Saturday, December 29, 2012

Water-Rich vs. Water-Poor

Some political experts predict that the next series of global wars will be fought over drinking water resources.


Boats cross the Blue Nile River near Tis Issat, Ethiopia.
Papyrus reed boats cross the Blue Nile River in Ethiopia, where some people claim their water is being handed over to Saudi business interests.
Photograph by Peter Guttman, Corbis
Fred Pearce in Ethiopia
Published November 30, 2012
This piece is part of Water Grabbers: A Global Rush on Freshwatera specialNational Geographic Freshwater News series on how grabbing land—and water—from poor people, desperate governments, and future generations threatens global food security, environmental sustainability, and local cultures.
The cows appear on the horizon like a mirage. Drive about a hundred miles (160 kilometers) through the Arabian Desert southeast from Riyadh, and you will come across one of the world's largest herds of dairy cattle. Some 40,000 Friesian cows survive in one of the driest places on the planet, with temperatures regularly reaching 110°F (43°C).
The cows live in six giant air-conditioned sheds, shrouded in a mist that keeps them cool. They churn out 53 million gallons (200 million liters) of milk a year, which heads off down the highway in a constant stream of tankers.
Welcome to Al Safi, one of the world's largest and most improbable dairy farms, the creation of the late prince, Abdullah al Faisal, eldest son of Faisal, the Saudi king from 1964 to 1975. It is not alone in one of the largest bodies of sand in the world, more than three times the size of Texas. Down the road is the Almarai dairy farm, almost as big, the creation of a racehorse-breeding Saudi prince and his Irish chum, dairy magnate Alastair McGuickan.
Saudi Arabia's Glass Is Four-Fifths Empty
Anyone flying over Saudi Arabia today will see the desert dotted with cow sheds and huge circles of green, where crops to feed both the cows and Saudis are grown. The water to irrigate those fields, and cool those cows, does not come from rivers. There are no rivers. It comes from what was once one of the world's largest reserves of underground water. More than a mile beneath the sand, the water was laid down tens of thousands of years ago during the last ice age, when Arabia was wet.
The sheikhs of Saudi Arabia have been farming the desert in this way for 30 years, spending hundreds of billions of dollars of oil revenues to pursue their dream of self-sufficiency in food. The Saudi government has been paying farmers five times the international price for wheat, while charging nothing for the water, and providing virtually free electricity to pump that water to the surface. Fortunes have been made as the giant pivots green the desert, and cows graze in their mist-filled sheds.
Saudi Arabia map

