1.3 Global water distribution and water balance

Earth's water is distributed across several reservoirs, but the vast majority of it is saltwater. Only about 2.5% is freshwater, and most of that is locked in ice or deep underground. Understanding where water exists and how it moves between reservoirs is foundational to hydrology and water resource management.

Water balance applies the principle of mass conservation to track inputs, outputs, and storage changes within any hydrologic system. This concept lets you assess water availability at scales ranging from a single farm field to the entire planet.

Global Water Distribution and Reservoirs

Global distribution of water resources

Earth's water sits in several interconnected reservoirs, but the distribution is extremely lopsided:

• Oceans hold roughly 97% of all Earth's water (Pacific, Atlantic, Indian, Southern, Arctic).
• Ice caps and glaciers store about 1.7% of total water, making them the second-largest reservoir. Antarctica alone contains about 60% of all freshwater on Earth.
• Groundwater accounts for roughly 1.7% of total water, but most of it is saline or too deep to access economically. Fresh groundwater makes up about 0.76% of all water.
• Surface freshwater (rivers, lakes, wetlands) contains only about 0.01%. Lake Baikal in Russia, for example, holds roughly 20% of the world's unfrozen surface freshwater.
• The atmosphere holds about 0.001% as water vapor, in clouds and humidity.

These percentages are approximate and vary slightly by source, but the takeaway is clear: almost all water is salty, and the freshwater that does exist is mostly inaccessible.

Water moves continuously between these reservoirs through the hydrologic cycle. The key transfer processes are evaporation, transpiration, precipitation, infiltration, and runoff. A water molecule might spend thousands of years in a glacier but only about 9 days in the atmosphere, so the residence time in each reservoir varies enormously.

Freshwater vs. saltwater availability

Freshwater has low concentrations of dissolved salts (below about 1,000 mg/L of total dissolved solids). It's what you need for drinking, agriculture, and most industrial processes. The problem is access:

• Only about 2.5% of Earth's water is fresh.
• Of that freshwater, roughly 69% is locked in ice caps and glaciers (Antarctica, Greenland, mountain glaciers).
• About 30% is groundwater, much of it deep and difficult to reach.
• Less than 1% of all freshwater is readily accessible surface water in rivers, lakes, and shallow aquifers.

Saltwater has dissolved salt concentrations above 1,000 mg/L and makes up about 97.5% of Earth's water. It's found in oceans, seas, and some inland bodies like the Dead Sea and Great Salt Lake. Saltwater can't be used directly for drinking or irrigation without desalination, which requires significant energy (through processes like reverse osmosis or distillation).

This mismatch between where water exists and what humans can actually use is why freshwater management matters so much.

Water Balance and Sustainability

Water balance components and scales

The water balance is built on a simple idea: mass conservation. Water entering a system must either leave it or be stored within it. The general form is:

$P = ET + R + G \pm \Delta S$

Where:

1. Precipitation (P) is the water input from the atmosphere (rain, snow, sleet, hail).
2. Evapotranspiration (ET) is water lost back to the atmosphere through evaporation from surfaces and transpiration from plants.
3. Runoff (R) is water that flows over the land surface into streams and rivers.
4. Groundwater flow (G) is subsurface water movement into or out of the system through aquifers.
5. Change in storage ($\Delta S$) is the net change in water stored within the system (soil moisture, groundwater levels, lake volumes, snowpack).

If $\Delta S$ is positive, the system is gaining water. If negative, it's losing water. Over long time periods, $\Delta S$ often approaches zero for large systems, meaning inputs roughly equal outputs.

You can apply this equation at different spatial scales:

• Local: a single farm, hillslope, or small catchment
• Regional: a major watershed or river basin (e.g., the Mississippi or Amazon Basin)
• Global: the entire Earth's surface, where total evaporation must equal total precipitation over time

Water balance for sustainability assessment

Water balance analysis is how hydrologists figure out whether a region has enough water to meet its needs. You compare precipitation and groundwater recharge (inputs) against evapotranspiration and runoff (outputs), then check whether storage is increasing or declining over time.

Water stress occurs when demands on water exceed the available supply. This can result from drought, population growth, agricultural expansion, or a combination of factors. A region where groundwater levels drop year after year (negative $\Delta S$) is drawing down its reserves unsustainably.

Sustainable water management aims to balance human water use with long-term supply and ecosystem health. Common strategies include:

1. Improving water use efficiency: drip irrigation, low-flow fixtures, recycling industrial water
2. Enhancing storage and distribution: reservoirs, managed aquifer recharge, pipeline infrastructure
3. Protecting natural water sources: preserving wetlands and forests that regulate water flow and quality
4. Demand management: tiered pricing structures, public education, water use restrictions

Water balance analysis also helps identify:

• Regions with chronic water surplus or deficit
• Seasonal and interannual variability (monsoon cycles, El Niño/La Niña effects)
• How land use changes like deforestation or urbanization alter the water balance (e.g., more impervious surfaces increase runoff and decrease infiltration)
• Opportunities for development or conservation, such as rainwater harvesting or dam construction