The flows and stores of the water cycle

What is the water balance?

It summarises the flows of water in a drainage basin over time. It states that precipitation is equal to evapotranspiration and stemflow, plus or minus entering or leaving storage.

What is the equation to find the water balance?

Precipitation = Evapotranspiration (E) + Streamflow (Q) plus/minus Storage

What are the flows in the WC?

• Precipitation

• Evaporation

• Transpiration

• Run-off

• Infiltration

• Percolation

• Throughflow

Explain the flow of precipitation

• This forms when vapour in the atmosphere cools to its dew point and condenses into tiny water droplets to form clouds.

• Eventually these droplets to form clouds. Eventually these droplets aggregate, reach a critical size and leave.

• Duration is the length of the time a precipitation event lasts as prolonged events, linked to depressions, may cause saturation of the soil which leads to overland flow and possibly river flooding.

Explain the flow of transpiration

• This is the diffusion of water vapour to the atmosphere from the leaf pores of plants.

• It is responsible for 10% of moisture in the atmosphere

• This is dependent on water availability

Explain the flow of condensation

• This is the phase change of vapour to liquid water

• It occurs when air is collected to its dew point.

• At this critical temperature air becomes saturated with vapour resulting in condensation. Clouds form through this.

What are cumuliform clouds?

Flat based and vertically tall clouds

How do cumuliform clouds form?

• When air is heated locally through contact with the Earth’s surface.

• This causes heated air parcels to rise freely through the atmosphere, expand and cool.

• As this cooling reaches dew point, condensation begins and clouds form.

How do Stratiform clouds (layered clouds)?

• When air mass moves horizontally across a cooler surface (often ocean).

• This process, together with some mixing and turbulence is called advection

What are clouds?

Visible aggregates of water or ice or both that float in the free air

How are clouds fomred?

1. Air, which is warmed by the ground/sea, rises freely in the atmosphere.

2. Pressure falls as it rises and cools by expansion (adiabatic expansion) - this vertical movement is called convection.

3. Air masses move horizontally across a relatively cooler surface (advection).

4. Air masses also rise as they cross a mountain barrier or as turbulence forces their ascent.

5. A relatively warm air mass mixes with a cooler one forming a cloud.

What are lapse rates?

Describe the vertical distribution of temperatures in the lower atmosphere and the temp changes that occur within an air parcel as it rises vertically away from the ground.

What are the three types of lapse rates?

• Environmental

• Dry adiabatic lapse rates

• Saturates adiabatic

What causes precipitation?

• This develops when tiny water droplets formed by condensation in saturated air, grow until they’re heavy enough to fall to the ground through rising air currents beneath them.

• Sometimes they collide with each other in turbulent air or by ice crystals growing within clouds as they rise and fall

Define evaporation

• The phase change of liquid water to vapour - this is the main way water enters the atmosphere

• Heat is needed to break the molecular bonds (this energy doesn’t increase temps)

• This energy is absorbed in latent heat and released in condensation, allowing huge quantitates of heat to be transferred around the planet.

Define Interception

• This is when vegetation intercepts water, storing it temporarily on branches, leaves and stems

• Eventually this moisture either evaporates (interception loss) or falls to the ground.

Define throughfall

• This is when rainfall is briefly intercepted before dripping to the ground

• During periods of prolonged/intense rainfall, intercepted rainwater may flow to the ground along branches and stems as stemflow

If rain doesn’t enter storage where else does it go?

• Infiltration by gravity into the soil and lateral movement or throughflow to streams and river channels

• Overland flow across the ground surface either as sheets or as trickles to stream and river channels.

How is overland flow linked to soil capacity?

• When rainfall intensity exceeds infiltration capacity, overland flow occurs

• Other theories state differently that water always infiltrate the ground until it becomes saturated causing it to raise the water table in a process called saturated overland flow.

What occurs where permeable rock is found?

Percolation as water seeps into rocks and migrates as groundwater flow - these can emerge as springs or seepages

What is the seasonal pattern of groundwater levels?

• By late Oct, the water table begins to rise as temps and evaporation falls. This recharge occurs until Jan

• Groundwater levels decline throughout winter, spring and summer reaching its lowest point in early autumn

What is the Cryosphere process?

• Ablation and meltwater is an important component of water flow in high latitudes and mountainous catchments in spring and summer

• Rapid thawing of snow in upland Britain in winter is a common cause of flooding in lowlands such as the Pennies