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Parched America: Satellite images reveal just how dry the ground has become across California, Oregon and Washington

Images reveal how water in mid September 2015 compares with averages for Septembers between 1948 and 2012
States near the coasts were much drier than normal, while the Southwest and Midwest regions was also affected
One map depicts moisture in the 'root zone,' or the top meter (39 inches) of soil, which another shows the state of groundwater in shallow aquifers

By Ellie Zolfagharifard For Dailymail.com

Published: 19:21 EST, 23 September 2015 | Updated: 20:22 EST, 23 September 2015

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Large swathes of the US continue to suffer the impact of a deep, ongoing drought.

Conditions have been particularly severe in California, Oregon, and Washington, where below-average precipitation has had a large, lasting effect on water supplies.

Now Nasa has created a series of maps, which look beneath the crops and deep into the ground, to highlight the severity of the situation.

The maps reveal how water in mid September 2015 compares with averages for Septembers between 1948 and 2012, with warmer colours showing drier-than-average conditions.

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The maps reveal how water in mid September 2015 compares with averages for Septembers between 1948 and 2012, with warmer colours showing drier-than-average conditions. This map shows moisture content in the top 2 centimeters (0.8 inches) of surface soil. States near the coasts were significantly drier than normal, while the Southwest and Midwest regions were also affected but to a lesser extent

They were created using data from the Gravity Recovery and Climate Experiment (Grace) satellites, alongside ground-based measurements.

The first map shows moisture content in the top 2 centimeters (0.8 inches) of surface soil. States near the coasts were significantly drier than normal, while the Southwest and Midwest regions were also affected but to a lesser extent.

However, it takes time for the effects of rainfall or drought to be felt deeper in the ground, which explains the differences between the three maps.

The second map depicts moisture in the 'root zone,' or the top meter (39 inches) of soil; the third map shows the state of groundwater in shallow aquifers.

While the eastern United States does not show as severe an anomaly at the surface, many aquifers have not recovered from previous deficiencies.

Western states, meanwhile, show far-below-normal conditions at all depths due to the long-term drought.

It takes time for the effects of rainfall or drought to be felt deeper in the ground, which explains the differences between the three maps. The map depicts moisture in the “root zone,” or the top meter (39 inches) of soil

Pictured is the state of groundwater in shallow aquifers. While the eastern United States does not show as severe an anomaly at the surface, many aquifers have not recovered from previous deficiencies

California, for example, is in its fourth year of severe drought. Early in 2015, the Los Angeles Department of Water and Power dammed the Los Angeles Aqueduct to conserve the limited snowmelt from the Eastern Sierra; for the first time since 1913, the aqueduct stopped carrying runoff from Owens Valley.

By April, state officials announced there was no snow in the Sierra Nevada for the first time in 75 years of measurements. The announcement spurred researchers to launch a study of tree rings in the Central Valley.

They found that mountain snow - which California relies on for water - has not been so low since the 1500s.

According to the U.S. Drought Monitor, areas in Washington and Oregon also saw severe or extreme drought conditions in mid-September. As of September 15, extreme drought spanned 67 percent of both states. Low winter snowpack played a role in the summer drought in these states as well.

Drought was ongoing at the same time that the region saw soaring temperatures. According to Weather Underground blogger Christopher Burt, summer 2015 was poised to become the warmest on record in Washington and Oregon.

In July, Nasa said it would take an entire year's worth of rain to replenish California's drought-stricken soils, according to a new Nasa report.

The space agency used 17 years of satellite data and ground-based precipitation measurements to study average rain and snowfall totals.

It found California created a 20 inch (50.8 cm) 'rainfall debt' between 2012 and 2015 - equal to the amount expected to fall in the state in a single year.

It would take an entire year's worth of rain to replenish California's drought-stricken soils, according to a new Nasa report. Pictured is California's accumulated precipitation 'deficit' from 2012 to 2014 shown as a per cent change from the 17-year average based on satellite observations

WHAT ARE ATMOSPHERIC RIVERS?

Atmospheric rivers occur all over the world.

They are narrow, concentrated tendrils of water vapor that travel through the atmosphere similar to, and sometimes with, the winds of a jet stream.

Like a jet stream, they typically travel from west to east.

The ones destined for California originate over the tropical Pacific, where warm ocean water evaporates a lot of moisture into the air.

The moisture-rich atmospheric rivers, informally known as the Pineapple Express, then break northward toward North America.

The debt was driven mainly by a lack of air currents moving inland from the Pacific Ocean that are rich in water vapour, according to Nasa.

In an average year, 20 to 50 per cent of California's precipitation comes from relatively few, but extreme events called atmospheric rivers.

These atmospheric rivers travel from over the Pacific Ocean to the California coast.

'When they say that an atmospheric river makes landfall, it's almost like a hurricane, without the winds,' said study lead author Andrey Savtchenko at Nasa's Goddard Space Flight Center.

The state as a whole can expect an average of about 20 inches (50.8 cm) of precipitation each year, with regional differences.

But, the total amount can vary as much as 30 per cent from year to year, according to the study.

In non-drought periods, wet years often alternate with dry years to balance out in the short term.

But from 2012 to 2014, California accumulated a deficit of almost 13 inches (33 cm).

The 2014-2015 wet season increased the debt another seven inches (17.8 cm), for a total 20 inches (50.8 cm) accumulated deficit during the course of three dry years.

The majority of that precipitation loss is attributed to a high-pressure system in the atmosphere over the eastern Pacific Ocean.

The debt was driven mainly by a lack of air currents moving inland from the Pacific Ocean that are rich in water vapour, according to Nasa. Pictured are dead plum trees that have been removed from the ground due to the lack of water for irrigation at the drought affected town of Monson, California

'The history of the American West is written in great decade-long droughts followed by multi-year wet periods,' said Nasa climatologist Bill Patzert, who was not involved in the research.

'Savtchenko and his team have shown how variable California rainfall is.'

Some forecasters claim the strengthening El Nino system off the Southern California coast could bring on a more active rainy season during the fall and winter, relieving the state's drought.

But Savtchenko says El Nino patterns only play a small part in controlling California's long term precipitation trends.

The research suggests dramatic rainfall changes and long droughts are a fact of life in the sunshine state.

'Drought has happened here before. It will happen again, and some research groups have presented evidence it will happen more frequently as the planet warms,' Savtchenko said.