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
– Past Event
Real Time
Historic
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
Rainfall Intensity in in/hour
Rainfall Hyetograph
Time in hours
Intensity in inches/hour
Intensity-Duration-Frequency
IDF-Curves
Time in minutes
Rational Formula
Q=CiA
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
1
Tr 
ProbExceedance 

Where TR is the return period and P is the
probability of exceedance.
Frequency

Frequency characteristics of storms are
generally summarized in IntensityDuration-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 in inches/hour
Intensity-Duration-Frequency
IDF-Curves
Time in minutes
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
n
R
AT
i=
C
(d + B)
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 Intensity in in/hour
Rainfall Hyetograph
Time in hours
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 fastmoving storms. Warm fronts give origin to slowmoving 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
Duration
hours
1
2
3
4
5
6
Intensity
in/hr
2.6
3.6
4.5
5.1
5.6
6