05. Water Resources and Aquatic Systems

Water Distribution and Availability

Water is renewable at the planetary scale because it circulates continuously, but usable freshwater is not evenly distributed or equally accessible.

Earth contains approximately $1.386$ billion cubic kilometers of water. About $96.5\%$ is saline water in oceans and seas. Of the freshwater fraction, approximately $68.7\%$ is stored in glaciers, ice caps, and permanent snow, while about $30.1\%$ is groundwater. Water in lakes, rivers, soil, wetlands, the atmosphere, and living organisms makes up only a small fraction of the total supply. Rivers contain roughly $0.006\%$ of Earth's freshwater, yet they are essential to societies and ecosystems.

Availability is not the same as accessibility

A water source may exist without being readily usable. Accessibility depends on:

• Location: Supply may be far from population centers.

• Physical state: Glaciers and ice caps contain freshwater but are difficult to use directly.

• Quality: Pollution may make water unsafe or costly to treat.

• Renewal rate: Some groundwater remains underground for thousands of years and is effectively nonrenewable on human time scales.

• Seasonal variation: Snowmelt, drought, and monsoon cycles can change supply substantially.

• Infrastructure and governance: Reservoirs, treatment plants, pipelines, laws, and water rights influence access.

A region with substantial precipitation can still face water scarcity when demand is high, infrastructure is inadequate, or water quality is poor. Conversely, a dry region may support a large population through reservoirs, groundwater pumping, conservation, water transfers, and reuse.

Takeaway: The amount of water on Earth does not by itself determine how much reliable freshwater people and ecosystems can use.

The Hydrologic Cycle and Water Budgets

The hydrologic cycle moves water among the atmosphere, hydrosphere, geosphere, cryosphere, and biosphere. Solar energy drives many upward movements of water, while gravity drives precipitation, runoff, infiltration, and groundwater flow.

Major processes

1. Evaporation changes liquid water into vapor.

2. Transpiration releases vapor from plant leaves.

3. Evapotranspiration combines evaporation and transpiration.

4. Sublimation changes ice or snow directly into vapor.

5. Condensation forms liquid droplets or ice crystals from cooled vapor.

6. Precipitation returns water as rain, snow, sleet, or hail.

7. Infiltration occurs when water enters the soil.

8. Percolation moves water downward through soil and permeable rock.

9. Runoff flows across the land surface into streams, lakes, and oceans.

10. Groundwater flow moves through saturated underground layers and may discharge into springs, streams, wetlands, or oceans.

11. Storage holds water temporarily in oceans, glaciers, lakes, soil, groundwater, the atmosphere, reservoirs, and organisms.

Water can follow many paths. For example, precipitation may infiltrate soil, enter plant roots, return to the atmosphere through transpiration, condense into clouds, and fall again as precipitation.

Watershed water budgets

A simplified watershed water budget is:

Here, $P$ is precipitation input, $Q$ is runoff and stream discharge, $ET$ is evapotranspiration, and $\Delta S$ is the change in storage in soil, groundwater, snow, lakes, and reservoirs.

Human activities alter this balance. Paved surfaces reduce infiltration and increase rapid runoff. Deforestation can increase runoff and erosion. Irrigation transfers water from rivers or aquifers to fields, where much of it may be lost through evapotranspiration. Groundwater pumping reduces underground storage and can reduce the base flow that sustains streams during dry periods.

Takeaway: The hydrologic cycle recycles water, but human land use and withdrawals change the timing, location, quality, and storage of that water.

Watersheds and Surface Water

A watershed connects land, surface water, groundwater, vegetation, and downstream ecosystems. A watershed is an area that drains water to a common outlet. Ridges and elevated landforms separating neighboring drainage areas are called drainage divides. Large watersheds contain many smaller ones.

A watershed includes more than a visible river channel. It may contain headwater streams, tributaries, rivers, lakes, reservoirs, wetlands, floodplains, riparian zones, soils, vegetation, urban surfaces, and hydraulically connected groundwater.

How land use affects water quality

Water transports sediments, nutrients, pathogens, and pollutants. Therefore, activities anywhere in a watershed can affect downstream water. Fertilizer carried from an agricultural field into a stream can stimulate algal growth. When the algae decompose, dissolved oxygen may decline and create hypoxic conditions.

Surface-water benefits and trade-offs

Surface water is generally easier to access than groundwater, but it is more exposed to contamination and seasonal changes. Reservoirs can support cities, agriculture, hydroelectric power, flood control, and recreation. Dams and reservoirs can also:

• Change the timing and amount of downstream flow

• Block fish migration

• Trap sediment

• Flood terrestrial habitats

• Increase evaporation

• Displace human communities

Wetlands store floodwater, slow runoff, trap sediment, remove or transform nutrients, provide wildlife habitat, and can help recharge groundwater. Riparian vegetation can also protect streambanks and filter runoff.

Takeaway: Protecting an entire watershed, including wetlands and riparian areas, can address water quantity and quality together.

Groundwater, Aquifers, and Overdraft

Groundwater is stored beneath Earth's surface in the pores and fractures of soil and rock. An aquifer is a permeable layer that can store and transmit useful quantities of groundwater. An aquitard is a less-permeable layer that restricts groundwater movement.

Recharge and discharge

Groundwater recharge occurs when water infiltrates the ground and moves downward into an aquifer. Permeable soils, vegetation, open land, and wetlands promote recharge. Asphalt and concrete reduce recharge by directing precipitation into storm drains.

