I know we need to conserve water and such but why? Doesn’t it eventually flow back into the ground or evaporated and rained back down regardless of how it’s consumed, so it’s a cycle? Do we lose a bit per cycle?
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You are not being asked to conserve water, because the water cycle means that for the most part water is never lost across the whole planet.
You are being asked to conserve purified, drinkable water. That’s a very small fraction of the water on the planet, and it’s quite expensive to take the water we have lots of (salt water in the ocean) and turn it into drinkable water.
Earth won’t run out of water, the planet’s surface is mostly flooded miles deep with the stuff.
What we run out of is easily accessible salt-free water near human population centers.
It takes a lot of money and energy to purify salt water or pump water long distances, so cities greatly prefer to draw their water from a nearby freshwater source. For a city like Chicago or Detroit that sits along a massive freshwater lake this isn’t a problem, but for a city like Las Vegas or Phoenix that’s in the middle of a desert trying to draw water from a single nearby river, the capacity of that river becomes a limitation.
It rains, but not always where we would like it to rain, so the amount of rain water in a specific location is limited, in addition the purification to make the water fit to drink/wash etc. uses up energy which drives up the cost of water.
The speed at which water evaporates and rains down is limited. With increasing population and industry, we can consume and pollute more water than gets refreshed, especially in drought-prone areas. Moving water from where it is more abundant is not practical. Fresh water still requires some treatment steps and pumping, which cost money. Discarded untreated wastewater can pollute ground water sources and make them unusable.
It’s a local issue rather than a global one. The world’s supply of water might not be running out but water that evaporates or runs off your hometown today may not return for hundreds of years. A location only gets a certain amount of water every year from rivers and rainfall and if that runs out then the place will be nearly unlivable.
It does eventually flow back into the ground. Water conservation is needed mostly during droughts.
Several good answers already, but an additional factor I don’t think has been mentioned:
Many parts of the US rely on pumping drinking and irrigation water at unsustainable rates from deep aquifers that built up over thousands of years. Every year, the aquifer recedes from being overdrawn, and in many areas, will be depleted soon.
It takes a very very long time for underground water sources to replenish. We are taking it out faster than that.
Removing salt from sea water is terribly expensive.
Collecting rainwater is iffy due to weather and requires a lot of space.
Some water flows back into the ground or rivers, but much of it ends up in oceans and becomes salt water, which is WAY more expensive to desalinate and treat for human use. Also, the demands for water and the locations of fresh water aren’t always the same. California has tens of millions of residents and a huge percentage of the country’s farmland, so there are HUGE needs for water in California. But floods in Florida do nothing to help draught conditions in California because there is no feasible to way to capture and transport water on the volume necessary.
Reservoirs under the ground take decades if not centuries to fill up again. Draining them completely is not a good long term sustainable goal. They should be for emergencies and not the only solution for drinkable water.
Easily accessible water like rivers or streams are good but we have run off of chemicals and oil into them to we need to purify it in order to drink it. Rivers change over time too. A river is not a static formation and a river can “move.” This was one of the many key factors that started the drought in California. Rivers tend to change all the time and there’s an entire science behind this fluid mechanics used by engineers when building structures to tame water.
Great answers in this thread and a great question too!
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