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## Precipitation

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### Develop IDF Curves

Precipitation

- Types
- Convective
- Cyclonic
- Orographic
- Important for Real Time Input and Forecasting

Convective

- 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.

Cyclonic

- 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.

Cyclonic

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

Orographic

- 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).

Precipitation

- Historic – Past Event
- Real Time
- Radar – Measurement – Prediction
- Mesoscale Model – Predication
- Hypothetical

Hypothetical Event

- Probabilistic (e.g. 100-year Event)
- Design Standard (Standard Project, Probable Maximum Precipitation.
- NRCS Type II

ProbabilisticRainfall Characteristics

- Intensity
- Duration
- Frequency
- Amount
- Time Distribution
- Spatial Variability

Rational Formula

Q = C i A

i is a function of the time of concentration

Good for Watersheds < 200 Acres

Duration

- 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.”

Duration

- 6-hour - Thunder Storm
- 12 hour
- 24 hour – 100-year
- 48 hour - PMP

Frequency

- 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

- 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

- 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.

Frequency

- 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.

Frequency

- 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

- 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

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 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 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

- TP-40
- Universities
- New National Weather Service Rainfall Atlas

Rainfall Amount, Duration, and Frequency

Fifty Year Rainfall

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