HYDROLOGY
Module 2: Precipitation
HYDROLOGIC CYCLE
HYDROLOGY IN ENGINEERING
• Planning and design of engineering projects for the
control and use of water
• Design and operation of hydraulic structures for the
protection against the ill effects of extreme conditions
• Data collection (measuring the quantities of water in
transit in the cycle)
• Frequency analysis (deal with the interrelations between
factors) of occurrence of extremes in the cycle
CHARACTERISTICS OF RAINFALL
MEASUREMENT OF PRECIPITATION
Rain Gauges: measures rainfall depths collected through
time
https://bagong.pagasa.dost.gov.ph/automated-rain-gau
ge
https://philsensors.asti.dost.gov.ph/
MEAN/ AVERAGE AREAL PRECIPITATION
estimate of the average depth of precipitation over a
region for a specific time interval
1. Arithmetic Mean
2. Thiessen Polygon
3. Isohyetal Analysis
4. Distance Weighting/Gridded
ARITHMETIC MEAN
THIESSEN POLYGON
ISOHYETAL ANALYSIS
DISTANCE WEIGHTING/GRIDDED
EXAMPLE
Determine the average areal annual precipitation
in the shaded area in the figure if the following
stations have the annual precipitation:
A=2800mm
B=2500mm
C=1800mm
D=2300mm
E=2700mm
a. Arithmetic Mean
b. Thiessen polygon
c. Estimated Rainfall at Point X
Note: each square cell has an area of 100sq.km.
EXAMPLE
EXAMPLE
EXAMPLE
EXAMPLE
STATISTICAL HYDROLOGY
Return Period is the average number of years during
which a storm of given magnitude (maximum depth or
intensity) may be expected to occur once, i.e., may be
equaled or exceeded.
STATISTICAL HYDROLOGY
1. If the probability of exceedance is 0.5%, the return
period is?
2. If the Return Period is 50 years, the probability of
non-exceedance is?
3. The probability that a 50 year return period daily rainfall
to occur within 40 years is?
PROBABILITY DISTRIBUTIONS
FREQUENCY ANALYSIS
NORMAL DISTRIBUTION
1. Get the mean and standard deviation of annual maximum
rainfall
2. Choose return period (T) and get its probability of
non-exceedance (Pnon-excd)
3. Estimate frequency factor using =NORMSINV(Pnon-excd
) in excel or consult a table of normal distribution
4. Evaluate XT(rainfall magnitude with return period T)
EXAMPLE
The data set of annual maximum rainfall in mm is shown in
the table below, calculate the probable rainfall with 70-year
return period.
EXAMPLE
EXAMPLE
The data set of annual maximum daily rainfall in mm is
shown in the table below, estimate the probable rainfall
with 100-year return period.
EXAMPLE
