ESS Water Cycle, Scarcity, Fisheries & Pollution – Key Review

The water budget quantifies the amount of water stored in oceans, ice caps, glaciers, rivers, lakes, groundwater and the atmosphere. About 97 % of Earth’s water is salt water, while only 3 % is fresh; of that fresh portion, roughly two‑thirds resides in ice caps and glaciers. The atmosphere holds merely 0.01 % of total fresh water, yet it drives the rapid turnover of water in rivers and clouds. Solar energy powers evaporation, condensation, and precipitation, creating a continuous loop: evaporation → condensation → advection → precipitation → infiltration or runoff. Turnover time varies—molecules linger for millennia in ice caps and oceans but circulate in days through rivers and the atmosphere.

Human Impacts on Water Systems

Urbanization spreads concrete surfaces that block infiltration, increase runoff, and heighten flood risk. Deforestation cuts evapotranspiration and reduces soil water storage, also promoting erosion and flooding. Intensive irrigation depletes rivers, exemplified by the Aral Sea’s collapse, and can cause salinization of soils. Sewage and plastic waste enter the seas mainly via river discharge, linking land‑based activities to marine pollution.

Water Scarcity and Security

Physical scarcity arises when natural rainfall and river flow are insufficient, while economic scarcity reflects limited infrastructure, storage, and transport capacity. Aquifers—porous rock layers—store groundwater; many are fossil aquifers that recharge extremely slowly, making them effectively non‑renewable. Hydropolitics emerges when nations contest shared water resources, such as the Nile’s Grand Ethiopian Renaissance Dam (GIRD) or the Colorado River’s allocations with the Rio Grande.

Management Strategies

Supply‑side approaches include building reservoirs, dams, rainwater harvesting systems, and desalination plants. Demand‑side measures focus on water metering, efficient appliances, and gray‑water recycling. In agriculture, drip irrigation, drought‑resistant crops, and organic fertilizers reduce water use. Desalination, often employing reverse‑osmosis, supplies fresh water but incurs high energy costs.

Fisheries and Aquaculture

Continental shelves support high productivity because sunlight and nutrient upwellings fuel plankton growth. Yet 75 % of wild fish stocks are overexploited or at their limit, illustrating the tragedy of the commons where unregulated harvest leads to collapse, as seen with Newfoundland cod. Aquaculture expands supply but can cause mangrove loss, disease spread, and pollution.

Water Pollution and Measurement

Direct water quality measures use probes for pH, temperature, and turbidity (e.g., Secchi disc). Indirect indicators include Biochemical Oxygen Demand (BOD) and the presence of macro‑invertebrate species. High BOD values signal that microbes are consuming large amounts of oxygen to decompose organic matter, indicating severe pollution. Nutrient runoff (nitrates, phosphates) triggers eutrophication: excess nutrients fuel algal blooms, which die and decompose, depleting dissolved oxygen and causing fish kills.

Plastic Pollution

Synthetic polymers are non‑biodegradable; they fragment into microplastics (< 5 mm) and nanoplastics that persist in ecosystems. These particles bioaccumulate and magnify through food chains, reaching higher trophic levels. Major sources include road dust, tire wear, synthetic clothing fibers, and microbeads from personal‑care products. Ten of the dirtiest rivers generate roughly 90 % of the world’s plastic pollution.