GEOG 408 midterm

Saturation

point of equilibrium where evaporation and condensation rate are equal

Atmospheric pressure

a measure of the force that the atmosphere exerts on each m^2 of the surface

Saturation vapour pressure over ice

lower than over water

Evaporation

water molecules turning into water vapour at the water surface, at any temperature

Boiling

liquid water turning into water vapour through the entire water volume, at a specific temperature

Boiling point temperature is

when saturation vapor pressure is equal atmospheric pressure

Vapour pressure

-          The partial pressure exerted by water vapour in the atmosphere

-          Units of Pa (kPa or others)

Absolute humidity

-          Also known as vapour density, it’s the mass of water vapour contained in a volume of air

-          Units: kg m^-3 (or g m^-3)

Mixing ratio

-          The ratio of the mass of water vapour to the mass of dry air

Usually reported as with units of g/kg

Specific humidity

-          The ratio of the mass of water vapour to the total mass of air

-          g/kg

Dewpoint temperature

–         the temperature to which air must be cooled, at a constant pressure, to reach saturation

Lower than the dewpoint = net condensation

Relative humidity

reports how close the air is to saturation

Vapour pressure deficit

difference between actual and saturation vapour pressure

Dew point depression

difference between air temperature and the dew-point temperature

Psychrometry

the difference between the wet-bulb and dry-bulb temperature is some indication of humidity

Hygrometer:

a device that absorbs water, producing some measurable change (length, temperature, mass)

Flux

Simply water moving through some component of the hydrologic cycle

different fluxes

Evaporation, precipitation, runoff, percolation/groundwater flow, transpiration, convection, and advection

Storage

An accumulation of water within any component of the hydrologic cycle

different storages

Lakes, aquifers, soil water, groundwater, snowpack, glaciers, vegetation

Catchment/River basins

Most common spatial unit in hydrology, It is an area of land where surface water flows to a single point, It is a flexible spatial unit, because a lot depends on where that point is

Watershed

-          Often used interchangeably with river basin

-          A little less specific, in that it can refer to all water on a landscape that drains to a certain body of water (lake, river, ocean)

What controls net evaporation rate

Vapour pressure deficit,  Availability of water, Turbulence (mixing), Available energy (heat)

Evaporation Pan

measures change in storage precipitation and assumes no run-off

Lysimeter

a somewhat complicated (weight scale type) device that mimics the soil/vegetation conditions, while trying to measure all the components of the water balance expect evaporation

Micromet

Basic instrumentation includes a sonic anemometer and a gas analyzer, Must operate at a high frequency, Integrates fluxes from a large upwind area, Measures a natural, undisturbed surface

micromet pros

Measures large area, Does not disturb the area

micromet cons

Expensive, Requires flat land, Requires specific atmospheric conditions

evaporation pan pros

Measures max evaporation

evaporation pan cons

Edge effect increases radiation, No vegetation interaction

lysimeter pros

Very good representation of the surface

lysimeter cons

Requires 3 measurements (lots of room for error)

Theoretical models

attempt to describe a phenomenon from “first principles”

Empirical models

attempt to describe phenomenon from observations

Semi-empirical models

combine theoretical models with measurements

What controls evapotranspiration

water vapour gradient, Mixing (turbulence), Available energy, Available water (restraints, soil water, stomata)

Potential evapotranspiration PE

The evapotranspiration that would occur if the water supply was unrestricted, Evapotranspiration over land is often restricted by the availability of soil water of willingness of plants to transpire’

Thornthwaite Pros

Easy to get required data, Works pretty well for certain regions

Thornthwaite cons

Calibrated got humid/temperate regions… appears to overestimate PE in arid/semi-arid regions, Can only provide estimates at the monthly time scale, Ignores many of the factors important to evaporation

Penman’s model – semi-empirical Pros

Pretty easy, need a few measurements, Can get evaporation estimates for shorter timescales, Includes more driving forces involved in evapotranspiration

Penman’s model cons

Provides “open water” evaporation estimates which is more useless than PE

Canopy resistance (stomatal resistance)

Describes how tightly plants hold on to their water

Rc = 0 in the case where water is freely available (assumed by PE)

Some plants (particularly crops) have low rc… they don’t hold their water tightly

Other plants, particularly those living in arid areas have high rc

Aerodynamic resistance

How efficient surface is at mixing the atmospheric boundary layer when wind blows over it

Basically, mixing is less effective over smooth surfaces, so ra is higher over those surfaces

Penman-Monteith Model Pros

Directly calculates actual evapotranspiration, which is what we want, Requires relatively few measurements (net radiation, temperature, wind speed, relative humidity), Has been rigorously tested and seems to work under a wide range of situations

Penman-Monteith Model cons

Relies heavily on good values for canopy and atmospheric resistance terms… those can be difficult to get

Snow water equivalent SWE

the amount of liquid water you would obtain if you melted a certain amount of snow

Snow crystals

Bergeron process, The shape of the initial crystal when it falls depends on temperature in the cloud and degree of supersaturation, Once on the ground crystals change

Destructive metamorphism

Snow density increases, Snow grain size deceases, Snow grain form changes

Constructive metamorphism – faceting

Snow density decreases, Snow grain size increases, Snow grain form changes

Net shortwave

A function of incoming shortwave and albedo, Once melt starts, K* will not be evenly distributed across the watershed

o   Slope and aspect will play an important role

o   Vegetation cover will play an important role

Cannot provide enough energy to initiate melt, Once melt is initiated, K* increases rapidly as albedo decreases, accelerating melt, Always positive

