Water is everywhere. In fact, it covers two-thirds of the Earth's surface. As a vital natural resource needed for basic survival, comfort, and operation, water is constantly in high demand. The Earth is a curiously organized ecological and geographical system that allows for water to operate as a renewable resource, but we're often told not to waste water. People living in desert climates have to worry about conserving water so they do not run out, and many areas of the world have to divert massive amounts of water from distant sources to support their populations. By many accounts, water’s status as a renewable resource has not prevented it from becoming a critically limited resource. Why do we have to worry about running out of it if water is supposedly perpetually reusable?

Water, despite often scarcity, is considered a renewable resource. Renewable resources are resources that replenish at reasonable rates to keep pace with human consumption. Water is renewable because it is recycled in reasonably short periods of time, and it allows water to be used without a long-term depletion in supply. The hydrological cycle (often called the water cycle) starts and ends with fresh water, sea water, and atmospheric water vapor all across the world.

People use freshwater sources for cleaning, manufacturing, drinking water, and a wide variety of other applications. Fresh water is often stored as surface water (e.g. lakes, streams, snow, and ponds) and groundwater (e.g. aquifers and underground lakes). After being used, fresh water is often repurposed for successive uses, or it simply undergoes evaporation and proceeds to the next stages of the water cycle.

Used fresh water and precipitation (from evaporated water) will often find a way to oceans and seas around the globe. Then the fresh water will become salt water as it disperses into the salty sea water.

Water will then evaporate from the ocean and later fall as rain or snow over land, replenishing freshwater supplies. These processes will repeat and interact, but the amount of water on Earth will always remain essentially constant. Water scarcity arises when the various forms of fresh water become less common or harder to access, and some parts of the world (like arid desert climates) find it difficult to locate long term water sources because of their geography and weather patterns.

Other examples of renewable resources are sunlight, which is consumed as solar energy. Sunlight replenishes instantaneously, and the energy from the sun actually provides the necessary energy input to drive the water cycle. 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. Biofuels, derived from organic biomass, still contribute emissions and air pollution, but they are still considered a renewable energy source because they replenish on a short time scale.

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 not relevant when considering human rates of consumption.

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.

Nonrenewable energy sources like coal, oil, and natural gas cannot replenish quickly enough in relation to human consumption. They also contribute heavily to climate change and global warming from countless human activities and emissions.

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 climates, the renewal time scale 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.

Water pollution and irresponsible use of water bodies contributes further to limited freshwater access and unfair water usage rights.