Aguas

Continuation…
Intensity, Duration and

Frequency of Rainfall
Albert Aguas BSABE 3B
Intensity, Duration and Frequency of Rainfall
Rainfall Intensity
• one of the most important rainfall characteristics
• defined as the ratio of the total amount of rain (rainfall
depth) falling during a given period to the duration of the
period.
• It is expressed in depth units per unit time, usually as mm
per hour (mm/h).
Storms
• Very intense storms are not necessarily more frequent in
areas having a high total annual rainfall.
• Storms of high intensity generally last for fairly short
periods and cover small areas.
• Storms covering large areas are seldom of high intensity
but may last for several days.
• The infrequent combination of relatively high intensity and long duration gives large total amounts of rainfall.
• These storms do much erosion damage and may cause devastating floods.
• These unusually heavy storms are generally associated with warm front precipitation.
• They are most apt to occur when the rate of frontal movement has decreased, when other fronts may passed by at close
intervals, when stationary fronts persists in an area for a considerable period, or when tropical cyclones move into an
area.
The first step in designing a water control-facility is:

• to determine the probable recurrence of storms of different and duration so that an
economically sized structure can be provided.
For most purposes it is not feasible to provide a structure that will withstand the greatest rainfall that has ever occurred. It
is often more economical to have a periodic failure than to design for a very intense storm.
When human life is endangered, however, the design should handle runoff from storms even greater
than have been recorded.
For these purposes, data providing return periods of storms of various intensities and durations are essential.
Rainfall intensity is classified according to the
rate of precipitation:
Light rain
● when the
precipitation rate is
< 2.5 millimetres
(0.098 in) per hour
https://images.app.goo.gl/hwgtGSKj44mbTGNx9
Moderate rain
● when the precipitation
rate is between 2.5
millimetres (0.098 in) -
7.6 millimetres (0.30
in) or 10 millimetres
(0.39 in) per hour
Heavy rain
rate is > 7.6 millimetres
(0.30 in) per hour
Violent rain
rate is > 50 millimetres
(2.0 in) per hour
Return Period
The return period sometimes called recurrence

interval, is defined as the period within which the
depth of rainfall for a given duration will be equaled or
exceeded once on the average.
The likelihood or probability of an event with a
specified intensity and duration is called the return period
or frequency. The intensity of a storm can be predicted for
any return period and storm duration, from charts based
on historic data for the location.
INTERPRETATION OF PRECIPITATION DATA
Point Measurement
Estimation of Areal Precipitation A single point
precipitation measurement is quite often not representative
of the volume of precipitation falling over a given catchment
area. A dense network of point measurements and/or radar
estimates can provide a better representation of the true
volume over a given area. A network of precipitation
measurements can be converted to areal estimates using
any of a number of techniques which include the following.
AREAL PRECIPITATION
Predicting watershed response to a given precipitation
event often requires knowledge of the average rainfall that
occurs over a watershed area in a specified duration. The
average depth of precipitation over a specific watershed area
is more accurately estimated for an area that is well
monitored. Three basic methods exist to drive area averaged
values from point rainfall data: the arithmetic mean, the
Thiessen polygon method, and the Iso-hyetal method,
distance weighting/gridded , MAPX, and index station.
Average Precipitation Over Area
The average depth of precipitation over a specific area (Arithmetic
Mean), on a storm, seasonal or annual basis, is required in many types of
hydrologic problems. The simplest method of obtaining the average depth
is to average arithmetically the gauged amounts in the area. This method
yield good estimates in flat country if the gauges are uniformly distributed
and the individual gauge catches do not vary greatly from the mean. The
method is not particularly accurate for larger areas where rainfall
distribution is variable. These limitations can be partially overcome if
topographic influence and aerial representatively are considered in the
selection of the gages sites.
The Thiessen method attempts to allow for non-uniform
distribution of gages by providing a weighting factor for each
gage. The stations are plotted on the map, and connecting lines
are drawn. Perpendicular bisectors of these connecting lines
form polygon around each station. The sides of each polygon
are the boundaries of the effective area assumed for the
station. The area of the polygon is determined by planimetry
and is expressed as a percentage of the total area. Weighted
average rainfall of an area is computed by multiplying the
precipitation at each station by its assigned percentage of area
and totaling. The result is more accurate than those obtained by
simple arithmetic averaging.
Any point in the watershed receives the same amount of rainfall as
that of the nearest gage. Rainfall recorded at a gage can be applied to
any point at a distance halfway to the next station in any direction.
Steps:
1. Draw lines joining adjacent gages.
2. Draw perpendicular bisectors to the lines created in step 1.
3. Extend the lines created in step 2 in both directions to form
representative areas for gages.
4. Compute representative area for each gage.
5. Compute the aerial average using the following formula:
One important aspect of hydrologic modeling is the
estimation of the total precipitation and its
distribution within a watershed. This problem is
commonly referred to as “areal estimation of
precipitation” and is best described as follows:
Given the coordinates of m precipitation stations
along with their respective recorded
precipitation values ( Pi ; i =1,…, m ), how can
we determine the area-averaged ( Pavg )
precipitation from these limited stations?
This problem is illustrated below, where
Pavg across the entire basin has to be estimated
from measured P1 , P2 , P3 , and 4 .
We will discuss four methods:
1) Arithmetic Average.
2) Theissen Polygons.
3) Isohyetal Method.
4) Grid Method.
The Grid Method, at its core, is an
instructional framework where the teacher
develops a tiered and aligned learning
pathway called a “Mastery Grid.” Learners
work through the Mastery Grid at their own
pace. The pathway is organized utilizing
Depth of Knowledge (DOK) and has
embedded assessments throughout.
Other Methods
Distance Weighting/Gridded - This is another station
weighting technique. A grid of point estimates is made based
on a distance weighting scheme. Each observed point value
is given a unique weight for each grid point based on the
distance from the grid point in question. The grid point
precipitation value is calculated based on the sum of the
individual station weight multiplied by observed station
value. Once the grid points have all been estimated they are
summed and the sum is divided by the number of grid points
to obtain the areal average precipitation.
MAPX - This is a NWS-specific gridded
technique. Areal runoff zone precipitation
estimates are made using the 4 x 4 km WSR-
88D 1-hourly gridded precipitation estimates.
The arithmetic mean calculation technique is
used to average the grid point estimates.
Index Stations - In some areas of the
country (primarily mountainous areas), pre-
determined station weights based on
climatology are used to compute basin
average precipitation.
Areal Precipitation Terminology As Used In
ABRFC Hydrologic Modeling
MAP - Mean Areal Precipitation -Areal runoff

