Precipitation + Evapotranspiration

Question 1

What is the water balance equation

Answer 1

P - E - ΔS - Q = 0

Question 2

What does each letter in the water balance equation stand for

Answer 2

P = Precipitation 
E = Evaporation 
ΔS = Change in storage 
Q = Runoff

Question 3

Explain each part of the water balance equation in terms of flux and storage of water

Answer 3

Precipitation - flux between rainfall and snowfall

Evaporation - flux between
open water bodies, soil surface and vegetation (included interception and transpiration)

Storage term - inclused soil moisture, groundwater, glaciers and snow cover

Runoff - movement of water above and below the surface of earth

Question 4

Drawback of the water balance equation?

Answer 4

processes occur over a spatial and temporal scale that may not coincide with the scale at which we make our measurement

Question 5

Precipitation formation

Answer 5

Warm, moist air is cooled → saturation with water vapour → Condensation nuclei allow water vapour to condense into liquid water or ice

Question 6

4 types of uplift mechanisms

Answer 6

Convective
Cyclonic/Frontal
Orographic

Question 7

Convective uplift

Answer 7

Earths surface heats → warm air rises and cools → upward movement creates downdraft expelling rain, hail + cool air

EXAMPLE
July 2003 convective thunderstorm leads to flooding (10mm) in essex village but 10km away in next town there was no rainfall recorded

Question 8

Cyclonic/ Frontal uplift

Answer 8

Heavy showers and thunder at cold from, light rain at warm front

cold air advances under warm moist air lifting it up → producing heavy cloud and intense rain

occurs mainly in mid-latitude regions where warm tropical air meets cold polar fronts

Question 9

Orographic uplift

Answer 9

warm moist air forced upwards over an obstruction → rises and condenses → creates clouds → often precipitation → at other side of obstruction air is hot, dry and cloudless

Question 10

what is the adiabatic process

Answer 10

Heating by compression of air → low latitudes

Cooling by expansion of air→ high latitudes

Air is more dense nearer the surface of earth so if a packet of air rises, it cools and expands into the less dense space above

NO heat is added or removed from the system but the observed temperature changes

Question 11

LAPSE RATES

Dry adiabatic lapse rate → Saturated adiabatic lapse rate

Answer 11

unsaturated air cools (3°C/1000ft) and condenses → passes dew point and clouds form → latent heat released → passes dew point → SALR (1.5°C/1000ft)

Question 12

Collision and Coalescence

droplets falling

Answer 12

growth of water droplets to overcome gravity and fall from clouds
droplets grow from 1 micron to 3,000 microns
raidndrops collide → come together (coalescence) to form larger droplet → collides with more before falling to surface

mechanism of water droplet formation not really known → proportion of condensation, collision or bergeron process formed depends on cloud circumstances + can vary

Question 13

Bergeron process

droplets falling

Answer 13

Vapour pressure = pressure exerted within a parcel of air with water vapour present

Saturation vapour pressure = Maximum vapour pressure the air parcel can hold

mechanism of water droplet formation not really known → proportion of condensation, collision or bergeron process formed depends on cloud circumstances + can vary

Question 14

Dynamic influences on precipitation

Answer 14

Variations in weather

At a global scale influences on precipitation are largely dynamic

Continental scale → differences in rainfall due a mixture of static and dynamic

Question 15

Static influences on precipitation

Answer 15

Altitude
Aspect
Slope

They do not vary between storm events

Question 16

Explain static influences

Answer 16

Altitude = orographic precipitation

Aspect = slopes with aspects facing away from the predominant weather patterns will receive less rainfall than their opposites

Slope = only relevant at small scale and is normally ignored

Question 17

Rain shadow effect

Answer 17

Due to a mix of static and dynamic influences when there is a large landmass rainfall can be found to be heavier on one side than the other.

Question 18

Explain direct through fall and the leaf area index

Answer 18

water falls to the surface directly through gaps in the canopy

amount of direct throughfall controlled by canopy coverage which is measured by the leaf area index → the ratio of leaf area to ground surface area

< 1 → direct through fall
> 1 → more than one layer of leaf above ground

Question 19

Explain
-Indirect through fall

• Canopy storage capacity
• Rainfall characteristics

Answer 19

water drips off leaves and branches

Canopy storage capacity →volume of water held in canopy before it starts dripping

Rainfall characteristics → dictates how quickly the canopy storage capacity is filled

HOWEVER

canopy characteristics are constantly changing so indirect through fall occurs before capacity is reached → difficult to gauge exact storage capacity

may eventually contribute to runoff or transpiration

Question 20

Explain stem flow

Answer 20

rainfall intercepted by stems and branches → flows down tree trunk into the soil like a funnel

may eventually contribute to runoff or transpiration

Question 21

Explain interception loss

Answer 21

water sits on the canopy → before through fall or stem flow occurs → it is available for evaporation

Question 22

Explain interception gain

Answer 22

trees intercept fog particles → creating for droplets → interception gain

Vegetation roughness → mixing of air and wind → rapid deposition of condensing water

Deforestation = less water in river

Question 23

What 3 things does evaporation need to occur

Answer 23

Water at the surface
Energy supply
Unsaturated air (ability of the atmosphere to receive vapour)

Question 24

Explain

• sensible heat
• net radiation

Answer 24

Sensible heat = energy transferred from a warm body to a cooler one

Net radiation = energy available due to the balance between incoming and outgoing

when outgoing > incoming there is an energy DEFICIT

when outgoing < incoming there is an energy SURPLUS

Question 25

Explain transpiration

Answer 25

internal consumption of water by plants is lost to the atmosphere through the hundreds of stomata in leaf surfaces → special case of evaporation → evaporation + transpiration = evapotranspiration

stomta open and close with daylight and are limited by water availability

Question 26

How is Evapotranspiration measured

Answer 26

Evaporation pans - has weighing device → can record how much water is lost over a time period
BUT doesn’t reflect transpiration

Thornwaite model - uses heat budget, estimated rate from open water systems

Question 27

Explain evaporation from snow (sublimation)

Answer 27

sublimination = evaporation direct from snow/ ice → significant in forest areas → snow remains in place for weeks