For Phoenix, it’s water is supplied through surface water sources. The northern end of the city gets water from the Central Arizona Project, which is supplied by the Colorado River and Lake Mead. The southern end gets its water from the Salt River Project.
Las Vegas gets 90% of its water from the Colorado River and Lake Mead and 10% from groundwater sources. The groundwater supplies up to 25% during the hot summer months when demand is high. In 1971 Las Vegas with a population of 126,000 residents began getting water from Lake Mead.
Today, Las Vegas has swelled to 2 million residents and increases on weekends by an additional 3 million people. Las Vegas gets a 300,000-acre feet allotment from the Colorado River each year. They only use about 240,000-acre feet and bank the rest in the groundwater table.