Net longwave

outgoing longwave radiation depends on the surface temperature of snow, a melting surface will stay at -315 W/m^2, a cold, clear, dry atmosphere will have low Ldown, and hence high net longwave losses (L* negative), since K* is small in winter (but positive), L* will typically overwhelm K*, making Q* negative, But, a moist, cloudy system will increase Ldown, making L* less negative and possibly pushing Q* positive

Sensible heat flux

if the atmosphere is warmer than the snow, there will be a transfer of energy to the snow (a chinook)

Latent heat flux

if the atmospheric humidity is very low, it will drive some sort of sublimation (transfer from solid to vapour phase), this will use up a lot of energy

Ground (conductive) flux

during melt, the ground heat flux is probably going to be a loss of heat from the snowpack (ground will be cooler than the snowpack)

Snow warming phase

Before we can melt snow, we need to warm it up to 0C, but before we can do that, we need Q* > 0, Change in incoming solar radiation, Change in albedo, Change in longwave energy balance

Snow ripening phase

once the snow is “isothermal” (all at 0C), melt begins, But, initially this melt is retained in pore spaces (doesn’t run off immediately)

Output phase

after snowpack is ripened, further energy input will produce runoff

Clouds

Typically formed by rising, cooling air, Convection, orographic lift, convergence, frontal system, divergence

“Cirr”

high cloud (>7000m)

“alto”

medium-height cloud (~2000 – 7000m)

No prefix

low cloud (<200m)

“cumulus”

puffy cloud

“stratus”

layered cloud

Terminal velocity

larger droplets have a smaller surface area-to-mass ratio, and therefore fall faster than small drops

Why don’t droplets fall

Since clouds are made by vertical motion, there is basically always some sort of updraft, A droplet needs to grow big enough that its terminal velocity exceeds updraft velocity

Growth by Collision & Coalescence

Most important in warm clouds, Droplets grow by smashing into each other

Bergeron-Findeisen processes

Most important in cold clouds, Ice crystals are surprisingly rare in clouds between ~0C to -20C, But they can form by spontaneous freezing of cloud droplets, or by accretion onto cold particles called ice nuclei, In clouds colder than -40C only ice crystals exist, Depends heavily on the fact that saturation vapour pressure over ice is lower than over water, Ice crystals grow at the expense of droplets

“graduated” rain gauge pros

Cheap, Simple

“graduated” rain gauge cons

Low frequency measurement, Evaporation, wetting, turbulence

Tipping bucket pros

Pretty cheap and simple, Higher frequency measurements

tipping bucket cons

Sensitivity varies, Wetting turbulence

Weighing bucket pros

High frequency measurements

weighing bucket cons

Wetting turbulence, Kind of expensive

laser gauge pros

High frequency, Type + amount

laser gauge cons

Super expensive, Power hungry

Point measurements biases/uncertainties

Evaporation loss, Wetting loss, Splash, Turbulence, Response rate

point measurement other issues

Sitting concerns, Occult precipitation (fog drip, rime) may be missed by the gauge, measuring under a canopy

Areal Measurements

Doppler radar, Satellite remote sensing

Thiessen Polygons

In this approach, the region is divided not “subregions” based in proximity to a gauge and weighting based on relative areas, The big problem with this method is that it’s really only valid if you have a good reason to believe that rainfall is consistent across sub-regions

Rainfall Intensity//Duration

The depth of rain is important for hydrology, but the rainfall intensity can also be important, The duration of a storm is important too

A hyetograph

plots rainfall intensity vs time

Return period

is the average time between storms with the same duration and the same or greater depth

Intensity duration frequency graph IDF

Tells you how likely it is to have a rainfall event with a certain average intensity over a certain period of time

to figure our rainfall intensity

From your rain gauge (tipping bucket) data, you can determine how long a storm lasted, When did the bucket start and stop tipping

air mass

a large body of air, thousands of km2 across, with similar temperature and humidity in horizontal directions

fronts

boundaries between air masses with different properties

type of front

depends on the air mass that is moving into an area

stationary front

a boundary between air masses with no movement, could become ware or cold front if one of the air masses starts moving

cold front

an area where cold air (usually cA/cP or mP) is replacing warm air (mT or cT)

properties of a cold front

displacement of the warm air by dense, cold air, creates uplift and precipitation, average 36km/hr

warm front

an area where warm air (usually mT or cT) is replacing cold air (cA, cP, mP)

properties of a warm front

warm air overrides cold air, again leading to precipitation , but different cloud types, average 18 km/hr

occluded front

an occlusion occurs when cold front catches up tom and overtakes, a warm front

cyclone

an area where the atmosphere is exhibiting circular motion, rotating in the same direction as Earth, around a low pressure centre

cyclone is associated with

surface convergence and uplift, so probably clouds and precipitation

cyclogenesis

the development or strengthening of a mid-latitude cyclone

cyclogenesis tends to occur

preferentially along topographic obstructions and big temperature gradients

jet stream

fast moving (>185 km/hr - >370 km/hr) air currents thousands of km long, a few hundred km wide and a few km thick

Rossby waves

waves several thousand kilometers long, occurring in the upper-air westerlies of the mid-latitudes

divergence/convergence aloft

waves in the jet stream create convergence/divergence aloft, which intensify surface highs and lows