(Bloomberg) -- Rain has finally been falling hard inCalifornia, where reservoirs are filling up fast.

After six years of punishing drought, that’s obviously a goodthing. But it creates a balancing-act challenge for the state’swater managers.

Keep too much holed up in storage and the system will overflowif the precipitation keeps coming. Open the hatches too much and,if Mother Nature doesn’t provide any more deluges, California willbe parched when the rain stops.

“There is always a trade off — and you never know if youare doing it right,” said Jay Lund, director of the Center for WatershedSciences at the University of California at Davis.

Recent flooding in Northern California

Floods already have washed over Northern California, inundatinghomes and farmland and spurring evacuations. The Sacramento Weirspillway was unlocked for the first time in a decade last week,sending water pouring out into nearby plains to take the pressureoff the Sacramento River and protect the city, which is the statecapital.

The storms started rolling in with 2017. Record rain and heavysnow saturated the state during the first two weeks, thanks toflowing ribbons of moisture rising out of the Pacific thatclimatologists call atmospheric rivers. There was so much so fastthat most reservoirs are holding more than average for the month,and the mountain snowpack is deep and wide.

By Jan. 12, the share of California that was abnormally dry orin drought had fallen to about 65 percent from almost 82 percentthe week before, according to the U.S. DroughtMonitor. California is the biggest U.S. agricultural producerof more than 400 crops as well as more than a third of thecountry’s vegetable production and two-thirds of fruit and nutoutput. Milk, almonds, grapes, cattle and lettuce were the state’sbiggest crops in 2015, data from the agriculture departmentshow.

Flood pools

The storms “have been a godsend,” said Robert Oravec, a seniorbranch forecaster at the WeatherPrediction Center in College Park, Maryland — but notnecessarily the closer needed to call the drought done. That willdepend in part on how reservoirs are managed to ensure there’senough water come summer for more than 38 million residents and a$40 billion agriculture industry.

“It’s the primary challenge we deal with,” said Mike Anderson, astate climatologist in Sacramento.

Western states depend on water captured in the rainy season andfrom runoff when mountain snows melt. California is dotted withhundreds of reservoirs, and the Army Corps of Engineers setsrules that determine how much spare capacity each must have, calledthe flood pool. When that level is reached, stored water has to bereleased and the outflows carefully calculated so rivers downstreamaren’t overwhelmed.

Groundwater depletions

When big storms are on the way, “drought is not really aconsideration,” said Mitch Russo, intelligence chief with theCalifornia Department ofWater Resources’ flood-operations center. It’s all aboutpreventing floods.

“Ideally we would love to have these come in as snow,” Russosaid. “‘The snowpack in the Sierra Nevada is by far and away ourgreatest water resource.”

So far, so good: On Monday, 68.7 percent of the Sierra Nevadawas snow-covered to an average depth of nearly 3 feet, according tothe National OperationalHydrologic Remote Sensing Center. A year ago, about the sameamount of cover was only about half as deep.

For all that, there’s no telling how much damage has been donein the past six years to groundwater resources, especially in thesouthern part of the state, which hasn’t shared in all theprecipitation and where big cities have big demands.

Related: Earth just experienced the hottest 5 years onrecord

“Groundwater depletions may be with us for years, maybe decades,maybe forever,” said Lund, a civil and environmentalengineering professor who writes the California Water Blog.

Still, 2017 started out with such a bang that the experts arecautiously optimistic. At the least, the atmospheric rivers haveput “a dent” in the drought, Anderson said.

In fact, according to the monitor, the percentage of the statein exceptional drought, the worst category, fell to just above 2percent last week from more than 18 percent the week before.

And there’s an atmospheric river in the forecast forWednesday.

Related: California sinking into arid earth as farmers suckwater dry

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