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

2. 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.

3. 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.

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

5. 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).

6. Precipitation Historic – Past Event Real Time Hypothetical
Radar – Measurement – Prediction
Mesoscale Model – Predication
Hypothetical

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

8. Probabilistic Rainfall Characteristics
Intensity
Duration
Frequency
Amount
Time Distribution
Spatial Variability

9. Time in hours
Rainfall Intensity in in/hour
Rainfall Hyetograph

10. Intensity-Duration-Frequency IDF-Curves
Time in minutes
Intensity in inches/hour

11. Rational Formula Q = C i A
i is a function of the time of concentration
Good for Watersheds < 200 Acres

12. 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.”

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

14. 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.

15. 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.

16. 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.

17. Intensity-Duration-Frequency IDF-Curves
Time in minutes
Intensity in inches/hour

18. 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.

19. 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

20. Frequency

21. 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

22. Time in hours
Rainfall Intensity in in/hour
Rainfall Hyetograph

23. 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.

24. 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.

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

26. National Weather Service
Rainfall Amount, Duration, and Frequency
TP-40
Universities
New National Weather Service Rainfall Atlas

27. Develop IDF Curves
Fifty Year Rainfall

28. Fifty Year Rainfall