But now many of the pumps are being silenced and the spigots turned off. The Saudi government says wheat-growing must cease by 2016, and the water-cooled cow sheds may be abandoned soon after.
The water is running out.
The mirage of water in the desert, and of food self-sufficiency for a desert nation, is fading. (See "Kingdom on Edge: Saudi Arabia" in National Geographicmagazine.)
Forty years ago, when the farming started, there was a staggering 120 cubic miles (500 cubic kilometers) of water beneath the Saudi desert, enough to fillLake Erie. But in recent years, up to five cubic miles (20 cubic kilometers) has been pumped to the surface annually for use on the farms. Virtually none of it is replaced by rainwater, because there is no appreciable rain.
Based on extraction rates detailed in a 2004 paper from the University of London, the Saudis were on track to use up at least 400 cubic kilometers of their aquifers by 2008. And so experts estimate that four-fifths of the Saudis' "fossil" water is now gone. One of the planet's greatest and oldest freshwater resources, in one of its hottest and most parched places, has been all but emptied in little more than a generation.
Parallel to the groundwater pumping for agriculture, Saudi Arabia has long used desalination of seawater to provide drinking water. But, even for the cash-rich Saudis, at about a dollar per 35 cubic feet (one cubic meter), the energy-intensive process is too expensive to be used for irrigation water.
But the Saudis have not abandoned their dream of growing their own food. If they no longer have water of their own, they are looking to someone else's. They are scouring the world for well-watered lands where their desert farmers can move to grow wheat, rice, and other crops that can be shipped home. But will their actions bring another tragedy, this time a human one as much as a hydrological one?
Grabbing the Headwaters of the Nile?
To find out, in mid-2011 I traveled some 1,500 miles (2,400 kilometers) south of Riyadh, across the Red Sea, to meet some poor Africans who say they are paying the price for the Saudi dream. They live in Gambela, the most impoverished corner of Ethiopia, at the headwaters of the Nile River, the world's longest. One of Ethiopia's nine kililoch (divisions), Gambela is a horn-shaped region that protrudes into South Sudan. (See a map of the region.)
Here, amid the wet pastures and forests, unrest is brewing. Locals say the Saudis want their water.
I met Omot Ochan, a tall, dark-skinned member of the Anuak tribe, wearing combat shorts and sitting on an old waterbuck skin in a forest clearing. He was angry. He said the lush forests and marshlands where he and his ancestors have hunted for generations were being taken by Saudi Star, a company owned by one of Saudi Arabia's richest men, Sheikh Mohammed Hussein Ali al Amoudi.
Yards from his hut, the company was digging a canal that Ochan said would drain the nearby wetland, where he fished. And nearby, al Amoudi's 24,711-acre (10,000-hectare) farm had taken over a reservoir built by Soviet engineers in the 1980s.
Government officials had told Ochan and hundreds of others that they had to move out of the forest and into government villages. Ostensibly the purpose was to provide better services, but Ochan believed the real reason was to clear the land for al Amoudi, a friend and sometime campaign financier of Ethiopia's prime minister at the time, Menes Zenawi.
Half an hour later, I drank tea in the shade of a huge mango tree with one tribal elder who spoke to me quietly about how he and his fellows had been forcibly moved from their fields. But he told me: "We have decided, each of us, that in the rainy season we will go back and cultivate our ancestral land. If they try and stop us, conflict will start."
And they were as good as their word. Months after my visit, in April this year, unnamed local gunmen invaded Saudi Star's company camp near the town of Abobo. They killed at least five workers. In an effort to root out the culprits, government soldiers allegedly went on a rampage in local villages, rounding up and torturing men and raping women.
The group Human Rights Watch interviewed some of those who fled to neighboring South Sudan afterward. The people said that their original raid was in retaliation for the company grabbing their land and water. A local churchman told me: "My son has gone but wants to come back and fight."
global water footprint

Wildlife at Risk?
This may be a wildlife tragedy, too. The waters of Gambela are vital to millions of white-eared kob, antelopes that cross from South Sudan in the dry season in search of the open water and wetlands at the head of the Nile. These animals—along with a scattering of elephants, an endangered antelope called the Nile Lechwe, and the giant shoebill stork—were the main reason for the creation back in the 1970s of the Gambela National Park. But the park has not been fully secured and much of its land has been given to Saudi Star. The migrating animals now face tractors, canals, and fenced pastures.
All this for water? The Saudis are determined that they will continue to feed their own people. They have plenty of land, but no water. They fear that, without water, even their oil will not save them from a perilous future of food insecurity. As one senior Saudi official told me: "We cannot eat oil." So they are determined to buy foreign land that has access to plentiful water.
Asked about the Saudi Star water grab in Gambela earlier this year, the Saudi minister for agriculture, Fahd bin Abdulrahman Balghunaim, said: "I honestly never heard any complaint coming out of Africa. What I read were some articles written by foreign correspondents about things happening in Africa, which we did not see happening."
Saudi Star declined to comment for this article.
Courting Foreign Governments
The King Abdullah Initiative for Saudi Agricultural Investment Abroad, launched in 2008, is providing government credit and diplomatic support for Saudi companies buying up foreign land and water to feed Saudis. Schemes are under way from the banks of the Senegal River in West Africa to the rain forests of Indonesian New Guinea. In most deals, Saudi investors have generous access to water and the right to export at least 50 percent of the harvest back to Saudi Arabia.
Some host governments are happy with these terms. Ethiopia's Zenawi, who died in August, had an instant answer to those who criticized his largesse toward his Saudi friend. "We want to develop our land to feed ourselves, rather than admire the beauty of fallow fields while we starve," he said.
Fair enough. But a 2012 report from one of Africa's biggest banks, Standard Bank in South Africa, suggests he was wrong and that Saudi investments may be bad value for the continent. "For African countries courted by Saudi agribusiness firms, a clear appreciation of the value of the asset on which they rest is necessary," it said. "Under-selling of agricultural assets (both land and, perhaps more critically, water) remains a profound threat."
Ochan and his fellows in the forest say they agree with that.
Fred Pearce is a journalist and author on environmental science. His books include When the Rivers Run Dry and The Land Grabbers, both for Beacon Press, Boston. He writes regularly for New Scientist magazine, Yale Environment 360, and The Guardian, and has been published by Nature andThe Washington Post.

