Katie Monahan, the last to go in an eight-rider timed jump-off last night, won the $35,000 International Open Jumper President's Cup, final event of the Washington International Horse Show at Capital Centre.

Monahan, 28, of Upperville, Va., rode Noren, a 9-year-old French-bred thoroughbred, over the seven-jump course in 35.8 seconds and earned $10,500 first-place money. It was her fourth victory of the week and led the United States Equestrian Team to a clean sweep in the team championship over Switzerland, Canada and France. Monahan won the opening night event, the pair relay and the match race.

Monahan's earnings during the eight-day show, first of the final three on the indoor circuit, were $14,250, which established her as the leading female rider of the show. For the season, she has won $135,000 in merchandise and money, including two Mercedes cars and a Rolex watch.

Monahan, who describes this season as her best ever, earned the International rider championship. She is currently leading the American Grand Prix rider of the year standings with earnings of $45,433, not including this week's take. Noren, owned by the Plain Bay Farm Ltd. partnership, meanwhile clinched horse of the year honors with last night's accomplishment.

Close behind on Calypso was Melanie Smith, last year's winner of this class.

Smith, 33, of Litchfield, Conn., the fifth to go in the jump-off, finished in a time of 36.1, good enough for second place.

Third place went to the only other rider to go clean in the final phase of the event, Kevin Maloney on Turf Fire, who went first in the timed jump-off. He took 39.3 seconds.

Maloney, 33, of Warrenton, Va., was honored as the Insilco amateur owner jumper champion of the year. Maloney led the amateur competition throughout the year on his other horse, Boing.