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NASA Earth Observatory
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NASA images and stories about climate and the environment.
NASA images and stories about climate and the environment.

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The Water is Wider

Most scientists who study rivers rely on measures of discharge, the volume of water transported through a given cross-section of a river. Much less studied, though critically important, is a river’s total surface area, particularly for scientists trying to understand how carbon dioxide moves between rivers and the atmosphere.

To calculate a river’s area, scientists have to know the length and width throughout the whole course—from the narrow headwater streams to the miles-wide stretches found in estuaries. In some places, it’s easy for hydrologists to visit in person and get accurate measurements of the width. But many rivers, especially those in the Arctic or remote tropical jungles, are difficult or expensive to reach. There is also the problem that the width of many rivers changes depending on the season and weather.

“Using Landsat data allowed us to get around—or rather above—these problems,” said George Allen, a geographer at Texas A&M University. Along with colleague Tamlin Pavelsky at the University of North Carolina, Allen developed a global database of river widths for large rivers based on roughly 7,300 satellite images collected over several years. To get the most accurate measurements, all of the Landsat scenes were acquired when the rivers were at mean annual discharge—not too high or low—which the researchers knew thanks to a global network of stream gauges that tracks discharge for all large rivers.

In these maps, the width of a river is depicted by the width of the line. As indicated in the map of the Lena River, most of the area of a river network is comprised of smaller tributaries that feed the main stem. While Allen’s technique measures wide rivers (at least 90 meters across) most accurately, he has also developed a model for estimating the width of the narrower streams.

There have only been a few other attempts to estimate the widths of all of the world’s rivers, and those efforts relied heavily on modeling and extrapolation rather than actual measurements. Using their Landsat-based technique to tally how much of Earth’s total surface is covered by rivers, Allen and Pavelsky came up with a surprisingly large number: 773,000 square kilometers (300,000 square miles), an area larger than Texas. That represents approximately 0.5 percent of Earth’s ice-free surfaces, nearly double the amount calculated in the best previous estimate.

The third map shows which river basins have the most surface area covered by streams and rivers. The Brahmaputra River in India and Bangladesh, the Amazon in Brazil, and the Lena in Russia are among the widest rivers—and the river networks with the largest surface areas.

Allen and Pavelsky found more river surface area in the Arctic because the terrain map for the Arctic used in the previous estimate was not entirely accurate. They also found more river area in undeveloped areas and less in highly-developed areas. “We think this is because water diversion—things like dams, irrigation, and levees — reduce the amount of water in river channels in our analysis,” noted Allen.

Carbon dioxide, methane, and other greenhouses gases move naturally from rivers into the atmosphere, particularly in upstream and mountainous stretches where rapids and waterfalls are common. Understanding the contribution is important for climate scientists trying to understand how carbon cycles through the atmosphere.

The new dataset has many other uses as well. It has already been used to improve flood models and to more accurately classify surface water bodies between lakes, canals, and rivers. Allen also expects that it will become a core data set for interpreting data from NASA’s upcoming Surface Water and Ocean Topography (SWOT) satellite, which will measure changes in river and lake heights globally.

https://earthobservatory.nasa.gov/images/92432/the-water-is-wider
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7/18/18
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Cape Town’s Reservoirs Rebound

After nearly running dry six months ago, Cape Town’s reservoirs have risen dramatically. Rain has poured down on southern Africa on several occasions in recent months. According to Cape Town’s Department of Water Affairs, water levels in the city’s main reservoirs stood at 55 percent of capacity on July 16, 2018.

The largest reservoir—Theewaterskloof—holds 40 percent of Capetown’s total water storage capacity, so the state of that reservoir serves as a good barometer for the amount of water available to the city. The Operational Land Imager (OLI) on Landsat 8 acquires new imagery of the reservoir every two weeks. This animation, based on Landsat imagery, shows the condition of the reservoir at two month intervals between 2015 and 2018. Parts of the reservoir with standing water appear dark blue; areas where the bottom of the reservoir was dry and exposed are light blue.

While Theewaterskloof was 55 percent full in 2015, it dropped to 40 percent capacity in 2016 following a year of light rainfall. As the drought worsened, the reservoir shrank to 20 percent capacity by July 2017 and 13 percent by January 2018. With the arrival of heavier rains in April 2018, the reservoir bounced back to 40 percent capacity by July 16, 2018.