Sunday, December 23, 2012

The Action of Groundwater

Give our recent discussion of the hydrologic cycle in class, I think the following article from ScienceDaily.com is rather timely.

Hawaiian Islands Are Dissolving from Within, Study Says

Dec. 21, 2012 — Most of us think of soil erosion as the primary force that levels mountains, however geologists have found that Oahu's mountains are dissolving from within due to groundwater.
Someday, Oahu's Koolau and Waianae mountains will be reduced to nothing more than a flat, low-lying island like Midway.
But erosion isn't the biggest culprit. Instead, scientists say, the mountains of Oahu are actually dissolving from within.
"We tried to figure out how fast the island is going away and what the influence of climate is on that rate," said Brigham Young University geologist Steve Nelson. "More material is dissolving from those islands than what is being carried off through erosion."
The research pitted groundwater against stream water to see which removed more mineral material. Nelson and his BYU colleagues spent two months sampling both types of sources. In addition, ground and surface water estimates from the U.S. Geological Survey helped them calculate the total quantity of mass that disappeared from the island each year.
"All of the Hawaiian Islands are made of just one kind of rock," Nelson said. "The weathering rates are variable, too, because rainfall is so variable, so it's a great natural laboratory."
Forecasting the island's future also needs to account for plate tectonics. As Oahu is pushed northwest, the island actually rises in elevation at a slow but steady rate. You've heard of mountain climbing; this is a mountain that climbs.
According to the researchers' estimates, the net effect is that Oahu will continue to grow for as long as 1.5 million years. Beyond that, the force of groundwater will eventually triumph and the island will begin its descent to a low-lying topography.
Undergraduate student Brian Selck co-authored the study, which appears in the journal Geochimica et Cosmochimica Acta. Unfortunately for him, he joined the project only after the field work in Hawaii took place.
Instead, Selck performed the mineralogical analysis of soil samples in the lab back in Provo. The island's volcanic soil contained at least one surprise in weathered rock called saprolites.
"The main thing that surprised me on the way was the appearance of a large amount of quartz in a saprolite taken from a 1-meter depth," Selck said.
After he graduates from BYU, Selck will pursue a career in hydrogeology. BYU geology professor David Tingey joins Nelson and Selck as a co-author on the new study.
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The above story is reprinted from materials provided byBrigham Young University.
Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference:
  1. Stephen T. Nelson, David G. Tingey, Brian Selck. The denudation of ocean islands by ground and surface waters: The effects of climate, soil thickness, and water contact times on Oahu, HawaiiGeochimica et Cosmochimica Acta, 2013; 103: 276 DOI:10.1016/j.gca.2012.09.046
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Brigham Young University (2012, December 21). Hawaiian Islands are dissolving from within, study says. ScienceDaily. Retrieved December 23, 2012, from http://www.sciencedaily.com­/releases/2012/12/121221114044.htm
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Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

Wednesday, December 19, 2012

Water as a Nonrenewable Resource



I thought the article below from National Geographic is particularly pertinent given our recent discussions of groundwater.  It paints a sobering picture of the status of the world's aquifers.


