosed over coming to rest inverted. The rudder was substantially damaged. The pilot reported that there were no mechanical failures or malfunctions that would have precluded normal operation. Examination of the carburetor heat system by an airframe & powerplant mechanic revealed no mechanical anomalies. Weather conditions reported at the time of the accident were conducive for serious carburetor icing at any power setting.