Amodel of the erosion cycle shows what

Well, assuming we're talking 3D model here, you'd definitely have water erosion within this model. You'd have to show runoff with water as well as snow depending on what environment you use. You could do the same with frost, but I'm not sure how you'd be able to show wind without just putting arrows as a direction mechanism.

Now, if we're talking about a "cycle" of erosion, you'd need to show precipitation occurring and then a landslide/runoff from that water. You could also show snow, ice, or wind. To end the cycle, you'd have to show that the runoff is traveled through streams to a larger body of water like an ocean. Then, of course, evaporation and transpiration would be shown. This is more of a hydrologic cycle, but it deals with erosion as well.

If this isn't the answer you were looking for, I'm terribly sorry. :(