Precipitation Types Important for Real Time Input and Forecasting 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 Hypothetical 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

Probabilistic Rainfall Characteristics Intensity Duration Frequency Amount Time Distribution Spatial Variability

Time in hours Rainfall Intensity in in/hour Rainfall Hyetograph

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

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.

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

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

Frequency

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

Time in hours Rainfall Intensity in in/hour Rainfall Hyetograph

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

Develop IDF Curves Fifty Year Rainfall

Fifty Year Rainfall