Surface water includes all water collecting on the ground or in a body of water open to the atmosphere, encompassing both freshwater and saline water. The classification depends entirely on its dissolved salt concentration, which varies widely across the globe. Therefore, not all surface water is freshwater. Understanding this distinction is important for grasping the planet’s water distribution and the availability of usable resources.
Defining the Two Categories
Water is scientifically categorized based on the concentration of total dissolved solids (TDS), which measures all inorganic and organic substances dissolved in the liquid. For water to be classified as freshwater, its TDS concentration must be less than 1,000 parts per million (ppm). This low concentration distinguishes it from brackish or saline water.
Surface water originates primarily from precipitation, which is inherently low in mineral content. However, the water’s final classification is determined by the geological and hydrological processes it undergoes. Water flowing over the land constantly absorbs minerals and salts, which contributes to its overall TDS level.
Key Types of Freshwater Surface Water
The most common examples of freshwater surface water are bodies that maintain a low salt content through constant replenishment and outflow. These include rivers, streams, most lakes, and wetlands like marshes and swamps. These systems are continuously flushed by precipitation and runoff, which introduces new, low-mineral water into the system.
This constant input, combined with the continuous flow, prevents the significant accumulation of dissolved solids. Rivers are characterized by moving water that carries away dissolved salts and other materials. Most lakes remain freshwater because they have an outlet that allows the water and its dissolved contents to escape.
Surface Water That Is Saline
A significant portion of the planet’s surface water is classified as saline, meaning it contains high levels of dissolved salts, far exceeding the 1,000 ppm threshold for freshwater. The oceans and seas represent the largest reservoir of saline surface water, containing approximately 97% of the Earth’s total water. The average salinity of the open ocean ranges from 33 to 37 parts per thousand (ppt).
This high salinity results from the natural weathering of rocks over billions of years, with rain runoff carrying minerals and salts into the bodies of water. Since oceans lack an outflow other than evaporation, the salts accumulate over time. Highly saline inland lakes, such as the Great Salt Lake or the Dead Sea, also exist because they are terminal basins with no outlet, causing high evaporation rates to concentrate the dissolved salts.
Global Accessibility of Usable Surface Water
While water covers about 71% of the Earth’s surface, the vast majority is saline, making it unsuitable for drinking or agriculture without expensive treatment. Freshwater accounts for only about 2.5% to 3% of the world’s total water supply. Even within this small freshwater reserve, much of it is not readily accessible surface water.
A substantial amount of freshwater is frozen in glaciers, ice sheets, and permanent snow, accounting for approximately 68% of the total freshwater. This leaves only a fraction of a percent of the planet’s water as easily sourced liquid freshwater on the surface, primarily in rivers and lakes. Although rivers and lakes make up a small percentage of all freshwater, they are the most important sources for human use.