A tanker fills a water reservoir in New Delhi, India.
Pumping and shipping groundwater in New Delhi and elsewhere in India has become big business, but water table levels are dropping.
Photograph by Findlay Kember, AFP/Getty Images
Fred Pearce in India
Published November 30, 2012
This piece is part of Water Grabbers: A Global Rush on Freshwater, a special National Geographic Freshwater News series on how grabbing land—and water—from poor people, desperate governments, and future generations threatens global food security, environmental sustainability, and local cultures.
Suresh Ponnusami sat back on his porch by the road south of the Indian textile town of Tirupur. He was not rich, but for the owner of a two-acre farm in the backwoods of a developing country he was doing rather well. He had a TV, a car, and a maid to bring him drinks and ensure his traditional white Indian robes were freshly laundered every morning.
The source of his wealth, he said, was a large water reservoir beside his house. And as we chatted, a tanker drew up on the road. The driver dropped a large pipe from his vehicle into the reservoir and began sucking up the contents.
Ponnusami explained: "I no longer grow crops, I farm water. The tankers come about ten times a day. I don't have to do anything except keep my reservoir full." To do that, he had drilled boreholes deep into the rocks beneath his fields, and inserted pumps that brought water to the surface 24 hours a day. He sold every tanker load for about four dollars. "It's a good living, and it's risk-free," he said. "While the water lasts."
A neighbor told me she does the same thing. Water mining was the local industry. But, she said, "every day the water is reducing. We drilled two new boreholes a few weeks ago and one has already failed."
Surely this is madness, I suggested. Why not go back to real farming before the wells run dry? "If everybody did that, it would be well and good," she agreed. "But they don't. We are all trying to make as much money as we can before the water runs out."
Ponnusami and his neighbors were selling water to dyeing and bleaching factories in Tirupur. The factories once got their water from a giant reservoir on southern India's biggest river, the Kaveri (see picture). But the Kaveri was now being pumped dry by farmers and industry farther upstream. The reservoir was nearly empty most of the year. So the factories had taken to buying up underground water from local farmers.
It is a trade that is growing all over India—and all over the world.
Draining Fossil Aquifers
We are used to thinking of water as a renewable resource. However much we waste and abuse it, the rains will come again and the rivers and reservoirs will refill. Except during droughts, this is true for water at the surface. But not underground. As we pump more and more rivers dry, the world is increasingly dependent on subterranean water. That is water stored by nature in the pores of rocks, often for thousands of years, before we began to tap it with our drills and pumps.
We are emptying these giant natural reservoirs far faster than the rains can refill them. The water tables are falling, the wells have to be dug ever deeper, and the pumps must be ever bigger. We are mining water now that should be the birthright of future generations.
In India, the water is being taken for industry, for cities, and especially for agriculture. Once a country of widespread famine, India has seen an agricultural revolution in the past half century. India now produces enough food to feed all its people; the fact that many Indians still go hungry today is an economic and political puzzle, because the country exports rice.
But that may not last. Researchers estimate that a quarter of India's food is irrigated with underground water that nature is not replacing. The revolution is living on borrowed water and borrowed time. Who will feed India when the water runs out?
Nobody knows how much water is buried beneath our feet. But we do know that the reserves are being emptied. The crisis is global and growing, but remains largely out of sight and out of mind.
The latest estimate, published in the journal Water Resources Research this year, is that India alone is pumping out some 46 cubic miles (190 cubic kilometers) of water a year from below ground, while nature is refilling only 29 cubic miles (120 cubic kilometers), a shortfall of 17 cubic miles (70 cubic kilometers) per year. A cubic kilometer is 264.2 billion gallons, or about enough water to fill 400,000 Olympic-size swimming pools.
Close behind India, Pakistan is overpumping by 8.4 cubic miles (35 cubic kilometers), the United States by 7.2 cubic miles (30 cubic kilometers), and China and Iran by 4.8 cubic miles (20 cubic kilometers) each per year. Globally, the shortfall is about 60 cubic miles (250 cubic kilometers) per year, more than three times the rate half a century ago. Egypt, Uzbekistan, Libya, Algeria, Morocco, Syria, Australia, Israel, and others are all pumping up their water at least 50 percent faster than the rains replenish. In some places, water that you could once bring to the surface with a bucket on a short rope is now a mile or more down.
Nile River

