At 6 a.m. today, a long line of dump trucks will begin forming at Capital Centre, bringing in more than 1,500 tons of dirt to form the floor for the Washington International Horse Show, which starts an eight-day run beginning at 5 p.m.

The floor of the Centre will be lined with 10 inches of sand and clay on which top horses and riders from around the world will vie for some $100,000 in prize money.

Canadian John Simpson on Texas, his California-bred quarterhorse, winner of last year's President's Cup, will head the list of entries in Olympic-style jumping. Simpson, who won the high jump class here in 1979, is also slated to ride Dallas. Joining him on the Canadian team will be Michel Vailancourt, Mark Laskin and Ian Millar on Bandit.

Representing Great Britian will be the father and son duo of Harvey and Robert Smith, who will share the riding on four different horses: Sanyo Hifi, Caldene Cresta Video, Alamamo Sanyo Tone and Sanyo Music Centre. Tim Grubb and Anne Smith (no relation) will also ride for Great Britain.

The four riders from Switzerland will be Walter Gabathuler, Gerhand Etter, Thomas Suchas and Kurt Naeder.

Michael Matz will lead the United States equestrianteam on Jet Run, winner of the gold medal at the Pan Am Games in Puerto Rico, and winner of a bronze medal in Mexico in 1975. Melanie Smith and Val de Loire, Grand Prix horse and rider of the year in 1978, will be joined by Norman dello Joio and Peter Leone.

Show veteran Rodney Jenkins, who for years has thrilled horse show spectators with his unequaled style, will ride Prime Time and Second Balcony, winner of the Upperville Grand Prix this summer.

In addition to the international jumping each day and evening, the world famous Budweiser Clydesdales -- the gentle giants of the horse world -- and the graceful Tempel Lipizzan Stallions will be at the show.

Tonight's attraction will be "Western Night," with a Texas-style rodeo, barrell racing and pole-bending part of the action.