Changes in the Andes Cause Flood Risks for South American Cities

December 8, 2012

Science

bogota-skyline-andes

Deforestation and a changing climate high in the Andes can adversely impact Colombia’s cities far below. Credit: Flickr/Matthew Rutledge

At more than 11,000 feet above sea level, very important ecosystems known as páramos exist. These kinds of ecosystems are only found in Central and South America and resemble a sort of alpine archipelago, each a link between distinct island ecosystems that exist nowhere else on the planet.

These páramos play a crucial role in maintaining a reliable water supply for millions of people in Bogotá and Medellín. And along with the forests below, they also protect the cities from flooding. Colombia’s second rainy season of the year began in September. Some parts of the country have had little respite from the deluge. Scientists believe that climate change is a factor in the flooding, and it has affected three quarters of Colombia in the past two years.

The degradation of the high Andean ecosystems is accelerating and there are growing concerns that the floods will become a chronic problem. Páramos and cloud forests are a part of the ecological engineering that manages the flow of water from the high mountains to the lakes and rivers below. They act like a sponge, absorbing and releasing water down the mountainside to the cloud forests. From there, the water is filtered and directed into rivers and reservoirs that supply the cities.

This system supplies 80% of the drinking water in Bogotá. The annual precipitation reaches 13 feet in some areas. The extreme, prolonged rainy season in 2010 and 2011 could offer a preview of the damage climate shifts may have on urban areas in the future. In Medellín, heavy rains have created rivers in the city center that have led to landslides. The Bogotá River is overflowing and flooding low-lying neighborhoods. Three million Colombians, about 7% of the total population, were displaced or suffered damage in 2011 as a result of flooding.

Climate models predict that warming in the Andes will contribute to more flooding and drought as mountain environments change. High altitude ecosystems like páramos are sensitive to warming, much like coral reefs and glaciers. One páramo research station showed increases in minimum temperatures that were almost twice that of lower elevations, while increases in maximum temperatures jumped to nearly three times the average at lower elevations.

Extreme rains are likely to cause more runoff and soil erosion, which is a recipe for more flooding. Dry periods are getting longer and wet seasons are getting more extreme. The conversion of mountain forest to pasture and croplands has raised temperatures in the region, prompting a lifting of the cloud level, enough to leave some cloud forests and páramos below the fog, increasing solar radiation. This causes plants found at lower elevations to move higher, which disrupts the delicate biological machinery that makes páramos and high forests function as a water storage and distribution system.

Páramos are receiving more precipitation as rain instead of fog. This affects plants adapted to collect moisture from fog and the soil. The heavier raindrops also compact the soil, causing it to absorb less water, increasing runoff and sediment, clogging rivers and contributing to lowland flooding.

[via Scientific American]

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