Pakistan's 2010 Floods: How Stuck Weather, La Niña and People Made a Disaster

Every summer the monsoon soaks India and Bangladesh, but in neighbouring Pakistan its rains are usually lighter, more on and off, and mostly in the northeast. The summer of 2010 was different. In July and August rain fell over most of the country, in places for weeks. The Pakistan Meteorological Department recorded nationwide rainfall 70 percent above normal in July and 102 percent above normal in August; in Peshawar, July's rain was 772 percent above normal, and in Khanpur, August's was 1,483 percent above.

How much more rain fell in July and August 2010 than the long-term average, estimated from satellites. NASA maps by Jesse Allen and Robert Simmon, using Global Precipitation Climatology Project data.

The Indus overflows

The Indus and its tributaries in the north burst their banks, and the surge swelled the river as it moved south through the central and southern provinces. In Sindh a dam failed and sent the river down another channel west of the valley, into a flood lake that merged with Manchhar Lake and spread over hundreds of square kilometers. Floods covered at least 37,280 square kilometers (14,390 square miles) at some point between July 28 and September 16, 2010. The water fell far more slowly than it rose: months later crops, homes, businesses and whole towns were still under water, and in places the only way out for it was evaporation.

The U.S. Agency for International Development estimated that the floods affected more than 18 million people, killed 1,985 and damaged or destroyed 1.7 million houses. It was perhaps the worst flood in Pakistan's modern history.

Why the rain would not stop

The monsoon works like this: sunlight heats the land of Central Asia, the warm air rises, and cooler, moister air is drawn in from the Indian Ocean. The Himalayas block the moist air from moving into Central Asia, so it rises, cools and falls as rain. In 2010 several things pushed this pattern over Pakistan.

• La Niña. A La Niña event, which went on to drench Australia and other places around the Pacific and Indian Oceans in late 2010 and early 2011, began around the time of the monsoon. It let the air carry more moisture, raising the chance of rain, though not by itself its intensity or persistence.
• A stuck jet stream. Meteorologists noticed that the jet stream had dipped over Eurasia and stalled. NASA Goddard scientists William Lau and Kyu-Myong Kim linked the same pattern to both the floods and that summer's heat wave and wildfires in Russia. A blocking event, a large stagnant weather pattern, stopped weather systems moving west to east and trapped hot, dry air over Russia.
• A clash of air masses. Downstream, the block set up unusual swirls of wind that carried cold, dry Siberian air south into the subtropics, where it met warm, moist monsoon air flowing north. The result was torrential rain in northern Pakistan.

High (red) and low (blue) pressure from July 25 to August 8, 2010. The high over Russia was a blocking event. NASA map by Jesse Allen and Mike Bosilovich, using MERRA data.

Lau's team concluded that the floods were triggered by disturbances pushing south from the block, and amplified by heat and monsoon moisture from the Bay of Bengal, with La Niña supplying extra moisture. Was it a sign of the future? "One event by itself is not evidence of a long-term shift," said Peter Clift, a geologist at the University of Aberdeen who studies the monsoon. Still, a longer, stormier monsoon may lie ahead if climate predictions hold.

What people added

Human activity probably made the floods worse than nature alone would have.

• Lost vegetation. Plants soak up rain, and decades of deforestation, especially in the Swat Valley, had left the land less able to absorb it.
• Diverted rivers. Most of the upper Indus comes from glaciers in the Himalaya and Karakoram, and dams, levees and channels divert it for irrigation. Diversions have grown in recent decades, and many landowners built their own embankments.
• Poor upkeep. "A system like that needs sustained maintenance," said Dath Mita of the U.S. Foreign Agricultural Service; silted-up channels probably carried less water.

Structures that could not contain the surge, Clift noted, then proved very good at holding the water in the wrong places. Yet Pakistan is usually short of water, not flooded: "Most of the time, they need that water infrastructure."

Flood water (blue) in western Sindh, September 2010… NASA image by Robert Simmon, using Landsat 5 data.

… November 2010 … NASA image by Robert Simmon, using Landsat 5 data.

… and January 2011. Roads and other structures held the water in place. NASA image by Robert Simmon, using Landsat 5 data.

The long aftermath

Cholera and other waterborne diseases threatened survivors; standing water bred malaria mosquitoes; crowded camps spread measles. "It is hard to remember a previous flood that has caused this type of impact over such a long period," said landslide specialist David Petley of Durham University; early reports called it the worst since 1929, "but this event was worse by almost every measure."

On the farms, the USDA estimated that almost 10 percent of the cotton crop and about a fifth of the rice crop were flooded. The staple, wheat, had already been harvested and stored, and Mita expected the wet soil to help the next crop. For many families the worst loss was livestock, their source of milk, biogas and ploughing power. The UN, aid agencies and the government built roughly 40,000 shelters, but many people rebuilt with poor materials. In early 2011 some land was still under water, UNICEF helicopters carried supplies into northwestern areas cut off by the flood and the 2005 earthquake, and the World Food Programme fed an estimated 4.4 million people in January and February.

Floods this size are rare in a human lifetime but not in geological time: Clift has found sandy layers left by ancient Indus floods. "Still," he said, "I've not seen anything like this."

Sources

• NASA Earth Observatory, "Heavy Rains and Dry Lands Don't Mix: Reflections on the 2010 Pakistan Flood"; rewritten in hubnx's own words. Maps and images from NASA; a flood map made with the UN's satellite centre is left out.