Water scarcity and water quality practice questions.

A country has abundant rivers and high annual rainfall, yet many of its rural households have no reliable safe supply because there is no money for storage, treatment or pipes. How is this best classified?

• Economic water scarcity
• Physical water scarcity
• Absolute water scarcity
• Seasonal water surplus

Answer: Economic water scarcity

Economic scarcity is a failure of investment and institutions, not of the resource; physical scarcity means the water itself is insufficient. The distinction changes the remedy completely, from infrastructure and governance to allocation and demand management.

In the Murray-Darling basin in Australia, irrigation has raised saline groundwater close to the surface in some districts, reducing crop yields. Which process is responsible?

• Secondary salinisation caused by a rising water table under irrigated land
• Eutrophication caused by nutrient enrichment of the river
• Acidification caused by dissolved carbon dioxide in rainfall
• Sedimentation caused by soil erosion from cleared catchments

Answer: Secondary salinisation caused by a rising water table under irrigated land

Applying more water than the crop uses raises the water table; once saline groundwater is within reach of capillary rise, evaporation leaves salt at the surface. It is a degradation problem created by the irrigation itself, which is why efficiency and drainage are the standard responses.

Nitrate and phosphate from fertiliser reach a slow-moving lowland lake. Which sequence describes eutrophication correctly?

• Nutrient enrichment, algal bloom, light cut off, decomposition, oxygen depletion
• Oxygen depletion, algal bloom, nutrient enrichment, decomposition, light cut off
• Algal bloom, oxygen enrichment, increased fish stocks, clearer water
• Nutrient enrichment, reduced algal growth, deeper light penetration, oxygen rise

Answer: Nutrient enrichment, algal bloom, light cut off, decomposition, oxygen depletion

The damage is done by decomposers, not by the algae directly: bacteria breaking down the dead bloom consume the dissolved oxygen that fish need. The chain has to be in this order for the fish kill to make sense.

Which sequence describes the normal order of treatment at a conventional surface-water treatment works?

• Disinfection, then screening, then filtration, then sedimentation, then coagulation
• Screening, then coagulation and flocculation, then sedimentation, then filtration, then disinfection
• Filtration, then screening, then disinfection, then sedimentation, then coagulation
• Sedimentation, then disinfection, then screening, then coagulation, then filtration

Answer: Screening, then coagulation and flocculation, then sedimentation, then filtration, then disinfection

Each stage removes what the next cannot handle: coarse debris first, then chemicals that clump fine particles so they can settle, then filtration of what is left, and disinfection last so the water leaves the works free of pathogens.

A student writes: "The Aral Sea in Central Asia shrank because the regional climate became much drier during the twentieth century." Which statement identifies the error in this explanation?

• The main cause was the diversion of the Amu Darya and Syr Darya for irrigation, so it was inflow, not rainfall, that collapsed.
• The main cause was a rise in the level of the Caspian Sea, which drew water out of the Aral basin.
• The Aral Sea did not shrink at all; its shoreline moved because sediment was deposited around the edge.
• The regional climate in fact became wetter, so the water must have been lost through increased evaporation alone.

Answer: The main cause was the diversion of the Amu Darya and Syr Darya for irrigation, so it was inflow, not rainfall, that collapsed.

The lake lost its inflow, not its rainfall: from the 1960s its two feeder rivers were diverted to irrigate cotton, so the water budget failed on the input side while evaporation from the lake surface carried on unchanged.

On a water balance graph for a Mediterranean station, the curve for potential evapotranspiration lies above the precipitation curve from May to September. What does the area between the two curves in those months represent?

• The soil moisture surplus available to recharge the groundwater store
• The total volume of rain that falls during the dry season
• The soil moisture deficit that accumulates through the dry season
• The rate at which the water table rises over the summer months

Answer: The soil moisture deficit that accumulates through the dry season

Where potential evapotranspiration exceeds precipitation, more water could be lost than arrives, so a deficit builds up; a surplus appears only when the reverse is true and the soil store has already been refilled.