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water > factfile > droughts and floods

Droughts and floods

Posted: 26 Mar 2008

Over the past decade, the number of droughts and floods have increased dramatically, as environmental conditions have deteriorated and the global climate continues to change due to accelerated greenhouse gas emissions. According to the World Meteorological Organisation (WMO), the number of people affected by floods alone from 1991 to 2000 is reported to be around 1.5 billion.

Nearly all of the world�s river systems have been altered by human activities. River modifications and excessive water withdrawals have contributed to and aggravated drought conditions in dry lands throughout the world. This has been exacerbated by loss of tree cover in watersheds, since trees and other vegetation help to soak up and store water during the wet season making more available during the dry season.

Many rivers no longer reach their deltas during dry periods. Rivers such as the Colorado, Huang-He (Yellow River), Ganges, Nile, Syr Darya and Amu Darya run dry in their lower reaches during the dry season � some of the are dried up for half of the year or more. The conversion of wetlands to agricultural and urban land also have impoverished the capacity of these natural sponges to soak up and store excess water during the rainy season.

Climate anomalies in East Africa, 2000. Credit: WMO
Climate anomalies in East Africa, 2000. Credit: WMO

What this means, according to the World Resources Institute, is that droughts are now becoming more frequent and more severe in drylands, while floods have also increased in intensity and frequency, particularly in flood prone regions throughout the world.

Consider the following recent examples:

  • Floods throughout South Asia in 2007 � across India, Pakistan, Bangladesh and Nepal � left 1,400 dead and displaced 31 million people. Some areas received 30-50% of their annual rainfall in just 30 days, from mid-June to mid-July.
  • Flooding in Mumbai, India in July 2005 left over 700 dead. Some low lying areas of the city were under 15 feet of water.
  • The devastating 2000 Mozambique flood, caused by heavy rains followed by a tropical cyclone, inundated most of the country, killing tens of thousands, and crippling the country�s economy. Flooding in early 2008 affected 60,000 people.
  • In September 2000, heavy rains in Southeast Asia resulted in unprecedented flooding along the Mekong River and its tributaries. Damage was widespread:
    - Flood waters inundated parts of northern Thailand, damaging more than half a million hectares of cropland
    - Nearly half a million people in the Mekong Delta (in Cambodia and Vietnam) had to abandon their homes
    - In Cambodia, rising flood waters submerged close to 400,000 hectares of cropland; emergency supplies were distributed to 1.4 million people
    - In Laos, over 18,000 families had to be evacuated from flood plains and the rampaging waters severely damaged just under 50,000 hectares of cropland.
  • By 2050 some 5 million people in the Nile delta could be displaced by rising sea levels.
  • In the United States, the Mid-Atlantic States flood of 2006 is considered the worst since the floods caused by Hurricane David in 1979.

The UN�s Economic and Social Commission for Asia and the Pacific (ESCAP) blamed the floods (and droughts) in the region on widespread deforestation in watershed areas, poor soil management practices, reclamation of flood plains and wetlands and the rapid expansion of urban areas.

The see-saw effect of droughts followed by floods is becoming more severe according to the UN. Destruction of forests and wetlands is the main reason. But as climate change sets in, these increasingly destructive cycles are expected to become even worse.

World Meteorlogical Organization

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