PrecipitationPrecipitation

Joan Wu and Jan BollJoan Wu and Jan Boll

(06/2012)(06/2012)

What Have You Known?What Have You Known?

The importance of understanding precipitation The importance of understanding precipitation as a hydrologic processesas a hydrologic processes

Mechanisms by which precipitation is generatedMechanisms by which precipitation is generated Uplift due to convergenceUplift due to convergence Uplift due to convectionUplift due to convection Uplift due to orographyUplift due to orography

Sources of precipitation data (NOAA-NCDC)Sources of precipitation data (NOAA-NCDC) Basic DDF or IDF analysisBasic DDF or IDF analysis

DDF: depth-duration-frequencyDDF: depth-duration-frequency IDF: intensity-duration-frequencyIDF: intensity-duration-frequency

What Are Covered Here?What Are Covered Here?

Critical temperatures for rain-snow Critical temperatures for rain-snow transitiontransition

Point measurements of precipitationPoint measurements of precipitation Filling in missing dataFilling in missing data Precipitation data generation using PRISMPrecipitation data generation using PRISM Precipitation data generation using Precipitation data generation using

CLIGENCLIGEN

Critical Temperatures forCritical Temperatures forrain-snow transitionrain-snow transition

If a precipitation record does not specify If a precipitation record does not specify the form of precipitation, what do we do?the form of precipitation, what do we do?

How do we decide whether precipitation How do we decide whether precipitation was rain, snow, or a mixture of rain-snow?was rain, snow, or a mixture of rain-snow?

Probability of occurrence of rain or snow as Probability of occurrence of rain or snow as a function of air temperature a function of air temperature (Auer, 1974)(Auer, 1974)

Partition of Precipitation (Partition of Precipitation (PP))into Rain or Snowinto Rain or Snow

Ps = P Ta <= Tmin

Ps = (Tmax − Ta)/(Tmax − Tmin) P Tmin < Ta < Tmax

Ps = 0 Ta >= Tmax

Pr = P − Ps

Pr, Ps = water equivalent depths of rain and snow, respectively,Tmin = threshold temperature below which all precipitation is snowTmax = threshold temperature above which all precipitation is rain

• Between the threshold values precipitation is a mix of rain and snow

• Typical threshold temperatures are −1.1C and 3.3C

Point MeasurementsPoint Measurements

Questions on gage setup and influence on Questions on gage setup and influence on measurement accuracymeasurement accuracy

What size orifice should be used?What size orifice should be used? How should the plane of the orifice be oriented?How should the plane of the orifice be oriented? How much should the gauge protrude above ground surface?How much should the gauge protrude above ground surface? Should the gauge be installed with a device to reduce wind Should the gauge be installed with a device to reduce wind

effects, e.g., with a wind shield?effects, e.g., with a wind shield? How far should the gauge be from other projections (trees, How far should the gauge be from other projections (trees,

buildings)?buildings)? How can we prevent water from splashing out?How can we prevent water from splashing out? How can we prevent evaporation of the collected water?How can we prevent evaporation of the collected water?

(a) Without windshielding; (b) rigidNipher-type shields;(c) hinged Alter-type shields(Dingman, 2002)

Wind effects on gage catch Wind effects on gage catch (Dingman, 2002)(Dingman, 2002)

Ideal rain gage for rainfall with egg-crateStructure (Dingman, 2002)

Point MeasurementsPoint Measurements

Other errors?Other errors? Instrument errorInstrument error Observer errorObserver error Errors due to different observation timesErrors due to different observation times Error due to occult precipitationError due to occult precipitation Errors due to low-intensity rainsErrors due to low-intensity rains

Point MeasurementsPoint Measurements

Checking the consistency of point Checking the consistency of point measurementsmeasurements Change of gauge locationChange of gauge location Change of gauge typeChange of gauge type Change of gauge environmentChange of gauge environment Change of gauge observerChange of gauge observer Change of gauge climateChange of gauge climate

