3. Precipitation Flashcards
Precipitation def
Water condensed in clouds that falls on land as rain, sleet, hail or snow
Different forms of precipitation (5)
- rain
- drizzle
- sleet
- snow
- hail
Steps of precipitation (3)
- Clouds form moisture (or water vapor) being present in the air
- Temperature determines the ability of air to hold/retain water
→ air that starts as cool is drier
→ air that starts as warm and moist, and then is chilled, will become saturated with water vaport - Condensation turns water vapor into liquid water (droplets) or solid water (crystals)
3 conditions required for precipitation to form + 1 sustaining condition
1) Air mass must cool to dew point
2) Condensation nuclei must be present
3) Droplets must grow
- Continuous import of vapor (needed to maintain the system)
Air mass definition
An air mass is a mass of air with relatively homogenous temperature and density
CONDITION #1: Air mass must cool to dew point
How is the air mass cooled?
(2 points)
- The dominant process for colling an air mass is adiabatic cooling by vertical lift.
- As air is vertically lifted, it expands, is chilled and becomes saturated with water pressure
Adiabatic def
Adiabatic means that the air mass changes some of its properties (pressure, volume or temperature) without any heat being added or withdraw from it
Dew point def
Temperature at which an air mass reaches 100% relative humidity
What can vertical lifting be caused by? (3)
- convection
- frontal effects
- orographic effects
Convective lifting (convection)
Warm air is less dense so it rises
Frontal lifting (fronts def + how frontal lifting happens)
- fronts are boundaries between air masses of different temperatures
- when fronts migrate, warmer air is pushed aloft, leading to adiabatic cooling
Orographic lifting
Orographic lifting occurs when air is forced to rise because of the physical presence of a mountain
CONDITION #2: Condensation nuclei must be present
what is condensation?
and what does it require? (2)
Condensation is a process driving the change from water vapor into liquid water.
It requires:
- air to be at or near saturation
-the presence of condensation nuclei
CONDITION #2: Condensation nuclei must be present
what is condensation nuclei?
Condensation nuclei are small particles or aerosols (e.g. dust, sea salt) upon which water vapor attaches to initiate condensation.
CONDITION #3: Droplets must grow
explanation of how droplets grow and become rain
- In clouds, small liquid water droplets collide and coalesce into larger water droplets.
- When those become to heavy, they fall as rain
CONDITION #3: Droplets must grow
explanation of how droplets become snow
- In clouds, ice crystal can grow in the air that has a mixture of both ice crystals and water droplets
- Small ice crystals colliding with small water droplets can coalesce to produce snow
SUSTAINING CONDITION: Continuous import of vapor
A constant supply of water vapor (through moist air rising) needs to be sustained
What can be used to measure precipitation? (1)
Bucket gages
How do tipping bucket rain gauges work? (5 points)
- The gauge registers rainfall by counting small increments of rain collected.
- When rain falls into the funnel, it runs into a container divided into 2 equal compartments by a partition.
- When half of the volume of the buck has drained from the funnel, the bucket tips the opposite way
- So number of times the bucket was tipped x half volume = precipitation
- The number and rate of the bucket movements are counted and logged electronically
What should be taken into consideration when finding a location to put a gauge? (1)
We want to make sure that it is far away as possible from obstructions (e.g. trees, buildings)
What parameters are recorded at a weather station? (5)
- wind velocity / direction
- rainfall
- relative humidity
- temperature
- radiation
Issue with standard range gauges
Undercatch of precipitation ranging from 5% to 50% from sources such as wind, evaporation, water splashing.
