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Precipitation. Types Convective Cyclonic Orographic Important for Real Time Input and Forecasting. Convective.

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Presentation Transcript
  • Types
      • Convective
      • Cyclonic
      • Orographic
  • Important for Real Time Input and Forecasting
  • 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.
  • 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.
  • Rainfall takes place along both fronts as the warm air rises at the interfaces.
  • 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).
  • Historic – Past Event
  • Real Time
      • Radar – Measurement – Prediction
      • Mesoscale Model – Predication
  • Hypothetical
hypothetical event
Hypothetical Event
  • Probabilistic (e.g. 100-year Event)
  • Design Standard (Standard Project, Probable Maximum Precipitation.
  • NRCS Type II
probabilistic rainfall characteristics
ProbabilisticRainfall Characteristics
  • Intensity
  • Duration
  • Frequency
  • Amount
  • Time Distribution
  • Spatial Variability

Rainfall Hyetograph

Rainfall Intensity in in/hour

Time in hours

rational formula
Rational Formula

Q = C i A

i is a function of the time of concentration

Good for Watersheds < 200 Acres

  • 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.”
  • 6-hour - Thunder Storm
  • 12 hour
  • 24 hour – 100-year
  • 48 hour - PMP
  • 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.
  • 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.
  • 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.
  • 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.
  • 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
time distribution
Time 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

Rainfall Hyetograph

Rainfall Intensity in in/hour

Time in hours

spatial distribution
Spatial 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 distribution24
Spatial 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 distribution25
Spatial Distribution
  • In most circumstances, it is assumed that rainfall is uniform over the entire watershed for the duration of the time increment.
national weather service
National Weather Service
  • TP-40
  • Universities
  • New National Weather Service Rainfall Atlas

Rainfall Amount, Duration, and Frequency

develop idf curves

Develop IDF Curves

Fifty Year Rainfall