Water is everywhere. In fact, it covers two-thirds of the Earth's surface. Water is a big part of life, and you can't go a day without it. Because of this, it is an incredibly valuable resource, and one that is often referred to as being renewable. But we're often told not to waste water, and people living in desert climates have to worry about conserving water so they don't run out. Despite the fact that water is a renewable resource, how come we have to worry about running out of it sometimes?

Water, despite occasional scarcity, is considered a renewable resource. Renewable resources are resources that replenish at reasonable rates so that humans won't run out of them. Water is renewable because it is recycled in reasonably short periods of time. To simplify the water cycle, fresh water is found in streams, reservoirs and underground. When people use this resource, waste water will flow into the ocean, and become salt water, which is a form that is not really available for most human uses.

Water will evaporate from the ocean and later fall as rain or snow over land, replenishing freshwater supplies. Other examples of renewable resources are sunlight, which is consumed as solar energy. Sunlight replenishes instantaneously. Wood is another renewable resource, because as it is used, new trees are growing back to replenish the supply. Alternative energy sources like hydropower and geothermal energy are renewable because they don't consume the resources they use, so nothing needs replenishing.

The key point is that renewable resources replenish either instantaneously or relatively quickly.

Non-renewable energy resources are resources that diminish in total supply as they are consumed, or if they replenish, they do so incredibly slowly. Fossil fuels like coal and oil are a good example of non-renewable resources, because as more of them are mined and burned, they become less available overall. Coal and oil are both fossilized plant material, which can theoretically replenish, but this is a process that takes millions of years, so the rate of renewal is negligible on a human timescale.

Non-fuel resources like phosphorus are non-renewable not because there are less of them overall, but because the way they are used as a resource means less of them are available over time. Phosphorus is used in fertilizers and naturally cycles locally throughout ecosystems. But as farmers use phosphorus-based fertilizer in their fields, that phosphorus ultimately finds its way into the ocean, and there is no natural mechanism for it to make its way back to land from there.

Just because a resource is renewable, that doesn't mean all use of that resource is sustainable. Of all the water on Earth, roughly three percent of it is available for human use at any given time. The rest is either locked in ice or full of salt. That three percent can renew fairly quickly, but unsustainable water consumption can outstrip renewal.

This is because the rate of this resource's renewal is more or less fixed. It relies on natural processes that operate independently of how the resource is used - this is why water conservation is important. In many places, especially in arid locations, the renewal time for water resources can be much slower than in other places.

Think about a city's water supply as a bathtub filled to the top. If two gallons of water flow into the bathtub every hour, but the city drains one gallon per hour, it is using water very sustainably. But if that city drains four gallons of water per hour while only two per hour flow in, it will run out of water eventually, regardless of the fact that it is being constantly renewed.