“A major reason for the decline is the upstream water diversion for agriculture and households,” Hall said. Populations in the states that rely on Great Basin water supplies have grown by 6% to 18% since 2010, according to the U.S. Census Bureau. “As the population increases, so does water use.”
There's a finite amount that can be done about agriculture, though you can do covered agriculture.
But we still have lawns, which is a significant chunk of water use.
Way back when the British colonists showed up in North America, they brought with them the tradition of the grass lawn. That was predicated on an England-like climate. That kind of works on the East Coast, but is a terrible mismatch for the American Southwest.
https://en.wikipedia.org/wiki/Lawn
It was not until the 17th and 18th century that the garden and the lawn became a place created first as walkways and social areas. They were made up of meadow plants, such as camomile, a particular favourite. In the early 17th century, the Jacobean epoch of gardening began; during this period, the closely cut "English" lawn was born. By the end of this period, the English lawn was a symbol of status of the aristocracy and gentry.
Wealthy families in America during the late 18th century also began mimicking English landscaping styles. British settlers in North America imported an affinity for landscapes in the style of the English lawn.
Before the mechanical lawn mower, the upkeep of lawns was possible only for the extremely wealthy estates and manor houses of the aristocracy. Labor-intensive methods of scything and shearing the grass were required to maintain the lawn in its correct state, and most of the land in England was required for more functional, agricultural purposes.
This all changed with the invention of the lawn mower by Edwin Beard Budding in 1830.
Lawns began to proliferate in America from the 1870s onwards. As more plants were introduced from Europe, lawns became smaller as they were filled with flower beds, perennials, sculptures, and water features. Eventually the wealthy began to move away from the cities into new suburban communities. In 1856, an architectural book was published to accompany the development of the new suburbia that placed importance on the availability of a grassy space for children to play on and a space to grow fruits and vegetables that further imbued the lawn with cultural importance. Lawns began making more appearances in development plans, magazine articles, and catalogs. The lawn became less associated with being a status symbol, instead giving way to a landscape aesthetic. Improvements in the lawn mower and water supply enabled the spread of lawn culture from the Northeast to the South, where the grass grew more poorly.
Maintaining a green lawn sometimes requires large amounts of water. While natural rainfall is usually sufficient to maintain a lawn's health in the temperate British Isles- the birthplace of the concept of the lawn- in times of drought hosepipe bans may be implemented by the water suppliers. Conversely, exportation of the lawn ideal to more arid regions (e.g. U.S. Southwest and Australia) strains water supply systems when water supplies are already scarce. This necessitates upgrades to larger, more environmentally invasive equipment to deal with increased demand due to lawn watering. Grass typically goes dormant during periods of cold or heat outside of its preferred temperature ranges; dormancy reduces the grasses' water demand. Most grasses typically recover quite well from a drought, but many property owners become concerned about the brown appearance and increase watering during the summer months. Water in Australia observed 1995 data that up to 90% of the water used in Canberra during summer drought periods was used for watering lawns.
In the United States, 50 to 70% of residential water is used for landscaping, with most used to water lawns. A 2005 NASA study estimated conservatively 128,000 square kilometres (49,000 sq mi; 32,000,000 acres) of irrigated lawn in the US, three times the area of irrigated corn. That translates to about 200 US gallons (760 L; 170 imp gal) of drinking-quality fresh water per person per day is required to keep up United States' lawn surface area.
In 2022, the state of Nevada pass a bill that not only banned the installation of new lawns in the state, but also mandated the removal of any lawn deemed "nonfunctional." This was in response to a years-long drought in the state.
Having lawns in the middle of the desert kinda requires pulling enough water out of aquifers and constantly dumping it on the ground to produce an English biome, which isn't really sustainable -- the aquifers recharge much more slowly than our present rate of extraction.
Maybe it's possible to do something like mass desalination and transport from the ocean, but that's gonna cost more...and even if we want to do that, there probably still has to be a reduction in lawn area at some point.