Why are dams causing recurring draughts? After an initial phase where the dam fills up shouldn’t there be same amount of water running out of it as before?

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Why are dams causing recurring draughts? After an initial phase where the dam fills up shouldn’t there be same amount of water running out of it as before?

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>After an initial phase where the dam fills up shouldn’t there be same amount of water running out of it as before?

That is not true.

The first point is that you can change when during the year water flow. So when water flows it is not the same. If you use the dam for hydroelectric power then you can lower the water level by using more water then is coming in to it when there is a high power demand. You can then release less when demand is lower and you need to refill it.

The next part is the surface area of the water above the dam has increased. This can be a lot. It looks like Lake Powel in the colorado river loses 6% of the total water flow by evaporation. Add to that the evaporation from Lake Meed at hoover dam. [https://www.glencanyon.org/lake-powell-reservoir-a-failed-solution/](https://www.glencanyon.org/lake-powell-reservoir-a-failed-solution/) The climate where the dam is and its surface area vs volume will have an effect on how much water evaporates

This is all assuming no water is removed as drinking water or for irrigation.

So when water flow during the year will change and evaporation will reduce the total amount of flow in a year.

There are a few differnt ways in which a dam might cause drought. Dams are built to serve a purpose, usually to create a resevoir of water. The water is needed for things like hydro power stations, irrigation or water supply. These may have their outlets far downstream in the river or even in different rivers altogether. That can cause droughts in the river downstream of the dam.

The amount of water needed also varies. Hydro power plants tend to be used for power spikes that slower power plants like coal or nuclear struggle with. So the hydro power plants often shut down to let the water reservoir refill so they can run when electricity prices are higher. That might cause droughts downstream of the power station, even if the power station and dam are located together.

You might also see droughts if there is high water usage from the resevoir. The water stored there is intended to be used when needed so when it is needed it is tapped down to its original level or even lower. This can cause droughts around the reservoir, places which used to have ground water supplied from the reservoir will become dry.