In the last part we came to the sobering realization that stretching ecosystem too far is dangerous for our own future. In this part, I want to introduce another such regulating service of ecosystem- the control and mitigation of floods.
Every year, an enormous amount of rain falls on earth’s surface. The water from rain is enough to cover the entire land area to the height of one meter! Most of this water gets absorbed in soil. Some flows into streams and rivers, and finally to the oceans. When the rain falls slowly, the soil and streams contain the water to normal levels. But sometimes a lot of water falls rapidly. At such times, the soil gets saturated, rivers get full and water level rises in the surrounding areas. This is what we call flooding.
The damage caused by floods throughout the world is enormous. Flood is the most common natural disaster. As many as 40% of natural disasters are floods. The economic loss from floods averaged 190 billion dollars a year in the period 2001-2010. The loss of and disturbance in every kind of life is of course incalculable. The instances of flooding have been increasing in the last fifty years. While there were 51 large floods in the 150 years from 1800 to 1950, there have been 31 floods in the ten years from 2000 to 2009, and as many as 17 floods in the two years 2010-11. We can now easily imagine the value of controlling floods.
Ecosystem is responsible for all the floods that DO NOT happen. As we have seen, the soil is responsible for absorbing most of the water that falls on surface. Because of the various kind of vegetation like grass, plants and trees, the speed of water flowing over soil is reduced, so more water is absorbed, preventing water-logging. The area around a river containing soil, vegetation and small water bodies is called floodplain. The floodplain acts like a giant dampener for the flowing water. It slows down the flowing water and absorbs it. The role of vegetation here is very important. If the vegetation in floodplain is destroyed, for example to build houses, the water speed will be high and soil will be loose. Then the soil will form mud, which will flow with the water. The mud fills the gaps in the earth. This reduces the water absorption even more and makes the flood more severe. It is the vegetation in the floodplain that stops this damaging process. The erosion of soil from deforested land, such as crop planted on hillside, is as much as 150 times that of similar forested land!
The flowing mud goes and deposits in the rivers, dams, lakes and other water reservoirs. This reduces the quality of drinking water and capacity of the reservoirs. India for example cannot draw water from 81 of its most important dams, as the silt deposits have accumulated. The Upper Vaitarana reservoir near Mumbai has lost 22% of its capacity due to silt deposit. We then have to spend big money on desilting the lakes and rivers or increasing the height of the dams. While we pay our taxes and fund these projects, it would be worth while to remember that it is the ecosystem that has saved all the money that we did NOT spend.
So much about the flood control ecosystem service. What the flowing mud carries away is probably the most precious commodity mankind ever had, or will ever get. It is called the topsoil. It is so important that I will dedicate the next part to the various services provided by only the topsoil.
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