On April 24, 2003, at approximately 0930 mountain daylight time, an Air Tractor AT-502, N318BR, was substantially damaged when it collided with an irrigation system during a forced landing near Sedan, New Mexico. The commercial pilot received minor injuries. The airplane was being operated by Riddell Flying Service Inc., of West Helena, Arkansas, under the provisions of Title 14 CFR Part 137. Visual meteorological conditions prevailed for the local aerial application flight that originated from a private airstrip near Dalhart, Texas, at approximately 0910. No flight plan had been filed. Prior to departure, the airplane was serviced with fuel and chemicals. The pilot reported that after "about one hour and forty-five minutes [a very slow fuel transfer rate]" both fuel tanks were filled to capacity. He said the fuel was "flowing extremely slow." The pilot stated he drained 2 to 3 gallons of fuel out of each tank to check for contamination before departing. While descending from 1,200 feet to 150 feet agl, the pilot looked down at a map on his lap to verify which field was supposed to be sprayed. The pilot said the engine (a Walther STC conversion) began losing power, dropping from 80 pounds of torque to 40 pounds of torque. He noticed the loss of power when he attempted to level off, and the airplane was near a stall. He dumped the chemicals, and pushed the power lever full forward, with no increase in power noted. The pilot performed a forced landing straight ahead due to his low altitude, and impacted a rotating irrigation system in a wheat field. The top of the hopper was crushed, and the canopy and its supporting structure were destroyed. The pilot received facial cuts, shoulder abrasions, and a broken thumb. Postimpact examination revealed water in the engine fuel filters and in the refueling system. An examination of the airplane systems revealed no anomalies. A few days after the accident, the manager of Air Service went to pump off the Jet A from the fuel tanks in the airplane, and he found approximately 6 ounces of water in the right fuel tank. He also checked the tank that the airplane was fueled from, and he found approximately 5 inches of water in that tank. The engine was test run at the operators facilities in Helena, Arkansas, and no abnormalities were noted.