Water practice questions.

A catchment is progressively covered with tarmac and roofs during a period of rapid urban growth. Compared with before, the storm hydrograph of its river will show

• a longer lag time and a reduced peak discharge
• a longer lag time and an increased peak discharge
• a shorter lag time and a higher peak discharge
• a shorter lag time and a reduced peak discharge

Answer: a shorter lag time and a higher peak discharge

Impermeable surfaces block infiltration, so rainfall leaves as surface runoff through drains instead of percolating into soil and groundwater storage. Water reaches the channel sooner (shorter lag time) and in greater volume per unit time (higher peak discharge).

In the hydrological cycle, which pair of processes are both transformations rather than transfers?

• Evaporation and condensation
• Infiltration and percolation
• Surface runoff and streamflow
• Advection and groundwater flow

Answer: Evaporation and condensation

A transformation changes the state or form of the water, while a transfer only moves it from one storage to another. Evaporation and condensation change liquid to vapour and back; runoff, infiltration and advection move water without changing its state.

Fresh water makes up about 2.5 % of all the water on Earth. Which store holds by far the largest share of that fresh water?

• Groundwater in aquifers
• Ice caps and glaciers
• Lakes and rivers
• Water vapour held in the atmosphere

Answer: Ice caps and glaciers

Roughly 69 % of fresh water is locked in ice caps and glaciers and about 30 % is groundwater, leaving well under 1 % in the lakes and rivers that people can use most easily. The atmospheric store is tiny but turns over in days, which is why it still delivers all precipitation.

Coastal upwelling off Peru supports one of the world's most productive fisheries. The systems explanation is that upwelling

• warms the surface water and raises its oxygen content
• removes nutrients from the surface into the deep ocean
• transfers nutrients from deep water to the sunlit surface
• raises the salinity of the surface water, which favours plankton

Answer: transfers nutrients from deep water to the sunlit surface

Productivity needs light and nutrients in the same place; the open ocean surface has light but is nutrient-poor because sinking organic matter exports nutrients downwards. Upwelling is the transfer that returns those nutrients to the photic zone, so phytoplankton growth and the food chain above it surge.

Since the industrial revolution the mean pH of surface ocean water has fallen from about 8.2 to about 8.1. The direct cause of this change is

• warmer water holding less dissolved oxygen
• acid rain formed from sulphur dioxide emissions
• run-off of nitrate fertiliser into coastal waters
• carbon dioxide dissolving to form carbonic acid

Answer: carbon dioxide dissolving to form carbonic acid

The ocean is a sink for atmospheric carbon dioxide; the dissolved gas forms carbonic acid, releasing hydrogen ions and reducing the carbonate ions that corals and shellfish need. The water is becoming less alkaline, so 'acidification' describes the direction of change, not an acidic ocean.

A district in the Sahel receives about 900 mm of rainfall a year, yet only 12 % of households have a safe supply because there is no capital to build storage, pipes or treatment works. This situation is best described as

• economic water scarcity
• physical water scarcity
• an absolute surplus of blue water
• salinisation of the household supply

Answer: economic water scarcity

Economic scarcity means the water exists in the catchment but investment, infrastructure or governance fail to deliver it; physical scarcity means the resource itself is insufficient, as in the Arabian Peninsula.