Double-mass curve techniqueDouble-mass curve technique

Double-Mass Curve TechniqueDouble-Mass Curve Technique

A plot on regular arithmetic graph paperA plot on regular arithmetic graph paper Successive cumulative annual Successive cumulative annual P P at the at the

gauge in question vs the cumulative avg. gauge in question vs the cumulative avg. annual annual PP at nearby gauges for the same at nearby gauges for the same periodperiod

A break in slope indicating inconsistenceA break in slope indicating inconsistence Adjustment maybe needed if the break in Adjustment maybe needed if the break in

slope persists over 5 yrslope persists over 5 yr

Double-Mass Curve TechniqueDouble-Mass Curve Technique

If If a, b a, b areare slopes of lines after and before slopes of lines after and before the break, respectively:the break, respectively: Adjustments to previous events by multiplying Adjustments to previous events by multiplying

each of them by each of them by a/ba/b, or, or Adjustments to recent events by multiplying Adjustments to recent events by multiplying

each of them by each of them by b/ab/a

Filling in Missing DataFilling in Missing Data

Station-average methodStation-average method Normal-ratio methodNormal-ratio method Inverse-distance weightingInverse-distance weighting RegressionRegression

Station-Average MethodStation-Average Method

PPXX is the missing precipitation value for station is the missing precipitation value for station X X

PP11, , PP22, …, , …, PPnn are precipitation values at the adjacent are precipitation values at the adjacent

stations for the same period stations for the same period nn is the number of nearby stations is the number of nearby stations

PX 1

nPi

i1

n

Normal-Ratio MethodNormal-Ratio Method

PPXX is the missing precipitation value for station is the missing precipitation value for station XX for a for a certain time periodcertain time period

PP11, , PP22, …, , …, PPnn are precipitation values at adjacent are precipitation values at adjacent stations for the same periodstations for the same period

NNXX is the long-term, annual average precipitation at is the long-term, annual average precipitation at station station XX

NN11, , NN22, …, , …, NNnn is the long-term precipitation for is the long-term precipitation for neighboring stationsneighboring stations

nn is the number of adjacent stations is the number of adjacent stations

PXNX

1

n

P1N1

P2N2

.....PnNn

PX

1

n

PiNii1

n NXor

Inverse-Distance WeightingInverse-Distance Weighting

1. di = (xi2 + yi2)0.5

PPXX is the missing precipitation value for station is the missing precipitation value for station X X for a for a

certain time periodcertain time period PPii are precipitation values at adjacent stations for the are precipitation values at adjacent stations for the

same periodsame period nn is the number of neighboring stations is the number of neighboring stations

PX 1

Wdi

bPii1

n3.

W di b

i1

n2.

• Distance from gage with missing data Distance from gage with missing data to the neighboring gagesto the neighboring gages

• Weight of distances where Weight of distances where bb is a is a proportionality factor (proportionality factor (bb = 1, 2 ) = 1, 2 )

RegressionRegression

PPXX is the missing precipitation value for station is the missing precipitation value for station XX for for

certain time periodcertain time period PP11, , PP22, …, , …, PPnn are precipitation values at the are precipitation values at the

neighboring stations for the same periodneighboring stations for the same period bb00, …, , …, bbnn are coefficients calculated by least-squares are coefficients calculated by least-squares

methodsmethods nn is the number of nearby gages is the number of nearby gages Method suitable when there is a large number of Method suitable when there is a large number of

days when observations are available for all gagesdays when observations are available for all gages

PX = bo + b1P1 + b2P2 + …. + bnPn

Precipitation Data Precipitation Data Generation Using PRISMGeneration Using PRISM

Click Products on top bar and choose Climatehttp://www.wcc.nrcs.usda.gov/

Click GIS Products

Go back and click PRISM

Click Prism Group at OSU

Click Internet Map Server

Click Zoom In

Click Show Chart

Precipitation Data Precipitation Data Generation Using CLIGENGeneration Using CLIGEN

Click Modeling Software/Erosion Modeling/ FS WEPPhttp://forest.moscowfsl.wsu.edu

Click Rock:Clime

Click SHOW ME THE CLIMATE

Choose a station, e.g., Spokane/Modify climate

Modify current values using PRISM

Use current values/Download climate