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2 major approaches to quantifying hydrologic cycle?
- water balance
- energy balance
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hydrological scale: temporal
- usually year (1 complete annual cycle)
- avg. across many yrs
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hydrological scale: spatial
- usually the watershed
- nationally/globally
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Water balance eqn
- P = ET + Q +/- change in storage
- P(+) + ET (-) + Q (-) + (+/-) change in storage = 0
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watershed
topographically delineated aarea of land that collects and discharges surface stream flow through one outlet
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3 components of ET?
evaporation, transpiration, interception
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storage
water held in the soil mantle of a watershed
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application of water balance knowledge?
- describe hydrology of area
- basis of understanding impacts of land use on hydrology
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precision of water balance estimations?
~ +/- 10-15% best that can be expected
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radiation
form of electromagnetic energy from rapid oscillations of electromagnetic fields
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shortwave (K) wavelengths
0.15-3.0μ
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longwave (L) wavelengths
3.0-100μ
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albedo
- symbol: α
- reflected incoming shortwave radiation
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emissivity
- symbol: ε
- absorbed or emitted incoming shortwave radiation
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radiation emitted by hot objects?
emit high energy, shorter λ
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radiation emitted by cold objects?
emit lower energy, longer λ
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net radiation eqn (2 versions)
Q* = (K↓) + (L↓) + K(↑) + (L↑)
Q* = Qh + Qg + Qe
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specific heat
- symbol: ρ
- like thermal capacity but mass-based (cal/g * °C)
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thermal capacity
- symbol: c
- volume based (cal/cm3 * °C)
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thermal conducitivity
- symbol: k
- how easy it is for energy to travel through a substance (cal/cm * min * °C)
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transfer rate eqn
- conductivity * potential energy gradient
- (conductivity=1/resistance)
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thermal gradient eqn
- = change in temperature/distance bw points
- = ΔT/ΔZ
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2 thermal properties of ground heat (Qg)
- thermal capacity (ρC): energy req. to raise temp of substance by 1°C
- thermal conductivity (k): ease of heat transport thru substance
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conduction of Qg eqn
Qg = (ρC) * -k(ΔT/ΔZ)
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factors involved with convection of sensible heat (Qh)
- turbulent transfer coefficient (Kh) velocity term
- thermal conductivity of air (k)
- wind speed (μ)
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convection of Qh eqn
Qh = (ρC) * -kh (ΔT/ΔZ)
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sensible heat
mass transfer in slowly moving air currents, changes temperature only
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latent heat
- Qi
- energy used or released during phase changes of water (L)
- (-) means energy required, (+) means energy released
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latent heat exchange (Qe) factors
- driving force: vapour pressure gradient
- wind (μ) helps maintain VP gradient
- C = coefficient to regulate for diff. situations
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Qe eqn
- Qe = LV * μC (Δe/ΔZ)
- LV = latent heat of vaporization
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latent heat of fusion
Lf = 3.3x105 J/kg
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latent heat of sublimation
Ls = 2.79x106 J/kg
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latent heat of vaporization
Lv = 2.45x106 J/kg
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evapotranspiration eqn
ET = Qe/Lv
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in an energy flux density vs time diagram, why would the Q* line drop below 0 at night time?
- bodies emit radiation as a function of their temperature
- since the atmosphere is cold at night, and the earth is warm from the previous day, at night there is energy being emitted from the earth (Q* no longer incoming, but outgoing)
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what does it mean if β<1
- means Qh<Qe
- evaporation dominates
- daytime, moist conditions, conversion of water and conditions are not moisture limited
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what does it mean if β>1
- means Qh>Qe
- heat production dominates
- often nighttime, dry conditions
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what happens during an oasis effect that changes the Bowens ratio from (+) to (-)
- before shower, sunny day
- after shower, evaporation of water on surfact needs energy, so steals energy from heat production
- therefore the surface becomes cooler than the atmosphere because of evaporation
- once evaporation is done, surface will warm up and Qh flux will cause Bowens ratio to go back to (-)
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ea
- ambient vapour pressure
- vapour pressure in air mass at current temperature
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es
- saturation vapour pressure
- max amount of vapour pressure that can exist in air mass at ambient/specified temperature
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Ta
- ambient temperature
- temperature right now, aka dry bulb temperature
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Td
- dew point temperature
- temperature where condensation occurs (es=ea)
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Tw
- wet bulb temperature
- when evaporation decreases temperature, usually measured with a psychometer
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relative humidity eqn
RH = (ea/es) * 100
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Vapour pressure deficit
- VPD (KPa) = (es-ea)
- affected by temperature
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relative humidity ____ when ____ is ____
peaks, ambient temp, lowest
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Adiabatic lapse rate
- lapse rate for a parcel of air that rises or falls without any significant energy exchange with the surrounding atmosphere
- does not lose or gain any water or energy
- changes in temperature of the air are due only to changing volume with pressure
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why does a moist adiabatic lapse rate process become less cold than a dry adiabatic lapse rate process?
