The Water Cycle

Cryospheric water

Water stored on Earth’s surface as ice

Discharge

The amount of water in a river flowing past a particular point expressed in cumecs

Greenhouse gas

Any gaseous compound in the atmosphere that allows short wave UV radiation from the sun to enter the atmosphere, but prevents outgoing infrared radiation from escaping

Hydrosphere

The total combined mass of water found on, above, and under Earth’s surface in liquid, solid, or gaseous forms

Terrestrial water

All water stored on Earth’s landmasses - groundwater, soil moisture, lakes, wetlands, rivers etc.

97

Percentage of the hydrosphere made up by oceanic water

3

Percentage of the hydrosphere made up of freshwater

A tiny fraction

Amount of the hydrosphere said to be made up of atmospheric water vapour

72

Percentage of Earth’s surface covered by oceans

Cryospheric, oceanic, and atmospheric

Water stores that are being pushed out of dynamic equilibrium by climate change

8.25 - 8.14

Rough change in oceanic pH over the last 250 years

2

Percentage of the hydrosphere made up by cryospheric water

75

Rough percentage of global freshwater that is cryospheric water (stored in ice)

Ice sheets

Areas of land ice that cover more than 50,000 km2

Antarctica and Greenland

The world’s 2 ice sheets

More than 99

Percentage of Earth’s freshwater ice stored in its only 2 ice sheets (Greenland and Antarctica)

Formation of ice sheets

When snow falls over a vast area in the winter and doesn’t entirely melt over the summer allowing layers of ice to accumulate over many years causing it to grow thicker and denser

60 metres

How much sea levels would rise by if the Antarctic ice sheet melted

Ice caps

Ice sheets that are smaller than 50,000km2, usually found in mountainous areas, distributed across the globe

Alpine glaciers

Thick masses of ice found in deep valleys or upland hollows, typically fed by ice from ice caps

15,000 glaciers

The Himalayan water store that supports perennial rivers such as the Indus and Ganges which are the lifeline of millions of people

Permafrost

Ground that remains at or below 0 degrees for at least 2 consecutive years, beginning to melt as climate warms causing the release of large amounts of stored CO2 and methane

Groundwater (lithosphere water store)

Water that collects in pore spaces of rocks

20

Percentage of global freshwater made up by groundwater

Water table

The depth at which gaps in rock/soil become completely saturated with water

Soil water

Water held in unsaturated upper layers of Earth

Biological water

All water stored in biomass, varies greatly around the world depending on vegetation cover and type, role of animals is minimal

Energy in the form of latent heat

What is absorbed or released when water changes state depending on the process

Rate of evaporation

What depends on the amount of solar energy, availability of water, air humidity, and air temperature

Decreases

Effect of increased humidity on evaporation rate

Cooling

Effect on the surroundings when evaporation uses energy in the form of latent heat

Transpiration

Water lost through plant leaves to the atmosphere as water vapour

Dew point temperature

The point where air becomes saturated with water as it has cooled sufficiently reducing its water capacity sufficiently, the point when excess water begins to condensate and form precipitation

When dew point temperature is often reached (and precipitation occurs)

When warm moist air passes over a colder surface or when adiabatic cooling occurs or when heat is radiated out of the atmosphere on a clear night, cooling the ground and hence the air above it

Adiabatic cooling

When air rises and expands in the lower pressure of the upper atmosphere and volume of air increases with no addition of heat

When adiabatic cooling usually occurs

When air is forced to rise over hills/relief or when masses of air with different temperatures/densities meet causing a frontal effect as the less dense air rises

Accumulation

Input of frozen water to a glacier/ice sheet/ice cap over time (cryospheric process)

Ablation

Output of frozen water/mass from a glacier from a glacier/ice sheet/ice cap over time

Cryospheric processes

Those that affect total mass of ice at any scale from patches of frozen ground to glaciers and ice caps

Saturated

When a water store has reached its maximum capacity

Evaporation

The process of liquid water changing into gas, requires energy, usually provided by the sun and aided by wind

Drainage basin (catchment area)

An area of land drained by a river and its tributaries - includes water found on the surface, in the soil, and the near surface geology

Evapotranspiration

The total output of water from the drainage basin directly back into the atmosphere

Groundwater flow

The slow movement of water through underlying rock

Infiltration

The downward movement of water from the surface into the soil

Interception store

Precipitation that is stored in vegetation or man made surfaces and can either be evaporated back into the atmosphere, absorbed by canopy surfaces, or ultimately transmitted to the ground surface

Overland flow

The tendency of water to flow horizontally across land surfaces when rainfall has exceeded the infiltration capacity of the soil and all surface stores are full or overflowing

Percolation

The downward movement of water within the rock under the soil surface - rate varies depending on the nature of the rock

Run-off

All water that enters a river channel and eventually flows out of the drainage basin

Stemflow

The portion of precipitation that is intercepted by the canopy and reaches the ground by flowing down stems, stalks, trees etc.

