Even multiplied by 40 million people, the amount of water used by homes amounts to relatively little. Farmland uses three to four times more than people use for cooking, drinking, washing, gardening and landscaping.
California’s farmers and ranchers irrigate their land with about 30 million acre-feet of water each year (mostly from surface supplies but also from underground). Residential water use runs about 8 million acre-feet per year.
Another 40 million acre-feet or so flows through river systems, supporting wildlife. That’s on average, spiking in wet years and ebbing during droughts.