Much of California’s water supply is a hidden asset: Deep below the surface, rocks, gravel and sand store water like a sponge, in an underground zone called an aquifer. 

In dry years, this groundwater has been tapped to save farms, keep grass green and provide drinking water to millions of Californians. But over time, people have taken more water out than nature has put back in. Estimates vary, but according to the U.S. Geological Survey, California pumped 41 trillion gallons of water fom the ground in about 100 years, through 2013. In some parts of the Central Valley, that means land has been dropping around a foot a year. 

The landmark Sustainable Groundwater Management Act, or SGMA, requires some of the state's thirstiest areas form local "Groundwater Sustainability Agencies" and submit long-term plans by Jan. 31 for keeping aquifers healthy. Together, those plans will add up to a big reveal, as groundwater managers finally disclose how badly they believe their aquifers are overdrawn, and a collective picture emerges. It's a major shift and arguably the most important new California water law in 50 years.

Here are some key things to know about the groundwater situation in California and how the law will impact the state. 

Until six years ago, California did not routinely regulate or monitor groundwater. 

The California Constitution decrees that water use has to be reasonable and beneficial, but the state has placed few limits on how water can be pumped from the ground. A 1914 law empowering the state water board to manage the resource omitted groundwater. You can blame the lack of regulation partly on 18th-century Spanish colonists who brought with them the idea that a landowner is entitled to all of the water below the surface, without any obligation to share it. 

At the beginning of the 20th century, water was still plentiful in California, and the idea of unfettered access to groundwater made sense in a state lush with wetlands. 

So for the last century, landowners continued to think of groundwater as pretty much a birthright. It’s become an essential component of California's water portfolio: State officials say 30 million residents rely on groundwater for at least some portion of their drinking supply, and in the driest years, people keep basically sticking a straw into the earth to slake their thirst. 

Read more of the full original article here.