salinity

Salinity is the total amount of dissolved salts in a body of water or in soil. In oceanography and hydrology, salinity is usually expressed as practical salinity units (PSU) or as parts per thousand (ppt). For seawater, typical salinity is about 35 PSU or 35 ppt, corresponding to roughly 3.5 percent by weight. Salinity in inland waters and soils varies more widely and can range from near zero in freshwater rivers to hypersaline conditions in some lakes and evaporation basins.

The major dissolved ions in seawater are sodium, chloride, sulfate, magnesium, calcium, potassium, and bicarbonate, with relative abundances that give seawater its characteristic composition. While the overall composition is relatively stable globally, local variations occur due to freshwater input, evaporation, mixing, and geological factors.

Salinity varies with location and depth. Evaporation concentrates salts, while precipitation, river inflow, groundwater discharge, and sea-ice processes reduce or modify salinity. Coastal zones, estuaries, and arid environments often show strong salinity gradients, creating brackish conditions.

Salinity has important physical and biological consequences. It strongly influences water density and buoyancy, osmoregulation in aquatic organisms, and nutrient cycling. In soils, high salinity can impair plant water uptake and reduce crop yields; salinity management is a major concern in arid regions and around irrigated lands.

Measurement and applications: salinity is commonly measured with conductivity sensors, refractometers, or sampling analyzed for ionic content. Conductivity at a standard temperature is converted to PSU. Salinity data are used in oceanography, climate studies, desalination technology, irrigation planning, and water quality assessment.