PrecipitationPrecipitation

TypesConvectiveCyclonicOrographic

Important for Real Time Input and Forecasting

ConvectiveConvective

• Heating of air at the interface with the ground. The heated air expands with a result of reduction of weight and the air will rise. Dynamic cooling takes place with precipitation resulting.

• High intensity, short duration precipitation develops as the warm air rises and cools rapidly. A summer thunderstorm is the typical convective storm.

CyclonicCyclonic

Air masses from high pressure regions to low pressure regions-cold fronts, warm fronts, stable fronts.

Cyclonic storms result from convergence of air masses of different temperatures and characteristics. Two fronts develop: a cold front in which cold air moves under the warm air, and a warm front in which warm air moves over the cold air.

CyclonicCyclonic

Rainfall takes place along both fronts as the warm air rises at the interfaces.

OrographicOrographic

Mechanical lifting of moist air masses over natural barriers such as mountains.

Orographic storms develop as the wind forces moist air to rise near a mountain range. The slope facing the wind (windward side) receives more precipitation than the opposite slope (leeward side).

PrecipitationPrecipitation

Historic – Past EventReal Time

Radar – Measurement – PredictionMesoscale Model – Predication

Hypothetical

Hypothetical EventHypothetical Event

Probabilistic (e.g. 100-year Event)Design Standard (Standard Project,

Probable Maximum Precipitation.NRCS Type II

ProbabilisticProbabilisticRainfall CharacteristicsRainfall Characteristics

IntensityDuration FrequencyAmountTime DistributionSpatial Variability

Time in hours

Rai

nfal

l Int

ensi

ty in

in/h

our

Rainfall Hyetograph

Intensity-Duration-FrequencyIntensity-Duration-FrequencyIDF-CurvesIDF-Curves

Time in minutes

Inte

nsit

y in

inch

es/h

our

Rational FormulaRational Formula

Q = C i A

i is a function of the time of concentration Good for Watersheds < 200 Acres

DurationDuration

• The duration of the storm is directly related to the volume of surface runoff.

• High intensities are generally associated with short duration storms. Large water volumes are generally associated with long duration storms. “It can rain like

cats and dogs for only a short time.”

DurationDuration

6-hour - Thunder Storm12 hour24 hour – 100-year48 hour - PMP

FrequencyFrequency

The frequency of occurrence of a storm of given magnitude and duration is important to establish a measure of risk.

For a given storm duration, the probability that an event of certain magnitude has of being equaled or exceeded in any one year is termed the probability of exceedance.

FrequencyFrequency

Frequency can be represented by the return period, which is the average number of years between events of a given magnitude or greater. The return period is related to the probability of exceedance by

Where TR is the return period and P is the probability of exceedance.

ExceedanceProb1

Tr

FrequencyFrequency

Frequency characteristics of storms are generally summarized in Intensity-Duration-Frequency (IDF) Curves.

In general, for the same return period, short storms are more intense than long storms. Similarly, for a given intensity, longer storms are associated with greater return periods.

Intensity-Duration-FrequencyIntensity-Duration-FrequencyIDF-CurvesIDF-Curves

Time in minutes

Inte

nsit

y in

inch

es/h

our

FrequencyFrequency

In hydrologic design, frequencies are needed to select appropriate rainfall values that will result in design streamflows.

A storm of a given frequency does not generally produce a peak discharge of the same frequency. However, these frequencies are commonly assumed to be the same, especially if models are used to estimate runoff from precipitation.

FrequencyFrequency

IDF curves provide a measure of risk. By selecting a return period for design, there is always a chance that a more severe event will occur within the life of the project. The probability of exceedance allows estimation of the risk.

Regional empirical equations can be derived for IDF curves. These equations have the form

FrequencyFrequency

i = A T

(d + B )Rn

C

Time DistributionTime Distribution

• A hyetograph is also used to describe the variation of the storm with time.

• The time distribution of the storm affects the shape of the direct runoff hydrograph.

• Early, Center, Late peaking precipitation

Time in hours

Rai

nfal

l Int

ensi

ty in

in/h

our

Rainfall Hyetograph

Spatial DistributionSpatial Distribution

A localized storm would likely produce smaller peaks and a shorter hydrograph than if the same storm covered the whole watershed.

A storm moving away from the outlet will produce an earlier and smaller peak than if the storm moves towards the outlet.

Spatial DistributionSpatial Distribution

Storm location, aerial extent, and storm movement are usually determined by the origin of the storm.

For instance, cold fronts produce localized fast-moving storms. Warm fronts give origin to slow-moving widespread precipitation.

A storm taking place far from the outlet would produce longer hydrographs and lower peaks than if the same storm occurred near the outlet.

Spatial DistributionSpatial Distribution

In most circumstances, it is assumed that rainfall is uniform over the entire watershed for the duration of the time increment.

National Weather ServiceNational Weather Service

TP-40UniversitiesNew National Weather Service Rainfall

Atlas

Rainfall Amount, Duration, and Frequency

Develop IDF CurvesDevelop IDF Curves

Fifty Year Rainfall

Fifty Year RainfallFifty Year RainfallDuration Intensity hours in/hr

1 2.62 3.63 4.54 5.15 5.66 6