- when volume expands w/ less pressure as you go up higher, less interparticle collisions, therefore lower temperature
- moist situation: gaseous vapour going to condense if cooled
- releases energy
- cools less than dry process because of state change, condensation releases energy that warms
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atmospheric stability
comparison of environmental lapse rate with that of the adiabatic lapse rate
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environmental lapse rate
existing (real) temperature profile with elevation that is present
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adiabatic rate
the rate at which air masses cool as they rise because of changes in atmospheric pressure
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stable atmosphere?
- Ta<Te
- temperature profile that does not demand more rising of air
- bc rising air ALWAYS cools at the adiabatic rate
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unstable atmosphere?
Ta>Te
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orographic storms
- forced lifting caused by topography
- air masses cool when riding up over the land surface (increase in elevation=cooling)
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cyclonic and frontal storms
- forced lifting, warm and cool air masses in collision
- often involves oceanic air masses
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cold front
- cold air moving under warm air and pushing it up fast
- produces short and high intensity precip. events
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warm front
- warm air going up over receding cold air
- produces long distance areas of low intensity precip.
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convective storms
- unstable atmospheres
- combination of forced lifting and unstable air
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#1 error in recording precipitation
- undercatch and overcatch
- ↑wind speed = ↓effective catch area of gauge opening
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precipitation gauges: recording
- total P vs. time/intensity (mm/hr)
- remote location, expensive
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3 types of precipitation gauges
recording, non-recording, installation
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precipitation gauges: non-recording
- total P/daily P, monthly, seasonal
- inexpensive, suitable for local use only
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precipitation gauges: installation
- single gauge vs. network of gauges
- considerations: $, line of sight, access
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average error due to exposure/wind (undercatch) when measuring precip?
-5 to -80%
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the more ______ in an area, the _____ gauges you need per km
spatial variation, more
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arithmetic mean
- avg. of point values
- not spatial average
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Thiessen polygons method
- spatial weighting
- point value is assigned to entire polygon
- acounts for spatial distribution of precip. bw stations
- accounts for non-uniform station distribution
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Isohyetal analysis
- spatial weighting
- linear/non-linear interpolations bw stations
- continuous gradient (surface) of precip.
- more spatial resolution than Thiessen polygons and accounts for variation in elevation
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intensity
precipitation/unit time (mm/hr)
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as storm duration____, intensity tends to ______
increases, decline
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probability eqn
p = m/(n+1)
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return period eqn
Tr = 1/p
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plotting flood frequency?
peak annual (or instantaneous) discharge against Tr
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calculating SWE
snow depth x snow density x water density
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influential factors of snow accumulation
- elevation
- wind
- slope and aspect
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characteristics of new snowpacks
- low density
- crystal structure
- high albedo
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characteristics of a mature snowpack
- increase in density
- granular structure
- lower albedo
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ablation
total loss of water from a snowpack by snowmelt plus evap/sublimation
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snowmelt
amt of liquid water produced by melting of snow that leaves the snowpack
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warming phase
increase in snowpack temp to 0°C
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cold content
measure of "energy" needed to raise the avg temp of a snowpack to the melting point
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ripening phase
snowmelt increases the water content in snow, but no output from the bottom of the snowpack
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liquid water holding capacity
water held against gravity on snow crystals and in capillary channels in snowpack
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output phase
- once ripe, any additional melt will be output from the bottom of the snowpack
- percolation water water through snowpack
- infiltrates soil if it's thawed/porous
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2 conditions needed for evapotranspiration
water and energy
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interception: amt reaching forest floor =
Th + Sf
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interception: canopy interception =
Ic = Pg - (Th + Sf)
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total interception =
Ic + Il
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interception: net ppt =
Pg - I
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interception storage capacity
- max amt of water held in all aerial portions of the vegetation and in the litter
- bucket metaphor
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vegetation controls on interception losses
- branch arrangement
- crown form
- bark form/roughness
- leaf habit
- crown density, crown length
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Precipitation controls on interception losses
- snow vs rain
- storm sizes and frequency
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interception measurement done my measuring
- gross precip. (Pg) in an open area
- throughfall precip (Th) under the canopy
- stemflow
I = Pg - (Th + Sf)
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measuring throughfall (2 methods)
- troughs: increased surface area, less rain gauges
- roving rain gauges: relocate after each "event", increases statistical power
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driving force of evaporation
VPD = Δe = (es-ea) across some distance
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Penmen eqn is a combination of:
mechanism to move water + supply of energy
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3 factors affecting soil evaporation
- net radiation
- vegetation and litter
- water availability
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gravimetric water potential
Θg = (mass wet-mass dry)/(mass dry)
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volumetric water potential
θv = (mass wet-mass dry)/(volume wet)
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total storage in a given soil profile
storage = θv x profile depth
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total water potential eqn
Ψt = Ψg + (Ψp + Ψm)
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driving force of soil water movement?
water potential gradient
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