Precipitation + Evapotranspiration Flashcards
What is the water balance equation
P - E - ΔS - Q = 0
What does each letter in the water balance equation stand for
P = Precipitation E = Evaporation ΔS = Change in storage Q = Runoff
Explain each part of the water balance equation in terms of flux and storage of water
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
Drawback of the water balance equation?
processes occur over a spatial and temporal scale that may not coincide with the scale at which we make our measurement
Precipitation formation
Warm, moist air is cooled → saturation with water vapour → Condensation nuclei allow water vapour to condense into liquid water or ice
4 types of uplift mechanisms
Convective
Cyclonic/Frontal
Orographic
Convective uplift
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
Cyclonic/ Frontal uplift
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
Orographic uplift
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
what is the adiabatic process
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
LAPSE RATES
Dry adiabatic lapse rate → Saturated adiabatic lapse rate
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)
Collision and Coalescence
droplets falling
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
Bergeron process
droplets falling
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
Dynamic influences on precipitation
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
Static influences on precipitation
Altitude
Aspect
Slope
They do not vary between storm events
Explain static influences
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
Rain shadow effect
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.
Explain direct through fall and the leaf area index
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
Explain
-Indirect through fall
- Canopy storage capacity
- Rainfall characteristics
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
Explain stem flow
rainfall intercepted by stems and branches → flows down tree trunk into the soil like a funnel
may eventually contribute to runoff or transpiration
Explain interception loss
water sits on the canopy → before through fall or stem flow occurs → it is available for evaporation
Explain interception gain
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
What 3 things does evaporation need to occur
Water at the surface
Energy supply
Unsaturated air (ability of the atmosphere to receive vapour)
Explain
- sensible heat
- net radiation
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
Explain transpiration
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
How is Evapotranspiration measured
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
Explain evaporation from snow (sublimation)
sublimination = evaporation direct from snow/ ice → significant in forest areas → snow remains in place for weeks