In June 2018, Cape Town authorities credited voluntary water conservation by residents, water use restrictions and tariffs, the installation of a city-wide pressure management system, a leak repair program, and the favorable rains for averting Day Zero, when most of the taps would have been shut off. Despite the recent increase in water stored in the reservoirs, the city plans to keep water-use restrictions in place until reservoirs are 85 percent full.

https://earthobservatory.nasa.gov/images/92428/cape-townrsquos-reservoirs-rebound
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One Year Adrift, but Not Far

In July 2017, a huge iceberg dramatically broke away from the Larsen C Ice Shelf on the Antarctic Peninsula. But the aftermath has been a bit more drawn-out, as the berg hasn’t moved very far.

The left image shows Iceberg A-68 on July 30, 2017, soon after it broke away from the shelf and then fractured into two pieces known as A-68A and A-68B. The right image shows the same area on July 1, 2018. Both images are false-color, acquired with the Thermal Infrared Sensor (TIRS) on Landsat 8. Colors indicate the relative warmth or coolness of the landscape, from orange (warmest) to light blue and white (coldest).

In a year, iceberg A-68A moved a relatively short distance from the edge of the ice shelf into the Weddell Sea. In the right image, the berg’s western edge is roughly 45 kilometers from the shelf. A-68B, the much smaller fragment of the original berg, is more than twice that distance from its prior location.

A-68A’s sluggishness is not surprising. When it calved, the berg was about the size of Delaware and weighed more than a trillion tons. Dense sea ice in the Weddell Sea has made it harder for currents, tides, and winds to move all of that mass. The iceberg has also become stuck at times when its north end encounters the shallow water near Bawden Ice Rise, an ice-covered rock outcrop.

Still, Iceberg A-68A has seen plenty of motion. Throughout the year, tide cycles have shuffled the berg back and forth like a driver trying to get out of a tight parallel-parking spot. Its north end has been repeatedly smashed against Bawden Ice Rise, fracturing and reshaping its northern edge. Also notice how the southeastern edge appears to have grown in area. This is not part of the original iceberg; it is fast ice that has come fastened to the edge of the berg as it shoves through the ice pack.

A-68A will continue this dance in moonlight, as the darkness of austral winter continues through early August. Thermal images offer one way that scientists can “see” the iceberg during polar night. Radar imagery from the Sentinel-1 satellite also has been an important tool for Adrian Luckman and the UK-based Project MIDAS, which has been monitoring the iceberg and how its calving affects the Larsen C Ice Shelf.

There’s no telling how much longer A-68A will stay “stuck” in the Weddell Sea. The smaller A-68B is a good example of the path taken by many Antarctic bergs, as they are carried by currents out of the Weddell and northward toward South Georgia and the South Sandwich Islands.

https://go.nasa.gov/2NSZ3YA
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Fires in a Dry, Hot Colorado Summer

On June 27, 2018, an illegal campfire caused the third-largest wildfire in Colorado state history, known as the Spring Creek Fire. According to news reports, more than 140 homes have been destroyed and 1,481 firefighters were on the scene as of July 10, 2018.

This image was acquired on July 6, by the Operational Land Imager (OLI) on Landsat 8. The image is false color (OLI bands 7-5-2) to better differentiate burned areas (red) from the surrounding landscape. The fire is located five miles northeast of Fort Garland and spanned on both sides of Highway 160.

As of July 12, the fire affected 107,967 acres of land, making it almost the second largest fire in state history. (The second largest fire in Colorado history, the West Fork Complex fire, burned 109,049 acres.)

The fire was 83 percent contained on July 12, with the fire completely contained for the area south of Highway 160. The northwest region of the fire remained uncontrolled as crews had trouble accessing areas of steep terrain with vegetation and other materials susceptible to burning. Officials estimated the fire would be completely contained by the end of July, more than a month after it ignited. Upcoming rainfall in the area could further help firefighters manage the blaze.

Spring Creek fire was one of 14 fires burning in Colorado on July 12. The state experienced hot summer days, high winds, and extreme to exceptional drought conditions for the past three months. Colorado has not faced similar drought conditions since 2013, according to data from the U.S. Drought Monitor. Officials instated bans against all open burning around the state in light of the dry, hot conditions.

https://go.nasa.gov/2JmqIxp
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Hail Cuts Swaths of Damage Across South Dakota

The long lines of damage visible in these satellite images of South Dakota may look like a product of tornadoes. However, the width of the damage swath—well over 10 kilometers (6 miles) in many areas—is a clue that it was hail that pummeled these croplands. According to Jordan Bell, a research meteorologist with NASA’s Short-term Prediction Research and Transition Center (SPoRT), tornado tracks rarely appear wider than a few kilometers.

