Put chickpeas, 1/4 cup each chickpea liquid and water, oil, tahini, 1 tsp. sumac, salt, lime juice, and garlic in a food processor. Whirl until smooth, scraping sides of bowl periodically, about 2 minutes. Add more chickpea liquid if you like your hummus looser and creamier. Just before serving, swirl in remaining 1/2 tsp. sumac.