Precipitation

1. Precipitation • Types • 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 • 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. ProbabilisticRainfall Characteristics • Intensity • Duration • Frequency • Amount • Time Distribution • Spatial Variability

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

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

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 in inches/hour Time in minutes Intensity-Duration-FrequencyIDF-Curves

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

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

27. Develop IDF Curves Fifty Year Rainfall

28. Fifty Year Rainfall