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Freshwater is an essential natural resource on our planet, characterized by its low concentration of dissolved salts compared to the saltwater in oceans and seas.

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About 71% of the Earth's surface is water-covered, and the oceans hold about 96.5 % of all Earth's water.
Water also exists as:

Water vapor

Rivers and lakes

Icecaps and glaciers

In the ground as soil moisture(permafrost) and in aquifers

Living beings

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Humans and other organisms rely on water for life. Freshwater is fundamental for human activities and for their progress throughout history.
Humanity:

Survival and health

Agriculture

Industry

Hydropower

Transportation

Recreation

Environment:

Climate regulation

Biodiversity conservation

Sustainable development

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Salt water is water that contains dissolved salts. The salt content in salt water can vary depending on the location, but it is typically around 35 parts per thousand (ppt).

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Salt water is formed when rainwater falls on land and dissolves minerals from the rocks and soil. The rainwater then flows into rivers and streams, which eventually carry it to the ocean.

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There are two main types of saltwater: ocean water and brine.

Ocean water is the salt water that covers most of the Earth's surface.

Brine is a type of saltwater that is found in lakes, ponds, and other bodies of water that are isolated from the ocean, in some special cases we can see a brine pool. A water so dense with salt that it won't mix with seawater and forms a clearly defined “lake” on the ocean floor, becoming a toxic environment where few organisms survive

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Biodiversity

Climate: Oceans play a crucial role in regulating global climate by absorbing and releasing heat and carbon dioxide.

Food

Transportation

Recreation and Tourism

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Oceans are dynamic ecosystems where various biogeochemical cycles play essential roles in regulating the distribution of elements and nutrients.

1. Water Cycle
2. Carbon Cycle
3. Nitrogen Cycle
4. Phosphorus Cycle
5. Sulfur Cycle
6. Iron Cycle
7. Oxygen Cycle
8. Silicon Cycle
9. Calcium Carbonate Cycle
10. Sedimentation Cycle

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Is a continuous natural process involving the evaporation of water from water sources, its condensation into clouds, and its precipitation as rain or snow, and is stored in the atmosphere, on the land surface, and below the ground.

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1. Rivers and streams

2. Lakes and ponds

3. Wetlands

4. Glaciers and ice caps

5. Groundwater

6. Estuaries

7. Mangrove

8. Peatlands

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An ecosystem is a complex, interconnected community of living organisms (biotic components) and their physical environment (abiotic components) in which energy and nutrients flow through various pathways.

Open Ocean (Pelagic Zone)

Coral Reefs

Intertidal Zone

Estuaries

Mangrove Forests

Kelp Forests

Deep Sea

Salt Marshes

Seagrass Beds

Rocky Shores

Open Ocean Gyres

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54% of groundwater is saline and exists mostly below oceans. The other 46% of groundwater is freshwater that exists below land surfaces.

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Water that collects or flows beneath the Earth's surface filling the porous spaces.

Unsaturated zone: portion of the subsurface above the groundwater table.

Water table: the upper level of the saturated zone of groundwater

Saturated zone: the area below the water table that holds water.

Aquifer: a body of rock or sediment that stores groundwater and allows the flow of groundwater.

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Unsaturated Zone

The soil and rock in the unsaturated zone contains air as well as water in its pores.

In some places this zone is absent (lakes and marshes).

In some places it is hundreds of meters thick (arid regions).

Not a source of readily available water for human consumption.

Provides water and nutrients that are vital to the biosphere.

Used for the cultivation of plants, construction of buildings, and disposal of waste.

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Saturated Zone

All of the pores, cracks, and spaces between rock particles are completely filled with water.

Also known as the phreatic zone after the Greek word for “well” or “spring”.

The top of the saturated zone is known as the water table.

Water pressure and atmospheric pressure are equal at the water table.

In an aquifer, there is enough groundwater that it can be pumped to the surface and used for drinking water, irrigation, industry, or other uses.

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Aquifers get water from precipitation that filters through the unsaturated zone. When the aquifer is full, and the water table meets the surface of the ground, water stored in the aquifer can appear at the land surface as a spring or seep. Recharge areas are where aquifers take in water; discharge areas are where groundwater flows to the land surface. Water moves from higher-elevation areas of recharge to lower-elevation areas of discharge through the saturated zone.

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Groundwater supplies drinking water for some population in different countries.

Groundwater helps grow our food.

Groundwater is an important component in many industrial processes.

Groundwater is a source of recharge for lakes, rivers, and wetlands.

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Pollution is the problem.

Groundwater can become contaminated when liquid hazardous substances soak down through the soil into groundwater. Contaminants that can dissolve in groundwater will move along with the water, potentially to wells used for drinking water.

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Is the water above the surface of the land, including lakes, rivers, streams, ponds, etc.

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There are three types of surface water perennial, ephemeral, and man-made.

1. Perennial, or permanent: surface water persists throughout the year and is replenished with groundwater when there is little precipitation.

2. Ephemeral, or semi-permanent: surface water exists for only part of the year. Ephemeral surface water includes small creeks, lagoons, and water holes.

3. Man-made surface water: is found in artificial structures, such as dams and constructed wetlands.

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Surface water participates in the hydrologic cycle, or water cycle, which involves the movement of water to and from the Earth’s surface. Precipitation and water runoff feed bodies of surface water. Evaporation and seepage of water into the ground, on the other hand, cause water bodies to lose water. Water that seeps deep into the ground is called groundwater.

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Precipitation occurs an as more rain falls, water begins to filter into the ground. How fast water soaks into, or infiltrates the soil depends on soil type, land use, and the intensity and length of the storm. Water infiltrates faster into soils that are mostly sand than into those that are mostly clay or silt.
Rain that cannot be absorbed into the ground collects on the surface, forming runoff streams.

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Since surface water is more easily accessible than groundwater, it is relied on for many human uses.

It is an important source of drinking water.

Is used for the irrigation of farmland.

Sustain ecological systems and provide habitat for many plant and animal species.

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When the soil is completely saturated, additional water moves slowly down through the unsaturated zone to the saturated zone, replenishing or recharging the groundwater. Water then moves through the saturated zone to groundwater discharge areas.

Evaporation occurs when water from such surfaces as oceans, rivers, and ice is converted to vapor. Evaporation, together with transpiration from plants, rises above the Earth’s surface, condenses, and forms clouds. Water from both runoff and from groundwater discharge moves toward streams and rivers and may eventually reach the ocean. Oceans are the largest surface water bodies that contribute to evaporation.

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Pollution

The accumulation of trash, oil, fertilizers and animal waste affects water quality.

Inflow of sediment can cloud water, blocking sunlight from reaching aquatic plants.

Sediment also settles to the bottom of streams, clogging the gravel beds used by fish for laying their eggs.

Nutrients, such as phosphorus and nitrogen from fertilizers enter the water and promote unusually rapid algae growth. As this algae dies, its decomposition reduces or eliminates oxygen needed for survival by fish and crabs and other aquatic life.

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There are nonpoint sources and point sources of pollution:
Point-source pollution is easy to identify. As the name suggests, it comes from a single place. Nonpoint-source pollution is harder to identify and harder to address. It is pollution that comes from many places, all at once.