( CNN ) -- About 11 trillion gallons of rain , or nearly 17 million Olympic swimming pools full . That 's how much water California needs to recover from its extreme drought despite downpours that caused flooding and mudslides this month , NASA said . This week , the space agency released a satellite data analysis of how much water the state 's reserves lack . It 's a lot -- more than 14,000 times the amount of water it would take to fill the Dallas Cowboys stadium , according to CNN calculations . It 's the amount of water that flows over Niagara Falls in about 170 days ' time . `` It takes years to get into a drought of this severity , and it will likely take many more big storms , and years , to crawl out of it , '' said NASA scientist Jay Famiglietti , who led the study . NASA climate satellites measured fluctuations in Earth 's gravitational field and the changing shape of the planet 's surface to determine the drop in water reserves . It 's the first calculation of its kind to determine the amount of water needed to break a drought , the space agency said . Virtually all of California is drought stricken , according to the University of Nebraska Lincoln drought monitor , most of it under the worst level -- `` exceptional drought . '' Deluges boosted state reservoirs . December 's deluges doused parts of California in just a few days with double the rain or more that they usually get in a year . An atmospheric river -- a moisture-laden stream of air from the tropics dozens of miles wide -- hosed the coast , and behind it , storm fronts battered the state . Nearly a foot of rain fell in some places in a single day . Creeks and rivers previously flowing at a trickle swelled over their banks and flooded nearby streets . The abundant rainfall gave the state 's reservoirs an upward bounce , according to measurements by the Department of Water Resources . One of the largest , Lake Shasta , went from a water level before the rains of 41 % of its historic average to 53 % on Tuesday . Another very large one , Lake Oroville , stepped up from a level of 44 % of its average volume to 54 % . Some of the state 's other 10 -- mostly much smaller -- main reservoirs felt a boost , too . Others , not so much . More rain is in the forecast this week , but it will be gentle , not torrential . Despite the new rain , California 's mighty reservoirs still resemble ponds surrounded by parched , elongated banks . Depleting groundwater . The drought has dug so deep in the past three years that it has tapped into groundwater . It has done this throughout the Southwest , where groundwater levels are at their absolute lowest levels in the last 65 years , said NASA 's Jet Propulsion Laboratory in Pasadena . The state 's two largest river basins , the Sacramento and San Joaquin , have lost 4 trillion gallons of water per year since 2011 , according to the analysis by NASA JPL . `` That 's more water than California 's 38 million residents use each year for domestic and municipal purposes . About two-thirds of the loss is due to depletion of groundwater beneath California 's Central Valley , '' NASA said . In September , Gov . Jerry Brown signed `` historic legislation '' that created `` a framework for sustainable , local groundwater management for the first time in California history , '' the governor 's office said at the time . Before the new legislation , California was the only Western state that did n't manage its groundwater , officials said . Brown declared a drought emergency , saying this was `` perhaps the worst drought that California has ever seen since records ( began ) about 100 years ago . '' He held up a chart of fluctuating rainfall levels over many years , with the graph plummeting off the bottom of the chart in recent years . Diminished snow caps . The atmospheric river and cold fronts loaded up the Sierra Nevada mountains with a couple of feet of snow at the highest altitudes , adding to the snowpack . It acts as a major water reservoir , storing up moisture in the winter months and then releasing it during melts . The storm 's extra padding doubled the level to 48 % of the historic average , a nice improvement . But there 's bad news here , too , based on studies done from airplanes this year , NASA said . `` The 2014 snowpack was one of the three lowest on record and the worst since 1977 , when California 's population was half what it is now , '' said NASA JPL scientist Tom Painter . That not only means less water from snow but also less reflection of sunlight , which means the Earth absorbs it and gets that much warmer , Painter said . The ground then also gets more parched , so when water does flow onto it , it soaks it up , leaving less of it to flow into reservoirs . -- -- . CNN 's Kevin Conlon contributed to this report .
Deluges during the atmospheric river give reservoirs a pleasant boost . But much more is needed after years of drought , NASA says in a new analysis . Satellites measure gravitational field and planet 's shape to find changes in water reserves . Water reserves have fallen so far , California is depleting its groundwater .