→ Undercatch is that we are measuring less precipitation than what has actually occurred
What are we interested in knowing about rainfall (2ish)
- total amount of rainfall
- amount of rainfall/time → to know intensity & max intensity
What are we interested in knowing about snowfall? (4ish)
- Amount of snow:
→ depth
→ density
→ SWE (snow water equivalent) - Distribution of snow
- Intensity
- Snowmelt
Snowpack (def)
The snowpack is a porous media (like soil) and is composed of solid, liquid, and vapor components
Snowpack depth (hs)
Snowpack depth can be defined by
hs = Vs/ A
where
- Vs = snowpack volume = Vi+Vw+Va (ice volume + water volume + air volume)
- A = snowpack area
DIMENSION: length
Snowpack porosity (Φ) (phi)
Snowpack porosity can be defined as
Φ = (Va + Vw)/ Vs
where
- Va = air volume
- Vw = water volume
- Vs = is the snowpack volume = Vi+Vw+Va (ice volume + water volume + air volume)
DIMENSION: Volume/Volume (no unit)
Snowpack density (ρs)
Snowpack density can be defined as
ρs = Ms/Vs
where
- Ms = weight of snow = Mi + Mw (ice weight + water weight)
- Vs = snow volume
DIMENSION: Mass / Volume
Liquid water content (θ)
Unfrozen water existing in a snowpack
Liquid water content can be defined as
θ = Vw/Vs
DIMENSION: 1
Relative density or fractional density (γs)
Fractional density is the ratio of snowpack density to water density
γs = ρs/ρw
Snow water equivalent (SWE) (hm)
Height of water that would be on the ground if the snowpack melted in place
hm = γs x hs
hm = (ρs/ρw) x hs
DIMENSION = length
Snowpack density (ρs): fresh snow vs old snow
which has higher density
Old snow has higher density than fresh snow
Three phases of snowmelt and their characteristics
- Warming phase
- snowpack is cold and/or subject to energy deficit
- energy is required to raise the temperature of the snowpack to 0°C - Ripening
- when the snowpack is “warm”, additional energy inputs melt the ice
- initial meltwater is retained in the snowpack until the water holding capacity of snow is exceeded - Output
- continued inputs of energy melt the remaining snow and ice
- water leaves by the base of snowpack
Energy balance for snowmelt (equation)
In - Out = Change in storage
(Sin - Sout) x t = ΔQ
S = fluxes (Joules/time)
Q = energy storage (Joules)
t = time
Snowmelt modelling when no info is given on energy (equation)
Δw = M x (Ta - Tb)
Δw = depth of melted water over the considered time
M = melt factor
Ta = mean daily temperature
Tb = base temperature (typically 0°C)
Measuring snow depth: 3 ways
- point measurements on a storm board with a ruler
- point measurements using an ultrasonic depth sensor (gives a continuous record)
- snow surveys
Snow surveys
- The basic principle guiding the measurement scheme is that measurements should be made consistently at the same locations, usually at monthly intervals during the winter, so that previous and subsequent measurements can be compared, month to month and year to year.
- The collection of measurements from snow surveys in a given region are used as indices that reflect the quantity of snow in that region.
- locations are chosen prior to the survey
- it records snow depth and sometimes SWE
- snow courses correspond to when multiple measurements of snow depth and SWE are made on a path between two fixed points
Two instruments that can measure snowfall as SWE
- weighing bucket gage
- snow pillows
Using a weighing bucket gage to measure snowfall as SWE
- how does it work?
- disadvantage?
- heated tipping bucket
- accumulated weight of water is recorded
- weight of water can be converted to SWE (length)
disadvantage: the bucket gauge can’t distinguish rain from snow
Using snow pillows to measure snowfall as SWE
- how does it work?
- 4 stainless steel panels are plumbed together and filled with antifreeze solution
- these constitute the snow pillows.
- the pressure of the snow on top of the snow pillows is measured
- the weight of water in the snow forces the fluid to the pressure transducer which converts the data to a signal for transmission
What can be used to represent temporal variance of rainfall? (3)
- rainfall hyetograph
- cumulative rainfall hyetograph or rainfall mass curve
- rainfall intensity
Rainfall hyetograph
- Plot of rainfall depth or intensity as a function of time
- usually a bar graph
Cumulative rainfall hyetograph or rainfall mass curve
Plot of the summation of rainfall increments as a function of time
Rainfall intensity
Depth of rainfall per unit time
(total depth of rain recorded at one point during an event / duration of the event)
What parameters can be used to compare rainfall events? (4)
- duration
- average rainfall intensity
- maximum rainfall intensity
- magnitude (total rainfall amount)
Frequency of a rainfall event of a certain intensity
(how to find) (eq)
The frequency of a rainfall event of intensity i is Fi
Fi = mi / T
T = observation period
mi = number of rainfall events of intensity i during the observation period
Return period (how to find) (eq)
The return period of a rainfall event of intensity i is Pi
Pi = 1 / Fi
(Fi = frequency = mi/T ; mi: # of events ; T: time)
Estimation methods for characterizing precipitation (rain or snow) over a large area (4)
- Arithmetic mean method
- Thiessen polygon method
- Isohyetal method
- Inverse distance weighting
Arithmetic mean method
Precipitation = sum of P at each gauge / # of gauges
Issues with the arithmetic mean method (3)
- gages must be inside the watershed
- gages must be ideally uniformly distributed
- gage measurements should not vary greatly about the mean (no outliers / extreme values)
Thiessen polygon method (steps) (5)
- Draw lines joining adjacent gages
- Draw perpendicular bisectors to the lines created in step 1
- Extend the lines created in step 2 until they intercept each other or hit the boundaries of the watershed.
- Compute the representative area for each gage
- Compute the areal average using the following formula:
Precipitation = ( P1 x A1 + P2 x A2 + … Pn x An ) / total Area
Advantage of the Thiessen polygon method (1)
Can use stations outside of the watershed, when helpful
Isohyetal method (steps)
- Construct isohyets
- Compute area between each pair of adjacent isohyets
- Compute average precipitation for each pair of adjacent isohyets
- Compute areal average using the following formula
Precipitation = ( P1 x A1 + P2 x A2 + … Pn x An ) / total Area
Isohyet def
An isohyet is a line of constant rainfall
(like contour lines for elevation)
Advantages of the isohyetal method (2)
- can use stations outside of the watershed if helpful
- less prone to error when stations are not uniformly distributed
What is the precision of the isohyetal method dependent on? (1)
The precision of the isohyets is dependent on the density of the stations within the area
Inverse distance weighting (def)
- prediction at a point is more influenced by nearby measurements than that of distant measurements
- the prediction at an ungauged point is inversely proportional to the distance to the measurement points
→ the further you are from a station, the less you are influenced by it
Inverse distance weighting (steps) (2)
- Compute distance (di) from each ungauged point to all the measurement points using
d12 = sqrt ( (x1-x2)^2+(y1-y2)^2 ) - Compute the precipitation using the following formula
Precip = (sum of (Pi / di^2 ) ) / ( sum of (1 / di^2) )
How do we relate point snow measurements to basin SWE?
