3.2.5.3 - Water availability, demand and stress

Sources, volume and quality

Water security depends on the amount, timing and quality of water that can actually reach users.

Sources include surface water in rivers, lakes and reservoirs, groundwater in aquifers, collected rainfall and treated seawater. Treated wastewater can substitute for fresh supply in suitable uses. A large volume of saline or contaminated water is not an equivalent potable supply: treatment requires energy, finance and monitoring.

Climate controls precipitation and evaporative losses. A dry climate usually supplies less renewable freshwater; high temperatures increase evaporation and irrigation need. Strong seasonality can create shortage during the dry season even where annual rainfall is high. Drought affects inflow and recharge, while intense rain may run off rapidly, carry pollutants or exceed treatment capacity.

Geology affects infiltration, storage and water chemistry. Permeable sand or fractured rock can transmit water; an aquifer stores it above less-permeable layers. Impermeable surfaces favour rapid runoff. Long residence in rocks can dissolve minerals, increasing hardness or salinity; naturally occurring contamination and human pollution both matter. Permeability alone does not establish a sustainable yield: recharge and connection to rivers must be investigated.

Drainage determines how water is collected and routed. A large catchment may gather more runoff but can include upstream abstractions and pollution sources. Steep slopes and a dense channel network can deliver short flood peaks; reservoirs can retain water but may lose it to evaporation or sedimentation. Basin boundaries are physical drainage divides, not necessarily political borders.

The same factor can affect volume and quality differently. A wet agricultural catchment can provide abundant flow while transporting fertiliser into an intake. Security therefore needs appropriate treatment, seasonal storage and access, rather than a rainfall total alone.

Demand, withdrawal and consumption

Water demand combines different uses, and the water removed from a source is not all permanently lost from it.

Domestic demand covers drinking, washing, sanitation and other household uses. Population, household size, appliances, income, prices and climate affect it. Agricultural demand includes irrigation and livestock; crop type, season, irrigation method and soil moisture matter. Industrial demand includes processing and cooling; energy production, manufacturing and service activity can change both volume and quality.

A power station may withdraw much cooling water and return most of it, warmer than before. Irrigation may withdraw less but consume a high proportion through evapotranspiration. Return flow can still be degraded or arrive downstream too late for another user. Compare quantities only with the same source, time and definition.