zone precipitation estimate normally based on point
precipitation observations. The distance weighting
calculation technique is used. MAP is used as input
to the river forecast model on a routine basis
MAPX - Radar Based Mean Areal Precipitation
- Areal runoff zone precipitation estimate based
on the 4 x 4 km WSR-88D 1-hourly gridded
precipitation estimates. The arithmetic mean
calculation technique is used to average the grid
point estimates. MAPX is used as input to the
river forecast model on a routine basis
FMAP- The Weather Forecast Offices (WFOs) develop
precipitation forecasts based on analysis is complete, the
WFO forecaster uses a computer program to draw isohyets
of forecast precipitation and then the program performs an
automated isohyetal analysis calculation technique to convert
to areal estimates. The forecast precipitation information
mosaicks the input from the WFOs so as to cover the entire
ABRFC area of responsibility. The HAS function also
coordinates any required changes in the individual WFO QPF
FMAP is used as input to the river forecast model on a
routine basis.
Sample rainfall event Precipitation Terms
1. Duration (TIME) How long it rains. Measured in units of time. From
intensity and duration, you can determine total rainfall depth.
2. Hyetograph - A plot of rainfall intensity vs. time 3. Intensity (RATE).
How hard it rains. Measured in units of depth per time, usually in/hr. or
cm/hr.
4. Isohyetal Map- Map with contours of constant rainfall
5. Isohyets = Contours of constant rainfall similar to contours of
constant elevation 6. Return Period (Frequency). How often, on
average, a storm of a certain intensity and duration occurs at a
particular location. Measured in units of time, generally years
Precipitation Facts
1. For any given return period, intensity (RATE)
decreases as duration increases. Short storms generally
have higher intensities.
2. For any given duration, intensity increases as the
return period increases. Severe events are less frequent
3. Higher intensity storms generally have larger
raindrops. Impacts the energy released when the drop
hits the ground. This is one of the forces which causes
erosion.
Thank you
👉👈

Aguas

Uploaded by

Copyright:

Available Formats

Aguas

Uploaded by

Document Information

Original Description:

Copyright

Available Formats

Share this document

Share or Embed Document

Sharing Options

Did you find this document useful?

Is this content inappropriate?

Copyright:

Available Formats

Aguas

Uploaded by

Copyright:

Available Formats

Continuation…

Intensity, Duration and

The first step in designing a water control-facility is:

The return period sometimes called recurrence

MAP - Mean Areal Precipitation -Areal runoff

You might also like