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Precipitation Joan Wu and Jan Boll (06/2012)

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What Have You Known? The importance of understanding precipitation as a hydrologic processes Mechanisms by which precipitation is generated Uplift due to convergence Uplift due to convergence Uplift due to convection Uplift due to convection Uplift due to orography Uplift due to orography Sources of precipitation data (NOAA-NCDC) Basic DDF or IDF analysis DDF: depth-duration-frequency DDF: depth-duration-frequency IDF: intensity-duration-frequency IDF: intensity-duration-frequency

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What Are Covered Here? Critical temperatures for rain-snow transition Point measurements of precipitation Filling in missing data Precipitation data generation using PRISM Precipitation data generation using CLIGEN

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Critical Temperatures for rain-snow transition If a precipitation record does not specify the form of precipitation, what do we do? How do we decide whether precipitation was rain, snow, or a mixture of rain-snow?

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

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Partition of Precipitation (P) into Rain or Snow P s = P T a <= T min P s = (T max − T a )/(T max − T min ) P T min < T a < T max P s = 0 T a >= T max P r = P − P s P r, P s = water equivalent depths of rain and snow, respectively, T min = threshold temperature below which all precipitation is snow T max = 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

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Point Measurements Questions on gage setup and influence on measurement 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 effects, e.g., with a wind shield? Should the gauge be installed with a device to reduce wind effects, e.g., with a wind shield? How far should the gauge be from other projections (trees, buildings)? How far should the gauge be from other projections (trees, 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?

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(a) Without wind shielding; (b) rigid Nipher-type shields; (c) hinged Alter- type shields (Dingman, 2002)

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Wind effects on gage catch (Dingman, 2002)

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

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Point Measurements Other errors? Instrument error Instrument error Observer error Observer error Errors due to different observation times Errors due to different observation times Error due to occult precipitation Error due to occult precipitation Errors due to low-intensity rains Errors due to low-intensity rains

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Point Measurements Checking the consistency of point measurements Change of gauge location Change of gauge location Change of gauge type Change of gauge type Change of gauge environment Change of gauge environment Change of gauge observer Change of gauge observer Change of gauge climate Change of gauge climate Double-mass curve technique

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Double-Mass Curve Technique A plot on regular arithmetic graph paper Successive cumulative annual P at the gauge in question vs the cumulative avg. annual P at nearby gauges for the same period A break in slope indicating inconsistence Adjustment maybe needed if the break in slope persists over 5 yr

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Double-Mass Curve Technique If a, b are slopes of lines after and before the break, respectively: Adjustments to previous events by multiplying each of them by a/b, or Adjustments to previous events by multiplying each of them by a/b, or Adjustments to recent events by multiplying each of them by b/a Adjustments to recent events by multiplying each of them by b/a

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Filling in Missing Data Station-average method Normal-ratio method Inverse-distance weighting Regression

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Station-Average Method P X is the missing precipitation value for station X P 1, P 2, …, P n are precipitation values at the adjacent stations for the same period n is the number of nearby stations

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Normal-Ratio Method P X is the missing precipitation value for station X for a certain time period P 1, P 2, …, P n are precipitation values at adjacent stations for the same period N X is the long-term, annual average precipitation at station X N 1, N 2, …, N n is the long-term precipitation for neighboring stations n is the number of adjacent stations or

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Inverse-Distance Weighting 1. d i = (x i 2 + y i 2 ) 0.5 P X is the missing precipitation value for station X for a certain time period P i are precipitation values at adjacent stations for the same period n is the number of neighboring stations Distance from gage with missing data to the neighboring gagesDistance from gage with missing data to the neighboring gages Weight of distances where b is a proportionality factor (b = 1, 2 )Weight of distances where b is a proportionality factor (b = 1, 2 )

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Regression P X is the missing precipitation value for station X for certain time period P 1, P 2, …, P n are precipitation values at the neighboring stations for the same period b 0, …, b n are coefficients calculated by least-squares methods n is the number of nearby gages Method suitable when there is a large number of days when observations are available for all gages P X = b o + b 1 P 1 + b 2 P 2 + …. + b n P n

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Precipitation Data Generation Using PRISM

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

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Click GIS Products

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Go back and click PRISM

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Click Prism Group at OSU

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Click Internet Map Server

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Click Zoom In

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Click Show Chart

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Precipitation Data Generation Using CLIGEN

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Click Modeling Software/Erosion Modeling/ FS WEPP

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Click Rock:Clime

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Click SHOW ME THE CLIMATE

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

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Modify current values using PRISM

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Use current values/Download climate

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