Farming's Big Thirst
Overwhelmingly, the problem is agriculture. Farming takes two-thirds of all the water we grab from nature, but that figure rises to 90 percent in many of the driest and most water-stressed regions.
This cannot go on, as the United States is already discovering. For more than half a century now, farmers have been pumping out one of the world's greatest underwater reserves, the Ogallala aquifer, which stretches beneath the High Plains from Texas to South Dakota. The pumping began in order to revive the plains after the horrors of the 1930s Dust Bowl. By the 1970s there were 200,000 water wells, supplying more than a third of the U.S.'s irrigated fields.
For a while it was a huge success. In a good year, the High Plains produced three-quarters of the wheat traded on international markets, restocking Russian grain stores and feeding millions of starving Africans. But the Ogallala water is drawing down, many wells are going dry, and the output of the pumps has halved. A quarter of the aquifer is gone in parts of Texas, Oklahoma, and Kansas, and over wide areas the water table has fallen by more than 100 feet. In some places, the sagebrush is returning because farmers are giving up on irrigated planting. (See "That Sinking Feeling About Groundwater in Texas.")
Other countries are heading in the same direction. Water tables are falling by more than a meter a year beneath the North China Plain, the breadbasket of the most populous nation on Earth. Saudi Arabia has almost pumped dry a vast water reserve beneath the desert in just 40 years.
Libya is doing the same beneath the Sahara. Muammar Qaddafi, Libya's late ruler, spent $30 billion of his country's oil revenues on giant pump fields in the desert, and a 2,000-mile (3,200-kilometer) network of pipes to bring underground water that is thousands of years old to coastal farms. Even though it was bombed by NATO forces last year, what Qaddafi called the Great Manmade River Project appears to still be functioning. But nature will eventually accomplish what the bombs did not. Water tables are dropping, pumping is getting harder, and the water is getting saltier.
Soon we may have a full global picture of how the world's underground water reserves are disappearing. Researchers are using NASA's GRACE satellite, which measures changes in the Earth's gravity field, to spot where the pores in rocks are being emptied of water. Jay Famiglietti, an earth science professor at the University of California, Irvine, is analyzing the findings. He says water security will soon rival energy security as the fastest-rising issue on the global geopolitical agenda.
More and more countries are so short of water for farming that they can feed their citizens only by importing crops grown using someone else's water. But the number of countries with spare water to export in this way is diminishing. The fear is that as the world's water supplies run on empty, the world's stomachs will as well.
Often, even before the water runs out, the pumps start to bring up water that is salty or toxic. In parts of India, there are epidemics of fluoride poisoning caused by drinking water containing high levels of this natural compound, which dissolves from hard rocks beneath water-bearing strata. I have seen villages full of severely disabled children, and adults suffering muscle degeneration, organ failure, and cancer caused by these poisons. Some communities call it "the devil's water."
We should not be doing this, says Brian Richter, freshwater strategist at The Nature Conservancy. "Falling groundwater levels are the bellwethers of the unsustainability of our water use," Richter said. "We're raiding our savings accounts with no payback plan."
We should not be stealing water from future generations, Richter said. We should instead use underground water sparingly and with caution.
Seeking Solutions
This can be done, starting with agriculture. Scientists are already working on new varieties of crops that need much less water to grow. And technologists are coming up with less wasteful ways to irrigate those crops. (See "Saving a River, One Farm at a Time.")
The truth is that, despite growing shortages, water is still usually so cheap that it is often wasted. The majority of the world's farmers irrigate simply by flooding their fields. But only a fraction of that water gets absorbed by the plants. Some of it percolates underground and can eventually be pumped to the surface again. But much of it is lost to evaporation.
Even spraying from pivots loses huge amounts of water to the air, where it may get carried out to sea or otherwise lost to local use. So the race is on to develop cheap drip irrigation, in which water is distributed across fields in pipes and dripped into the soil close to plant roots. That way we may be able to save our underground water reserves for future generations.
Meanwhile, communities across the world are running out of water. Where are things worst? The UN Environment Programme (UNEP) nominates the Gaza Strip, the Palestinian enclave on the shores of the Mediterranean between Israel and Egypt. It looks as though it will become the first territory in the world to lose its only water supply.
Gaza has no rivers. It cannot afford desalinated seawater. So its 1.7 million inhabitants drink from the underground reserves. But pumping is being done at three times the recharge rate, water tables are falling fast, and what comes through the wells is increasingly contaminated by seawater seeping into the emptying rocks. A UN report this year said Gaza's water probably will be undrinkable by 2016. What then?
Gaza is an extreme case. And water is only one of its many problems. But it offers a warning for the world. It shows what can happen as the water runs out—what will happen in many other places if we continue to steal water from our children and their children.
Fred Pearce is a journalist and author on environmental science. His books include When the Rivers Run Dry and The Land Grabbers, both for Beacon Press, Boston. He writes regularly for New Scientist magazine, Yale Environment 360, and The Guardian, and has been published by Nature andThe Washington Post.

Saturday, December 15, 2012

The Drought Continues...

Two ranchers walk across the plains of drought-stricken West Texas in July 2011.

We heard so much about the widespread and lingering drought affecting over 69% of the contiguous United States.  Whether or not the drought continues through the winter remains to be seen.  Many western areas in the United States depend upon abundant winter snow to provide much of their water in spring and summer.

I was catching up on reading and one link lead to another.  Ultimately I arrived at the article below (click the link) about the US drought and the NASA-JPL GRACE Mission.  Fascinating technology.  It's also a timely post as we just finished up Chp. 6 - Running Water and Groundwater.