Throughfall

The portion of precipitation that reaches the ground directly through gaps in the vegetation canopy and drips from leaves, twigs, and stems

Throughflow

The movement of water downslope through the subsoil under the influence of gravity, occurs below the surface after infiltration

Water balance

The balance between inputs (precipitation) and outputs (evapotranspiration, run-off etc.) in a drainage basin

Soil porosity

Main determinant of infiltration rate/capacity

Infiltration capacity

The maximum rate at which rain or water can be absorbed by a soil in a given condition

When infiltration capacity has been reached

When rainfall intensity exceeds the infiltration rate causing water to build up as surface storage or flow as surface run-off

Slows

Effect of greater volume of vegetation on throughflow which is typically far slower than overland flow

Triple

Amount of precipitation over land compared to the net transport from ocean to land. Indicates a considerable recirculation of water over land which is larger in summer and over tropical land areas

River regime

The annual variation in the discharge (flow) of a river, typically measured in cumecs

Water balance formula

Precipitation (P) = Discharge (Q) + Evapotranspiration (E) + Changes in storage (S)

Bankfull

Maximum discharge that a river channel is capable of carrying without flooding

Base flow

The normal day to day discharge of a river. The consequence of slow moving soil throughflow and groundwater seeping into the river channel

Lag time

The time between peak rainfall and peak discharge

Hydrograph

Graph showing river discharge against time

Flashy flood hydrograph

High peak discharge, steep rising/falling limbs

Subdued flood hydrograph

Low peak discharge, long lag times, gentle rising/falling limbs

More circular drainage basins

Physical factor increasing the flashiness of the flood hydrograph because all points in the river basin are roughly equidistant from the river channel where discharge is recorded

Steep sided drainage basins

Physical factor increasing the flashiness of the flood hydrograph because water reaches the river channel far quicker whether by throughflow or overland flow

High drainage density (lots of surface streams acting as tributaries)

Physical factor increasing the flashiness of the flood hydrograph because all the water arrives at the river channel at the same time

Already saturated drainage basin (by recent rainfall)

Physical factor increasing the flashiness of the flood hydrograph because infiltration capacity is already reached and overland flow is significant

Thick vegetation cover

Physical factor decreasing the flashiness of the flood hydrograph through increased interception, evaporation, and transpiration

Higher peak discharge

Effect on the flood hydrograph of a larger drainage basin

Shorter lag time

Effect on the flood hydrograph of a smaller drainage basin

Deforestation

Human factor increasing the flashiness of the flood hydrograph through decreased interception and decreased infiltration due to lack of roots causing rapid overland flow

Afforestation

Human factor decreasing the flashiness of the flood hydrograph by counteracting deforestation impacts on the flood hydrograph

Livestock overgrazing

Human factor increasing the flashiness of the flood hydrograph through soil degradation causing decreased infiltration and increased overland flow

Increased impermeable surfaces (urban growth)

Human factor increasing the flashiness of the flood hydrograph through lack of infiltration and interception causing rapid overland flow

Water abstraction

Human factor impacting the flood hydrograph through lowering the water table and hence the base flow meaning more water is needed to reach bankfull

Subsurface soil drainage

Form of land use change that lowers the water table artificially and allows heavy machinery and livestock to work on the soil above without danger of compaction and increased overland flow

Increased throughflow speed

Negative impact of subsurface soil drainage (land use change) which increases the flashiness of the flood hydrograph

Saltwater intrusion

Impact of water over-abstraction that degrades groundwater supplies, occurring in many places globally such as Malta

Subdued

Flood hydrograph of the River Itchen Catchment

60mm

Average monthly rainfall in February-September in the River Itchen catchment

100mm

Average monthly rainfall in October-January in the River Itchen catchment

900mm

Mean annual precipitation in the River Itchen catchment

748Ha

Size of River Itchen drainage basin - small basin causing shorter lag time due to less distance to river channel

Fairly circular

Characteristic of River Itchen drainage basin that causes a slightly more flashy flood hydrograph as many points in the the basin are roughly equidistant from the river channel

Land use in the River Itchen

Mainly rural land, with some small areas of woodland and urban land and some industrial and agricultural presence

Chalk

Dominant rock type in the River Itchen drainage basin making it a very permeable catchment - main factor of its subdued flood hydrograph

60mm (month’s rainfall in 5 days)

Amount of rainfall that fell in 5 days during the 2014 River Itchen floods due to repeated storms

£68.5 million

Estimated cost of damage in the 2014 River Itchen floods

£1.6 million

Amount spent on flood defence repairs by local council after 2014 River Itchen floods

Sewage contamination

Secondary impact of 2014 River Itchen floods experienced in St Cross and Water Lane