The storms that produced these hail swaths came in a one-two punch, according to a summary from the National Weather Service. The first arrived on June 27, 2018, charting a southeasterly path of destruction as it moved across Sully and Stanley counties, narrowly missing the city of Pierre, South Dakota. A second storm left an even longer swath, stretching from the Wyoming-South Dakota border for hundreds of miles before ending east of the Missouri River. Some areas reported hail larger than 4 inches (10 centimeters) in diameter, about the size of a grapefruit. The large hail—in conjunction with strong winds—stripped corn stalks bare and pummeled soybean leaves.

“Crops and grasslands can present a very uniform and green background when observed from space. With wind-driven hail capable of shredding the vegetation, the storm damage becomes visible in satellite imagery,” explained Bell. Following a similar event in South Dakota in 2003, researchers noted that the hail scar remained visible in satellite imagery for about six weeks.

The first image was captured on July 7, 2018, by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite. The damage was pronounced enough that it even elevated land surface temperatures, visible in the map above. “In the damaged areas, land surface temperatures are higher than in the surrounding areas because the bare soils heat up faster than in the non-damaged, vegetated areas,” said Bell.

The warmer temperatures within the damage swath are likely altering how and where warm air rises—a process called convection. Since convection is a critical ingredient in the formation of thunderstorms, some research suggests that these changes may cause or intensify thunderstorms in the region for the coming days and weeks.

The third image shows a more detailed view of damage just north of Onida, as observed two days later in both natural and false color by the Operational Land Imager (OLI) on Landsat 8.

https://earthobservatory.nasa.gov/images/92401/hail-cuts-swaths-of-damage-across-south-dakota
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Powerful Typhoon Heads for China

Once a super typhoon, the still powerful Typhoon Maria is expected to make landfall in eastern China on July 11, 2018, with damaging winds and heavy rains. Schools and factories in the city of Fuzhou have been closed; more than 140,000 residents have been evacuated from coastal and low-lying areas; and fishing boats have returned to port in anticipation of the typhoon’s arrival. Around 1,500 workers from Fujian Expressway Group are standing by to repair potential damage from the typhoon.

This image of Typhoon Maria was acquired on July 10, 2018, by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Aqua satellite. The storm already passed by Guam, knocking out power before passing over Japan’s southern Ryukyu Islands. The storm was headed for the northern tip of Taiwan and towards the Fujian and Zhejiang provinces of China.

Maria went through one of the fastest intensifications on record, growing from a tropical storm to a super typhoon in one day. The storm was at its most powerful on July 6 and July 8, when winds exceeded 135 knots (155 miles/250 kilometers per hour). The storm was equivalent to a category 4 hurricane on the Saffir-Simpson scale. The storm has since been downgraded to a typhoon and is expected to weaken some more as it approaches land. Even so, Typhoon Maria is formidable, bringing the potential to damage buildings and knock out power lines.

https://go.nasa.gov/2NI3AwP
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Severe Rainfall and Flooding in Japan

After being soaked in just a few days with double the amount of rain that falls in a normal July, parts of Japan are facing their worst flooding disaster in 35 years. Storms and flooding caused deadly landslides and numerous fatalities, while leading millions of people to evacuate their homes and businesses. Prime Minister Shinzo Abe has called for 73,000 nationwide rescue workers to provide emergency assistance as forecasts predict additional landslides and rain this week.

The map above shows rainfall accumulation from 3 a.m. (Japan Standard Time) on July 2 to 3 a.m. on July 9, 2018. Thirteen prefectures on Japan’s mainland received deadly amounts of rain. Hiroshima and Okayama, in the southern part of Honshu Island, were among the worst flooded areas.

These rainfall data are remotely-sensed estimates that come from the Integrated Multi-Satellite Retrievals (IMERG), a product of the Global Precipitation Measurement (GPM) mission. The GPM satellite is the core of a rainfall observatory that includes measurements from NASA, the Japan Aerospace Exploration Agency, and five other national and international partners. Local rainfall amounts can be significantly higher when measured from the ground.

The rains appear to have been caused by warm, humid air flowing from the Pacific Ocean and by remnants of Typhoon Prapiroon, both which intensified the seasonal rain front.

https://go.nasa.gov/2J9VU31
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Satellites Investigate Irrigation in a Stressed Aquifer

The High Plains Aquifer, also known as the Ogallala Aquifer, is under stress. Farmers today have to drill ever deeper wells in order to pump water for irrigation, and one recent study found the aquifer to be under more strain than any other in the United States. About 30 percent of the water once stored beneath Kansas is already gone, and another 40 percent will be gone within 50 years if current trends continue.

Nonetheless, pumping continues. Data collected by the MODIS sensors on Aqua and Terra satellites and other sources show that irrigation is increasing in some parts of the aquifer. One study found that 519,000 more hectares were irrigated in 2007 compared to 2002—97 percent of the total added in the United States during that period.

