Let's start by thinking about where Earth's water is located.
Saltwater – 97%
The majority of water on Earth is salty! Chloride and sodium are the most abundant ions found in salt water. Other ions or elements, particularly in the oceans, include magnesium, sulfur, calcium, potassium, and much more. These ions form salts, giving oceans, seas, and some lakes their salty or saline characteristic. Organisms that in these habitats have adaptations that allow them to live in the water and salt that constantly surrounds them. Humans cannot use saltwater directly – which you probably know if you’ve ever swallowed ocean water and felt sick afterward. Before humans can use saltwater for drinking or farmland irrigation, it must be treated to remove the salts, through a process called desalinization. Desalinization is costly and requires a lot of energy, but for places that are extremely dry this process can provide people with much-needed freshwater.
Ice – 2.06%
Most of the freshwater on the Earth is frozen! So much of our water is frozen, that if all of it melted at once, the sea would rise about 6 meters (20 feet)! Ice is made of freshwater, even ice floating in oceans and seas. This is because saltwater freezes at much lower temperatures than freshwater and very cold saltwater is so dense it sinks away from the surface where freezing takes place. Ice is found at the north and south poles as ice caps, as glaciers on high mountains and at high latitudes, and in regions with permanent snow and permafrost (frozen soils). Approximately 90% the Earth’s ice is in Antarctica. Ice seems motionless but many ice features move and flow, just very slowly. Ice caps and glaciers form in layers, as snow and frozen water is deposited over long periods of time. As each layer forms, gasses, dust, and other molecules get trapped, forming a record of the climate conditions that year. Climate scientists can drill deep into ice caps and glaciers to analyze how the Earth’s climate has changed over millions of years.
Groundwater seepage through sandstone. Image by Wayne Wurtsbaugh.
Groundwater – 0.9%
Nearly anywhere you can stand on the Earth’s surface, there is water in the ground underneath your feet. Water from rain trickles downward through the soil until it reaches material that is already saturated with water. Depending on depth of this area and the how fast the water has filtered through the soil, groundwater can be days to thousands of years old. Places where groundwater collects in water wells are called aquifers. Aquifers can be quite large – the Great Artesian Basin in Australia is one of the deepest and largest in the world, covering 1.7 million square kilometers (660,000 square miles) or 23% of the Australian Continent. In the United States, the Ogallala Aquifer covers 450,000 square kilometers (174,000 square miles) of the Great Plains, occurring in parts of Wyoming, South Dakota, Nebraska, Colorado, Kansas, New Mexico, Oklahoma, and Texas! Humans rely heavily on groundwater for drinking, farming, and other uses but over-use, pollution, and sea level rise threaten this precious resource.
Faselfad Lakes in Austria. The bright color in the lake on the left is due to inputs of glacial sediments, while the lake on the right has a high turbidity.
Lakes – 0.008%
Lakes are just one type of surface-water – water that is easily accessible and visible on the surface of the Earth. Lakes form where water runoff from rain and snow accumulates. In some places, lakes form in areas where groundwater seeps up to the surface. Lakes come in a variety of shapes and sizes, and can contain salty or fresh water. The Earth’s oldest and largest lake, Lake Baikal in Siberia, has a depth of over 1,500 m or 1 mile! Freshwater lakes are highly valued by people, as places of recreation and water supply. If your city or town has a reservoir, this is a natural or man-made lake used for its water – the source of water coming out of your faucets.
Wetlands – 0.001%
Wetlands occur in areas where water covers the soil for varying periods of time. This phenomenon can occur along coastlines where tides move water back and forth over the land, and in areas that are prone to flooding such as low lying areas around lakes and rivers. You might also know them as deltas, estuaries, marshes, swamps, and bayous. Wetlands contain a large diversity of organisms as they are areas where both land-dwelling and aquatic organisms live. This diversity of organisms, from microbes to mammals, interact with the environment and each other to create some of the most active aquatic habitats on Earth. Wetlands also fill important roles such as nursery habitats for fish, resting places for migrating birds, and buffer zones from storm damage. Unfortunately, many wetlands are also located in areas that are considered prime real estate for humans which has resulted in more than 50% loss of this important habitat.
Perdenales River, Texas cuts through sandstone.
Rivers – 0.0002%
Rivers form where water flows downhill, due to gravity, making a journey from the tops of mountains to the sea. Many different plant and animal species can be found along rivers. Although, rivers make up a small proportion of Earth’s water resources they have and continue to be an important resource for humans, serving as transit systems for exploration and transport of goods, power generation, recreation, and a source of freshwater.
Water moves between these reservoirs through processes that transfer energy across the globe.
Earth’s Water Resources are Connected through the Global Water Cycle
As you read above, water on Earth comes in many different forms and is found in many different places, from the oceans to deep below the soil surface. The globalwater cycle, or hydrologic cycle, constantly transforms and moves water around the Earth.
Warm air temperatures cause evaporation, the loss of water from the oceans, lakes, rivers, soil, and even plants into the atmosphere. Water in the atmosphere exists as vapor and as it rises and is pushed around by wind it encounters cooler temperatures. The cooler temperatures cause condensation, the transformation of water vapor into larger droplets – you see large accumulations of droplets as clouds and fog. Eventually water accumulations grow and collide forcing droplets to fall as precipitation; rain, ice, and snow. This precipitation is crucial to adding water back into the oceans, lakes, rivers, and groundwater reserves.
The global water cycle is powered by the sun; solar energy (heating) drives the processes described above. The global water cycle is an important driver of climate too. Human activity such as deforestation, water consumption, the channeling and diverting of waterways, and climate change influence the global water cycle.