Satellites can calculate groundwater disappearance by measuring a decrease in gravitational attraction. With less water in the ground, there is less mass and less attraction. The Colorado River Basin has lost 17 trillion gallons since 2004, mostly groundwater. Our water use is steadily growing, but groundwater losses take millennia to be restored by nature. Seventy percent of the basin's water supply goes to water-intensive agriculture, and much goes to population growth in cities. Solutions:

• municipal conservation
• reuse
• grey water treatment
• irrigation efficiency

For more information, read this Common Dreams article.