Over the last three years, California has been in the grips of the worst drought in at least 1,200 years. Reservoirs are shriveling. Crops are wilting in the fields. Cattle herds are thinning out.

California likely needs near-record amounts of rainfall this year to pull out of drought

Then, in December, the rain came: A weather phenomenon known as the "Pineapple Express" brought moisture all the way from Hawaii and dumped massive amounts of precipitation on the state. The San Francisco Bay Area got nearly 9 inches, and there were floods and landslides throughout northern California.

So how much did that help the drought?

Sadly, only a tiny bit. A recent analysis by Tom DiLiberto of NOAA explains that California's drought has receded somewhat, but 98 percent of the state is still in drought — and 32 percent still facing "exceptional" drought. Worse, it could take near-record amounts of rain this year to pull the state out of its drought:

December storms barely dented California's drought

The big problem here is that California is really, really dried out — and it takes a lot more than a few storms to fill the shortfall. Back in December, an analysis from NASA led by Jay Famiglietti estimated that the state needed nearly 11 trillion gallons of water to fill its rain deficit — a process that could take years.

Another way to look at this, as NOAA did, is that the state needs rainfall far, far above the historical average between now and September to even begin to make up the shortfall:

California needs near-record rain to lift its drought

On the left is a map showing how much rain the state would need — compared with what it gets normally — simply to bring total precipitation over the last four years up to the bottom 20 percent historically. (This is one key threshold the government uses to declare drought.) For this to happen, every part of the state would need more than 100 percent of normal precipitation. The San Joaquin Valley, a key agricultural area, would need a near-record 27.74 inches by September.

The map on the right shows what the state would need to bounce back to normal levels, or 50 percent of the historic threshold for precipitation over a four-year period. Getting here would require record amounts of precipitation in multiple parts of the state this year. The San Joaquin Valley would need to get 38 inches — obliterating its previous record.

A few storms won't be nearly enough

Now, there are complications here. For instance, California usually gets most of its rain between December and February, during massive storms that dump snow in the mountains. That snowpack then melts in the spring and summer to supply water elsewhere. So the type of precipitation matters almost as much as the amount. "If winter precipitation falls as rain instead of snow," NOAA's Tom DiLiberto writes, "it may not go as far toward alleviating drought as the precipitation totals would otherwise suggest. Indeed, in many areas of the state, the snowpack is just 50% of normal."

But the bottom line is pretty simple: A few normal storms won't end the drought. California will need an extraordinary amount of rain this year. And if that doesn't happen, the drought is likely to linger through the summer.

And what about California's groundwater?

Meanwhile, even if the drought does eventually end — whether it takes one year, two years, three years or more — it could take even longer for California's water resources to fully recover.

Farmers have replaced 75% of the lost rain with groundwater

The reason is groundwater. Facing low rainfall, many of California's farmers have instead been sucking out freshwater from beneath the ground to irrigate crops such as almonds. (After all, almond farmers can't just choose not to plant in dry spells — if they don't water the trees, the trees die.) According to one UC Davis study, California's farmers are replacing about three-fourths of lost rainfall with groundwater.

There's nothing stopping this from happening: While some other states have laws regulating how much water can be pulled out from underground aquifers, California lets pretty much anyone draw as much out as they want, as long as it's for a useful purpose. As a result, satellite surveys have shown, California's groundwater has been vanishing at a shocking rate:

The problem here is that these underground aquifers aren't easily refilled, since they were built up over centuries. A few storms won't bring them back, either. That means farmers are losing a crucial buffer against future droughts. (Some climate scientists have argued that California has seen 200-year "megadroughts" in the past, though there's much debate over whether the state is currently in the midst of another.)

Back in September, California passed a law that would set the stage for future groundwater regulation — but it will take another six years to take effect, and it faces a lot of opposition from farming interests. In the meantime, those aquifers are shrinking, and so long as the drought persists, they're likely to keep shrinking further.

Further reading

— Tom Philpott has an excellent new piece in Mother Jones on California's drought, its water-hungry almond farms, and the politics around groundwater regulation. You should read it in full.

— Could water markets help solve the West's water crisis?