Preheat oven to 450 degrees. Place carrots and cauliflower on a rimmed baking sheet; toss with cumin and 2 tablespoons oil. Season with salt and pepper. Spread half the vegetables on a second baking sheet. Roast until browned and tender, 25 to 30 minutes, rotating sheets and tossing halfway through. Cool to room temperature.
Meanwhile, in a medium saucepan, bring 1 1/4 cups salted water to a boil. Stir in couscous; cover and remove from heat. Let stand until tender, 5 minutes. Fluff with a fork; set aside to cool, uncovered.
Make dressing: In a small bowl, whisk together lemon zest and juice and remaining tablespoon oil; season with salt and pepper.
In a large bowl, combine roasted vegetables with couscous, chickpeas, and scallions. Place arugula on a serving platter, and drizzle with 1 tablespoon dressing. Add remaining dressing to couscous mixture, and toss; serve over arugula.