- Basin-wide snow surveys should be designed to capture the max variability and be performed at the time of max snow acccumulation
- Models can predict the overall basin distribution of snow (i.e. fill the gaps) based on terrain characteristics
- It is however difficult to assess the representativity of the point measurements
Relation between snow fall and water resources
Snowfall is out of phase with water demand:
- snow imposes a time lag to the precipitation-runoff problem
- snowmelt enters the watershed where it melts, not where it falls
- there is a thermal dependance of runoff
Remote sensing of snow cover (3 points)
- RADAR satellites can be used to monitor both rainfall and snow
- MODIS provides daily snow-covered area at a 500m resolution
- Direct remote sensing of SWE not available
Interception (def)
Fraction of the gross precipitation which:
- does not reach the ground
- wets and adheres to above ground objects until it is returned to the atmosphere via evaporation
Gross precipitation (P)
Precipitation measured above the canopy or in a clearing
Canopy
Canopy is the cover resulting from the leaves at the top branches of a tree.
Throughfall (T)
Precipitation reaching the surface directly or via canopy drip
Stemflow (S)
Water reaching the surface by running along the trunks and stems
Total Interception loss (It)
Sum of all canopy interception and losses
Net precipitation (N)
Gross precipitation minus total interception loss
N = P-It
(will always be smaller than gross precipitation)
Difficulty of the measurement of interception during rain events (2)
It is difficult to measure because:
- Spatially variable as a function of vegetation density and type, wind, etc.
- Temporally variable: interception increases exponentially during a storm, until the interception capacity is achieved, and the wright of rain overcomes the surface tension holding the water on the plants.
How do we measure the interception during a rain event?
Usually estimated by approximating canopy storage during an event
Why is interception a critical hydrological process? (3)
- it can be a significant source for evapotranspiration
- it has a strong influence on runoff
- canopy drip gives rise to larger drops, which can increase local erosion
Canopy storage by vegetation type:
Which of the following has the largest storage?
conifers, deciduous trees, tropical forests, or grasses?
Tropical forests have the largest storage (1-5mm)
Factors that influence interception (5)
- precipitation intensity
- duration
- wind speed
- type of precipitation (rain vs snow)
- precipitation frequency
How does precipitation intensity influence interception?
The lower the intensity, the higher the interception
How does duration influence interception?
The shorter the duration, the higher the interception
How does wind speed influence interception?
More wind = more interception
(increases interception by blowing water into the interior of plants and plastering wet snow against trees and shrubs)
HOWEVER more wind could also blow the water away or to the trees to be intercepted…
How does type of precipitation influence interception?
(rain vs snow)
- liquid water has a higher surface tension than snow → rain = more interception
- rain can freeze to plants → rain = more interception
- snow is more easily blown off by plants → snow = less interception
How does precipitation frequency influence interception?
The more frequent precipitation events are, the less interception there is
→ time is needed for plants to dry out and for canopy storage capacity to increase between precipitation events
Depression storage (def)
- Gross precipitation or throughfall retained in puddles, stock ponds, ditches, and other depressions on the ground surface.
- It can be of considerable magnitude
- It can attenuate the impact of flooding
- It can either evaporate or contrivute to soil moisture and/or subsurface flow later on
Retention
Retention means that storage is held for a long period of time and depleted by evaporation
Detention
Detention rather refers to short-term storage depleted by flow away from the storage location
Wetlands
Transitional systems between terrestrial and aquatic systems where the water table is usually at or near the surface or the land is covered by shallow water
Criteria to be considered a wetland
(at least 1/3 has to be met)
- The land supports predominantly hydrophytes, at least periodically
- The substrate is predominantly undrained hydric soil
- The substrate is saturated with water or covered by shallow water at some time during the growing season of the year.
Hydrophytic vegetation (hydrophytes)
- Plants that are typically adapted to wetland and aquatic habitats
- Plants which grow in water or on a substrate that is at least periodically deficient in oxygen due to excess water
Hydric soils
Soils that are saturated, flooded, or ponded long enough during the growing season to develop oxygen-free conditions in the upper six inches
Hydrologic conditions of wetlands
Groundwater (water table or zone of saturation) is at the surface or within the soil root zone during all or part of the growing season
Benefit of wetlands (1)
They trap excess water in their depressions, leading to less runoff and less intense flooding
