Topic 3/4- IDF Curves/Evaporation Flashcards
explain i = a*t / (b+D)
i = intensity a,b = coefficients D = duration
-intensity and duration are inversely related to each other
whats the purpose of idf curves
- intensity-duration-frequency curves help specify a storm using the parameters of intensity (mm/hr) and duration (minutes, hours)
- estimates statistically
explain h = k*D^m.
what can u do with this eqn?
h = total storm depth
k , m = parameters
D = duration
You can take the ln of both sides, to get
ln(h) = ln(k) + mln(D)
- can get the parameters k,m for a SPECIFIC return period using data
- so ex, plug in h1 D1, h2 D2, 2 eq, 2 unknown, you get a k and an m for a specific return period
What do IDF curves reflect?
What do we assume?
- the general climatology of a catchment (area)
- that intensity is constant
What’s a return period?
For example, a Tr = 100 yr (return period of 100yrs) means that approx a storm of this intensity and duration on the AVERAGE occurs once every 100 yrs
-higher return period, higher intensity (statistically, really big storms only happen once in a while)
What is the time of concentration?
tc, time of concentration, is the time taken for water falling on the remote part of a drainage basin to reach the basin outlet, where remoteness refers to the travel time than distance
What are the assumptions when applying IDF curves?
When are these assumptions usually not valid?
What else can we do if these assumptions arent valid?
- assuming constant intensity (I) for given duration (D)
- not valid when storm duration is very long, say several hours
- we can use rainfall hyetographs, which show the time distribution of a rainfall volume (depth per each hour of the storm, so see which hour had the most/least rainfall)
When using NRCS tables (TYPE 1 ,2 ,3) , how do we make a cumulative rainfall diagram and a rainfall hyetograph?
-NRCS table gives the time, and the cumulative rainfall
SUM of DeltaP / P
-to get a cumulative rainfall diagram, we do
Sum of DeltaP / P times P, to get P (Y axis)
-to get a rainfall hyetograph, we find the difference between each delta P, so not the sum of all delta P’s, but just the amount of P for each given hour
What is Tc?
If a watershed has a given Tc, how can the Qpeak calc vary?
Tc = time to concentration
- its how long rain will take to cover a whole watershed if it starts at one end and ends at the other end (how long it will take for the rain to cover the whole water shed)
- watershed specific
Qpeak = intensity * duration
-BUT IF duration is less than time of concentration Tc, then we get
Qpeak = intesntiy * duration *fraction of duration/Tc
What is potential ET? (conditions for PET)? How is PET related to ET
how much evapotranspiration would occur if there was unlimited moisture supply. so
ET <= potential ET
What is evaporation
a COOLING process (b/c solid —> liquid requires heat, so it takes heat from atmosphere) by which water is transferred from the land and water masses of the earth to the atmosphere as water vapor
Describe transpiration
- consists of vaporization of liquid water contained in plant tissues and the vapor removed to the atmosphere
- nearly all water taken up the plants is lost by transpiration
Why is evaporation process intensive?
b/c breaking hydrogen bonds is very hard???
what is latent heat of vaporization?
energy needed to turn liquid water to water vapor at a given temp
Factors effecting evapotranspiration? 4
- net solar radiation
- primary source of energy that provides the latent heat needed for the phase change from water to water vapor - vapor pressure
- difference between the atmosphere vapor pressure and land/water surface vapor pressure (relative humidity) - air and or water surface temperature
- heat transfer by wind (velocity)
what type of radiation does earth emit?
entirely infrared
Evaporation Pans to estimate ET
What does pan coefficient depend on?
- under same climatic conditions, plants and vegetation evaporates < evaporation pan
- pan evaporation Ep is adjusted for predicting crop evaporation by pc = pan coefficient
- so Potential ET = pc*Ep
Pc depends:
- crop type surrounding
- upwind distance of crop type
- wind speed
- relative humidity
Water budget approach
-the delta S stuff from introduction
Empirical models using climate data
Thornthwaite’s temp model
- PET(month,latitude)=1.6(10*ti/TE)^a
- TE = SUM [I]
- a=…. long ass eqn
- PETi = K*PETi(0)
Energy budget model
Rn = lvE + G + H
- Rn (net radiation) = Rsn + Rln (net short and longwave radiation)
- E=evapotranspirative flux from soil and vegetation
- G = ground heat flux
- H = sensible heat flux
- heat flux is the rate of heat energy transfer through a given surface per unit time
- En (kgm^-2s^-1, Rn, G, H in W/m^-2)
- W = J/s
- J = Nm
- to adjust from the effect of cloud cover on solar insolation, we use Rs = R[a+b(n/N)]
- Rs = solar radiation received on horizontal surface, a and b are emperical coefficients
- R = extra terrestrial solar radiation received, given in langley (gives value of radiation before cloud reduction)
- 1 langley = 1 calorie/cm^2
- Rsn = Rs[1-albedo]
- Rln = atmospheric sensetivity * stefan boltzmann constant * (Ta^4 - Ts^4) [temps in kelvin]
Mass transfer/Aerodynamic method
-estimates evaporation from modelling mass and momentum transport of water vapor from evaporating surface by convection
-Ea = f(U)(es-ea)
-note: use Ta for ea and Ts for es
-U = wind speed
-f(U) = mass transfer coefficient
=f(U) = 0.622(von karman’s constant)^2paU / Ppwln[height where measurements are taken / roughness height of vegetation]^2
-P is the atmospheric pressure
Penman combination method
- by assuming a thin free-water surface, this eqn combines both the aerodynamic and energy mtethods
- most physically based evaporation model derived from basic eqns
- Rn = lv + G(assume 0) + H
- bowens ration Beta = H/lvE=gamma (Ts-Ta)/(es-ea)
- saturation vapor pressure gradient delta = des/dT
- mass transfer Ea = f(U)(es-ea)
- so we get E = deltaEn + gammaEa / (delta + gamma)
- delta is the saturation vapor pressure gradient
- gamma is the psychrometric constant (Pa/degree celcius)
Priestley and taylor eqn
-found that Ea is about 30% of the first, so E=a *deltaEn/(delta + gamma)
Can penman’s eqn be used to calc both free water surface evaporation and potential ET of a water shed?
look and understand notes, me too tired (last page of notes)
When is Qpeak when referring to a drainage basin?
happens at time of concentration, tc, when all the rainfall hits the area,
so t1 < t2< t3< tc > t4 > t5 ….
What is a dimensionless rainfall diagram?
Rainfall hyetograph?
- sum deltaP / p versus t/D (are region specific, such as NRCS TYPE 1 , 2, 3)
- gives time distribution of a rainfall volume (depth vs time (in incraments of 1hr) , so it doesnt assume constant intensity
Thornwaite’s model, what constant K a function of? What does it do?
PETi(0)
- 0 = latitude
- i = month
K corrects for latitudes other than 0 degrees
1 Langley =?
= 1 calorie / [cm^2*day ]
What did all these evaporation methods assume
that we have unlimited water supply, that is, our calcs were PET