Groundwater can leave an aquifer through springs, seepage into streams and wetlands, evapotranspiration by deep-rooted plants, submarine discharge to oceans, and pumping from wells. Groundwater often supplies base flow, the portion of streamflow that continues between precipitation events. As a result, pumping can reduce river flow and dry wetlands even when the wells are some distance from surface water.

Consequences of excessive pumping

If withdrawals exceed long-term recharge, an aquifer experiences overdraft. Possible consequences include:

• Falling water tables

• Higher pumping costs

• Reduced spring and stream flow

• Drying wetlands

• Land subsidence

• Saltwater intrusion in coastal aquifers

• Permanent loss of aquifer storage capacity

Land subsidence occurs when reduced groundwater pressure allows sediments to compact. The resulting loss of pore space can permanently reduce storage capacity. In coastal areas, heavy pumping can lower freshwater pressure and allow seawater to move inland into wells.

Takeaway: Surface water and groundwater are connected systems, so groundwater management must account for effects on streams, wetlands, coastal water, and long-term storage.

Water-Resource Management Strategies

Water management must balance withdrawals with ecological needs and long-term supply. Because surface water and groundwater are connected, effective decisions consider the whole watershed and aquifer system.

Main strategies

1. Conservation and efficiency: Efficient appliances, leak detection, metering, drought-tolerant landscaping, and behavior changes reduce demand, energy use, treatment costs, and wastewater volumes.

2. Improved irrigation: Drip or microirrigation delivers water near plant roots. Scheduling irrigation by soil moisture, lining canals, using mulch, and selecting drought-tolerant crops can reduce evaporation and losses.

3. Water reuse and recycling: Treated wastewater can support irrigation, industrial cooling, agriculture, groundwater recharge, and, when treated to stringent standards, potable supplies.

4. Stormwater management: Green infrastructure slows runoff, filters pollutants, increases infiltration, and may store water for later use.

5. Source-water protection: Riparian buffers, wetland conservation, erosion control, land-use planning, inspections, zoning, and pollution prevention protect water before it reaches a drinking-water intake or well.

6. Reservoirs and transfers: Reservoirs store water during wet periods, while aqueducts and pipelines move water between regions. These systems can also cause ecological damage, political conflict, evaporation losses, and high construction costs.

7. Desalination: Desalination removes dissolved salts from seawater or brackish water. It requires substantial energy and produces concentrated brine, so it supplements rather than replaces conservation and watershed protection.

8. Managed aquifer recharge: Treated stormwater or other suitable water can be placed into an aquifer for storage and later recovery, provided water quality and aquifer chemistry are monitored.

Integrated decision-making

Integrated water management considers quantity, quality, ecosystems, energy, land use, and social equity together. For example, evaluating a dam requires more than calculating delivered water. The analysis should also consider habitat loss, sediment transport, downstream communities, construction and reservoir emissions, and alternatives such as conservation or reuse.

Takeaway: The most resilient approach combines demand reduction, pollution prevention, reuse, ecosystem protection, and carefully monitored storage.

Agricultural, Industrial, and Municipal Water Use

Water use differs by sector, and withdrawals do not always equal consumption. A power plant may withdraw a large volume and return much of it, whereas irrigation may withdraw less but consume a larger fraction through evapotranspiration.

Agriculture

Agriculture uses water for irrigation, livestock, and aquaculture. Irrigation supports crops where precipitation is insufficient or unreliable, but it can cause salinization, waterlogging, erosion, and aquifer depletion. When irrigation water evaporates, dissolved salts may remain in the soil and reduce productivity. Better drainage, irrigation scheduling, crop selection, and efficient delivery can reduce these effects.

Agriculture accounted for approximately $72\%$ of global freshwater withdrawals in $2021$, making it the largest water-withdrawing sector. Water applied to fields may return as runoff or drainage, but that return flow can carry sediment, fertilizer, pesticides, salts, and animal waste.

Industry and energy

Industry uses water for processing, washing, cooling, manufacturing, mining, mineral processing, and waste treatment. Thermoelectric power plants often withdraw large volumes for cooling. Once-through cooling generally has high withdrawals but relatively low consumptive use, while closed-loop cooling reduces withdrawals but may consume more water per unit withdrawn. Industrial pollution may include heavy metals, acids, solvents, heated water, and organic wastes.

Municipal systems

Municipal water serves homes, businesses, institutions, public services, firefighting, parks, schools, hospitals, and commercial activities. A typical system includes source-water collection, treatment, distribution, wastewater collection, and wastewater treatment followed by discharge or reuse.

Leaks, outdoor irrigation, and lawn watering can account for substantial losses, especially in dry climates. Low-flow fixtures, efficient toilets, drought-tolerant landscaping, rainwater capture, tiered pricing, leak repair, and treated wastewater reuse can reduce demand.

Interpreting withdrawal data

In a United States estimate for $2015$, total water withdrawals were approximately $322$ billion gallons per day. Thermoelectric power and irrigation accounted for about $133$ and $118$ billion gallons per day, respectively, while public supply accounted for approximately $39$ billion gallons per day. Together, these three categories represented about $90\%$ of total withdrawals. Irrigation represented approximately $42\%$ of total freshwater withdrawals, while thermoelectric power had the largest total withdrawal category because it included substantial saline-water withdrawals.

These comparisons are meaningful only when withdrawals, consumptive use, and return flows are kept distinct.

Takeaway: Water policy should evaluate not only how much water a sector withdraws, but also how much it consumes, what quality returns to the environment, and how withdrawals affect connected ecosystems.

• Water stress

• water table