hydrology presentation on rainfall

1. Analyses of Rainfall
Hydrology and Water Resources
RG744
Institute of Space Technology
October 09, 2015

2. Mean Rainfall
Mean Annual Rainfall: determined by averaging the total rainfall
of several consecutive years at a place
Mean Monthly Rainfall: by averaging the monthly total rainfall
for several consecutive years

3. Example:MeanMonthlyRainfall

4. InterceptionandNetPrecipitation
Vegetation influence on pattern of deposition and amount of
precipitation reaching the soil surface
Interception losses are less in arid and semiarid regions that have
sparse vegetation
Net precipitation is gross precipitation (measured by gauge) minus
interception loss
Net precipitation is the amount available either to replenish soil
water deficits or to become surface, subsurface, or ground water
flow.

5. EffectiveRainfall
Rainfall component causing perceptible change in the stream
flow
That is only a portion of the total rainfall recorded over the
catchment
Rainfall that does not infiltrate the soil and moves into the
stream as overland flow

6. Index of wetness
Ratio of rainfall in a given year and average annual precipitation
When it is
Less than 1 - bad year/deficient year/ dry year
More than 1 – good year / surplus year / wet year
Equal to 1 – normal year

7. ProbableExtremeRainfallEvents(Standard
DesignStorms)
Probable Maximum Precipitation
Standard Project Storm
Frequency based storm
Important for water resources engineering

8. ProbableMaximumPrecipitation(PMP)
Assumed physical upper limit of rain that will fall over a specified
area in a given time (that’s physically possible)
Rainfall for a given area and duration that can be reached or
exceeded under known meteorological conditions
Used to provide an estimate of the Probable Maximum Flood
(PMF) hydrographs
For design of major structure with the threat of loss of life PMP is
used (where no risk of failure accepted)

9. StandardProjectStorm(SPS)
Storm which is reasonably capable of occurring over the basin
under consideration (actually occurred)
The heaviest rain storm occurred in the region during the
period of rainfall records
Used for design project with economic considerations and low
risk

10. Frequency Based Storm
Frequency analysis of long term rainfall data

11. Flood/StormFrequency
Flood frequency: number of times a particular flood/storm can
be expected in a river/watershed in a given number of years
Example: 100 year flood – a flood that can be expected once in
100 years. (usually worked out by extrapolating the observed
data)
Frequency is the percentage of years, during which a storm of a
given magnitude may be equaled or exceeded
Recurrence interval/return period: Time interval after which a
similar flood can be forecasted or expected.

12. FrequencyAnalysis
Objective: to develop a frequency curve
Precipitation frequency curves can be developed to evaluate
maximum events
Used for planning water resources structures
Relationship between the magnitude of events and either the
associated probability or the recurrence interval
Weather systems vary year to year and hence magnitude of future
events can not be predicted accurately
Have to rely on statistical analyses of rainfall amounts over certain
period
Frequency distribution of past events
Probability or likelihood of having certain events occurring over a
specified period is estimated

13. RecurrenceIntervalofaStorm
Number of years within which a given storm may equal or
exceed once - Also known as return period
Means this precipitation value or more than that occurs m times in n years

14. Probability of Exceedance
The probability of occurrence of a flood (having a
recurrence interval T-year) in any year,
Probability: Reciprocal of the return period
p = 1/ T

15. Frequency
Frequency: Probability expressed in terms of
percentage
Frequency = p * 100
Frequency of a rainfall of a given magnitude = the
number of times the given event may be expected to
be equaled or exceeded in 100 years

16. Example:

18. The probability of having a 24 hr rainfall event of 100mm or
more in any given year is ?
about 0.024 (or a 40 year recurrence interval)

19. Once the frequency curve is developed, the probability of
exceeding certain rainfall amount over a specified period can be
determined
The probability that an event with probability p will be equaled
or exceeded x times in N years is determined by:
If x=0 (no occurrence in N years) then
Prob (no occurrence in N years) = (1-p)N
Therefore;
Prob (at least 1 occurrence in N years) = 1-(1-p)N
Q: the probability of having a 24 hr rainfall event of 100mm or greater over a 20 year
period is?

20. Solution to example
• What is the p for 100 years flood?
• Means probability of occurrence in one year = ?
• Prob of occurrence of 100 years flood in 100 years?

22. Intensity Duration Analysis
Study of intensity and its duration is called Intensity Duration
Analysis
Usually most intense storms last for shorter duration
As intensity reduces duration increases

23. Intensity Duration Curve
Graph of duration vs. intensity for an area
Normally follows the following equation
Where;
I = intensity in mm/hr
t = duration in minutes
C, a, b = constants for the specific area

24. Intensity Duration Curve

25. Example: A storm occurred over a catchment area as under:
Time (min) Precipitation (mm)
0 0
10 19
20 22
30 7
40 20
50 23
60 33
70 28
80 8
90 6
Plot maximum intensity duration curve?

26. *60/30

27. Intensity Frequency Duration Analysis
When sufficient data (say 50 years) for a catchment area is
available
Analyze data for each storm
Analyze for:
Intensity
Frequency
Duration
Different graphs for different catchment areas depending
on their hydrologic character

28. Intensity-Frequency-DurationCurve
To know rainfall intensities of different duration and
different return period

29. Isopluvial Maps
Combined map for large area for maximum rainfall depth for
various combination of a return period and duration
These are Isohyets shown on regional rainfall map

31. Depth-Area-Duration (DAD)Analysis
Average depth of storm and its duration for a specific area
Also called DAD study
Horton’s Equation
Pa =Poe-(KA)^n

32. DAD Curve
Based on records of several
storms on an area
Maximum areal precipitation for
different durations
corresponding to different areal
extents