Water systems practice questions.

Rainwater first passes from the ground surface into the soil, and later moves downward from the soil into the underlying permeable rock. In order, these two flows are

• infiltration then percolation
• percolation and then infiltration
• throughflow then infiltration
• percolation then throughflow

Answer: infiltration then percolation

Infiltration crosses the surface into the soil; percolation continues down from the soil into the rock and recharges groundwater. Throughflow is the separate lateral movement through the soil towards a channel, and confusing the three makes any hydrograph explanation incoherent.

Two catchments of equal area receive an identical storm. Catchment A is steep, with thin soils and sparse vegetation; catchment B is gently sloping and forested. In catchment A the storm hydrograph will show

• a longer lag time and a lower peak discharge
• a shorter lag time and a higher peak discharge
• a longer lag time and a considerably higher peak discharge
• a shorter lag time and a lower peak discharge

Answer: a shorter lag time and a higher peak discharge

Steep slopes, thin soils and little interception send a larger share of the rainfall to the channel as rapid overland flow, so the water arrives sooner and more of it arrives together. Vegetation and deep soil act as storages that delay and spread the same volume.

The deep global circulation sometimes called the ocean conveyor is driven mainly by differences in

• the direction of the prevailing surface winds
• the gravitational pull of the Moon and Sun on the oceans
• seawater density, set by its temperature and salinity
• the depth of the ocean floor beneath each basin

Answer: seawater density, set by its temperature and salinity

Cold, salty water is dense and sinks, chiefly in the high-latitude North Atlantic and around Antarctica, and that sinking pulls the rest of the circulation along. Winds drive the surface currents, but the deep limb is thermohaline, which is why adding fresh meltwater at high latitude is thought capable of slowing it.

A student writes: 'Water stays in the deep ocean for far longer than in the atmosphere because much more precipitation falls on the ocean each year.' Identify the error in this reasoning.

• Residence time depends on the size of a store relative to its throughflow, not on the flow alone.
• Precipitation is a transfer rather than a transformation, so it cannot change the amount of water held in any store at all.
• The deep ocean in fact holds each molecule of water for a shorter time than the atmosphere does.
• The atmosphere is a flow and not a store, so no residence time can be worked out for it.

Answer: Residence time depends on the size of a store relative to its throughflow, not on the flow alone.

Residence time is storage divided by throughflow. The atmosphere holds a tiny volume but cycles it in about nine days, while the deep ocean holds an enormous volume relative to the water entering and leaving it, so a large flow on its own says nothing about how long water stays.

In a drainage basin, infiltration increases while every other flow of the water cycle stays unchanged. Identify the storage that grows as a result.

• Interception on the leaves and stems of the vegetation
• Soil water in the pore spaces above the bedrock
• Channel storage in the river itself
• Groundwater in the permeable rock, because water that infiltrates the surface passes straight down into it

Answer: Soil water in the pore spaces above the bedrock

Infiltration is the transfer of water from the surface into the soil, so the soil water store is the one it fills. Reaching the rock below needs a second, separate flow, percolation, which this question holds constant.

A hydrologist first treats one sloping field as the system, then redraws the boundary around the whole catchment in which that field lies. Identify the flow that stops being an output of the system when the boundary is widened.

• Evapotranspiration from the crop growing on the field
• Percolation down into the deep regional aquifer
• Overland flow leaving the foot of the field
• River discharge passing the gauging station at the mouth of the catchment, which carries water out of the area studied

Answer: Overland flow leaving the foot of the field

Water running off the field still leaves the field, but inside the wider catchment it only moves between internal stores, so it becomes a transfer rather than an output. Discharge at the mouth and evapotranspiration cross the boundary at both scales.