NASA Earth Observatory Article - GRACE and Drought

Monday, December 10, 2012

Green Energy

Below is an interesting article from ScienceDaily.com.  I've heard varying opinions on the matter.  This article takes a positive stance with regards to the viability of wind and solar power.


Wind and Solar Power Paired With Storage Could Power Grid 99.9 Percent of the Time

Dec. 10, 2012 — Renewable energy could fully power a large electric grid 99.9 percent of the time by 2030 at costs comparable to today's electricity expenses, according to new research by the University of Delaware and Delaware Technical Community College.
A well-designed combination of wind power, solar power and storage in batteries and fuel cells would nearly always exceed electricity demands while keeping costs low, the scientists found.
"These results break the conventional wisdom that renewable energy is too unreliable and expensive," said co-author Willett Kempton, professor in the School of Marine Science and Policy in UD's College of Earth, Ocean, and Environment. "The key is to get the right combination of electricity sources and storage -- which we did by an exhaustive search -- and to calculate costs correctly."
The authors developed a computer model to consider 28 billion combinations of renewable energy sources and storage mechanisms, each tested over four years of historical hourly weather data and electricity demands. The model incorporated data from within a large regional grid called PJM Interconnection, which includes 13 states from New Jersey to Illinois and represents one-fifth of the United States' total electric grid.
Unlike other studies, the model focused on minimizing costs instead of the traditional approach of matching generation to electricity use. The researchers found that generating more electricity than needed during average hours -- in order to meet needs on high-demand but low-wind power hours -- would be cheaper than storing excess power for later high demand.
Storage is relatively costly because the storage medium, batteries or hydrogen tanks, must be larger for each additional hour stored.
One of several new findings is that a very large electric system can be run almost entirely on renewable energy.
"For example, using hydrogen for storage, we can run an electric system that today would meeting a need of 72 GW, 99.9 percent of the time, using 17 GW of solar, 68 GW of offshore wind, and 115 GW of inland wind," said co-author Cory Budischak, instructor in the Energy Management Department at Delaware Technical Community College and former UD student.
A GW ("gigawatt") is a measure of electricity generation capability. One GW is the capacity of 200 large wind turbines or of 250,000 rooftop solar systems. Renewable electricity generators must have higher GW capacity than traditional generators, since wind and solar do not generate at maximum all the time.
The study sheds light on what an electric system might look like with heavy reliance on renewable energy sources. Wind speeds and sun exposure vary with weather and seasons, requiring ways to improve reliability. In this study, reliability was achieved by: expanding the geographic area of renewable generation, using diverse sources, employing storage systems, and for the last few percent of the time, burning fossil fuels as a backup.
During the hours when there was not enough renewable electricity to meet power needs, the model drew from storage and, on the rare hours with neither renewable electricity or stored power, then fossil fuel. When there was more renewable energy generated than needed, the model would first fill storage, use the remaining to replace natural gas for heating homes and businesses and only after those, let the excess go to waste.
The study used estimates of technology costs in 2030 without government subsidies, comparing them to costs of fossil fuel generation in wide use today. The cost of fossil fuels includes both the fuel cost itself and the documented external costs such as human health effects caused by power plant air pollution. The projected capital costs for wind and solar in 2030 are about half of today's wind and solar costs, whereas maintenance costs are projected to be approximately the same.
"Aiming for 90 percent or more renewable energy in 2030, in order to achieve climate change targets of 80 to 90 percent reduction of the greenhouse gas carbon dioxide from the power sector, leads to economic savings," the authors observe.
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The above story is reprinted from materials provided byUniversity of Delaware.
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Journal Reference:
  1. Cory Budischak, DeAnna Sewell, Heather Thomson, Leon Mach, Dana E. Veron, Willett Kempton. Cost-minimized combinations of wind power, solar power and electrochemical storage, powering the grid up to 99.9% of the timeJournal of Power Sources, 2013; 225: 60 DOI: 10.1016/j.jpowsour.2012.09.054
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 MLA
University of Delaware (2012, December 10). Wind and solar power paired with storage could power grid 99.9 percent of the time.ScienceDaily. Retrieved December 10, 2012, from http://www.sciencedaily.com­/releases/2012/12/121210133507.htm
Note: If no author is given, the source is cited instead.
Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.