While MODIS can monitor regional trends, it cannot easily examine irrigation patterns at the level of individual fields. “The higher resolution of the sensors on Landsat give us a more nuanced understanding of the annual and seasonal rhythms of irrigation than is possible with MODIS,” explained Jillian Deines, a hydrologist at Michigan State University. Deines and colleagues David Hyndman and Anthony Kendall authored a study in which they compiled nearly two decades of Landsat data to study irrigation trends along the Republican River Basin, which runs through Colorado, Kansas, and Nebraska.

The map at the top of this page shows irrigation frequency in the basin between 1999 and 2016. Areas watered nearly every year are purple; those watered only rarely are yellow. The extent of the Ogallala Aquifer is shown with gray. The second image highlights the variability in irrigation between center-pivot irrigation fields in an area along the Colorado-Nebraska border. The most widely grown crops in the basin are corn and wheat.

Water use in the Republican River Basin is a sensitive issue. While there is a compact in place that details how the states should share water, litigation about water use is common. Given the legal context, Deines says it would be helpful for hydrologists and water managers to have a good understanding of exactly when and where irrigation occurs. Ground-based irrigation statistics tend to be decentralized and of varying quality, so Deines looked to Landsat for a more consistent view.

Examining natural-color and infrared imagery, the team produced high-resolution annual irrigation maps for the entire basin. The maps detail how frequently a field was irrigated, as well as the first and last year it was irrigated.

After analyzing some economic data, Deines and her colleagues believe some of the variability in irrigation they found was driven by crop prices. (Farmers expand irrigation when prices are high to increase yields and profits.) Rainfall also played a role in the variability. “Farmers ended up irrigating more intensely on a smaller number of fields during drought years,” noted Deines.

Landsat also detected an increase in the number of fields irrigated over the study period. Most of the increase was centered on the eastern part of the basin near the Platte and Republican Rivers, an area where irrigation depends more on drawing water from rivers than drilling groundwater from the aquifer.

https://go.nasa.gov/2J7Ysyn
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Marseille, France

An astronaut aboard the International Space Station (ISS) shot this photograph of Marseille, the second largest city in France. Known as Massalia in the days of the Roman Empire, the city sits along the Mediterranean coast.

From above, Marseille has a distinct red hue due to the clay terra cotta tiles covering the roofs of most buildings. Clay deposits are mined locally in Var, northeast of Marseille. Those signature roof tiles have influenced architectural styling in parts of Australia and New Zealand since the late 1800s.

The international spread of French culture and products can be attributed to Marseille’s coastal location. The city has been a major trading port since 400 BC, and the current Port of Marseille-Fos serves as the second largest port on the Mediterranean Sea. Today, the city is known for international trade and commerce of hydrocarbon products, iron, steel, ships, construction materials, alcohol, and food.

Adjacent to Marseille lies Calanques National Park, Europe’s first peri-urban national park—it is located at the transition between town and country. Founded in 2012, the park encompasses both land and water, while protecting the region’s natural landscapes, terrestrial and marine biodiversity, and cultural heritage.

https://go.nasa.gov/2J4ClJs
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Makgadikgadi Salt Pans

Northeast of Africa’s Kalahari Desert and southeast of the Okavango Delta lies one of the largest salt pans in the world. It was once the site of one of the largest inland seas on Earth.

On June 10, 2018, the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite acquired this natural-color image of the Makgadikgadi Salt Pans. The collection of salt flats covers roughly 30,000 square kilometers (10,000 square miles) amidst desert and dry savanna in Botswana. Located in Makgadikgadi National Park and Nxai Pan National Park, the salt pans are rivaled in extent only by the Salar de Uyuni in Bolivia.

For much of the year, the salt pans glimmer in white, parched by the sun and the salt and allowing little more than algae to grow. But during the rainy season (roughly November to March), the area can be transformed into a crucial wetland. Water can flow in from the Boteti and Nata rivers, filling ephemeral ponds, watering holes, and shallow lakes and creating short-lived but abundant grasslands. The event draws migrating wildebeest and zebras, as well as the predators that hunt them. The waters fill with ducks, geese, pelicans, and flamingos—one of just two breeding spots in southern Africa for the long-legged birds.

The pans are the salty remains of ancient Lake Makgadikgadi. Scientists estimate that the inland sea once spanned anywhere from 80,000 to 275,000 square kilometers. The Okavango, Zambezi, and Cuando rivers likely emptied into this lake until tectonic shifts changed the elevation of the landscape and a changing climate dried up the rains.

https://go.nasa.gov/2KX6gVE
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