Water exists in various forms and locations across Earth. Understanding its classifications is important for comprehending its availability and utility for natural processes and human endeavors.
What is Surface Water?
Surface water refers to any water body found on Earth’s land surface. This includes natural formations like rivers, streams, lakes, ponds, and wetlands. Oceans and seas are also categorized as surface water, covering the vast majority of the planet.
This category is distinct from groundwater, which resides beneath Earth’s surface in aquifers. Surface water can be perennial, existing year-round in large lakes and rivers, or ephemeral, appearing seasonally in creeks and waterholes. Artificial lakes and reservoirs also contribute to surface water.
What Defines Freshwater?
Freshwater is characterized by a low concentration of dissolved salts, particularly sodium chloride. The accepted threshold is typically less than 1,000 parts per million (ppm) of total dissolved solids (TDS). For drinking, a concentration below 500 ppm is often preferred.
The primary natural source of freshwater is precipitation, which includes rain, snow, and mist. As water evaporates from oceans and land, it leaves dissolved salts behind, and then condenses to form clouds that release relatively pure water. This precipitation replenishes freshwater bodies, either directly or through meltwater from glaciers and ice caps.
The Relationship Between Surface Water and Freshwater
The relationship between surface water and freshwater is not a simple equivalence; rather, it is a conditional one. Some surface water is indeed freshwater, while other surface water is not. The defining factor separating these categories is the concentration of dissolved salts.
For instance, most rivers, many lakes, ponds, and wetlands are classified as freshwater because their salinity levels fall below the established thresholds. Conversely, oceans and seas, despite being prominent forms of surface water, contain high concentrations of salt, averaging around 35,000 ppm, and are therefore not freshwater. Some inland lakes can also be highly saline, exceeding freshwater definitions. The continuous movement of water through the global water cycle plays a significant role in replenishing freshwater surface bodies. Water evaporates from all sources, including saline ones, but the salts are left behind during this process, leading to the formation of freshwater precipitation. This precipitation then falls onto land, contributing to surface runoff and groundwater recharge that feed freshwater rivers and lakes.
This distinction between freshwater and saline water within surface water carries substantial implications. Freshwater surface bodies are important for human consumption, providing a primary source of drinking water for communities worldwide. They are also vital for agriculture, with irrigation relying on these resources to support crop growth. Freshwater ecosystems, such as wetlands and rivers, support diverse plant and animal life, contributing to biodiversity and ecosystem health.