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rainfall series in the region for required storm durations. Factors that in fluency calculations of PMP values

are rainfall, dew point, temperature, wind speed, temperature and pressure.

2.1. Physically or maximization method

2.1.1. Depth-Area-Duration

An important step probable maximum estimates is the analysis of major storms. This analysis includes;

collection precipitation data from various sources. The objective of the storm analysis was to obtain

Deph-Area-Duration (DAD) information upon which to base PMP estimates as well as generalized

relations for other areas or other basin with similar climate and topographic characteristics. The first step

in order to development of DAD relations requires that rainfall amounts to all areas in the storms. Then

point precipitation amounts interpolated by assigning a particular precipitation gauge to a region.

Estimation of spatial distribution of rainfall is one of the basic steps in PMP studies. In this study in order

to interpolation were used kriging methods.

2.1.2.Maximum of storms humidity and dew point factor

Storm maximization, it is assumed that rainfall can be determined from the product of an available

moisture and storm mechanism. After selection of storms and estimation of mean rainfall depth for each

sub-basin, it was necessary to estimate maximum humidity source in order to maximize selected

storms. Storm moisture maximization factor was determined using the surface dew point temperature,

in conjunction with an assumed saturated atmosphere above surface level. Surface dew point was

used as a measure of moisture potential for server storms because it is the critical factor for server

storm development in small areas. Maximum dew point for any location is chosen as the highest value

persisting for 12-hour duration. In this study recorded surface dew points 7 station could be considered

as representative of in flow humidity source. There fore, to calculate the maximum in flow humidity to

storm, it is needed to investigate maximum persisting 12-hour dew point duration for statistical period of

7 station and also dew point values of those station at storm event. For investigation of maximum

persisting 12-hour dew point duration in statistical period, the 10-day recorded data for each year were

Figure (1) Southwest basins of Iran

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extracted and then maximum persisting 12-hour dew point values for each 10-day period were

calculated with 50-year return period by using frequency analysis. Having used the mean monthly

pressure data recorded at each station were transferred to 1000 millibar pressure level such that they

can be compared at different elevations. The calculated dew point temperature at the 7 station during

all storm events and maximum 12-hour persisting condition were reduced to equivalent mean sea level

(Msl, i.e.1000 millibar air pressure) dew point temperatures, using figure 2.

Figure(2):Peseoud-adiabatic chart for dew point reducation to 1000 hpa at high zero

In next step, variation of 10-day maximum 12-hour duration dew point of each station with 50-year

return period (at level 1000 millibar) are used the corresponding typical curves were plotted and then by

reading points from those curves, the humidity coefficients were estimated. Mentioned curves are

shown in figure (2). Usually recorded dew points in stormy days of a 12-hour period, which have the

biggest values, are selected to investigate 12-hour duration dew point at the selected storms. Then the

smallest of these values during storm event were used as maximum persisting 12-hour duration of dew

point.

These values were also transferred to 1000 millibar level by using stations air pressures data during

storm event to become comparable with each other. Then, the humidity coefficient calculated by using

equation1 FM=Wm/Ws (1)

in this equation Wm is maximum precipitable water at 1000 to 200 millibar levels which can be obtained

on the basis of maximum 12-hour duration dew point with 50-year return period in a simultaneous

period with storm.

Ws= maximum perceptible water at 1000 to 200 millibar levels which can be obtained on the basis of

maximum 12-hour duration dew point in a simultaneous period with storm.

The relevant values of dew point temperature and humidity coefficients are presented in table (1) and

(2).

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Table (2) the Moisture and Wind Maximization Factors for Some Selected Storms

Maxim12hr

return

Maximum persisting12hr wind (knot)

Moisturefactor

Precipitablewater(mm)

Maximumpersisting 12hr

dew point in1000 mb level

(100 year returnperiod)

()

Precipitablewater(mm)

Maximumpersisting 12hr

dew pointsin 1000 mblevel (in thestorm time)

()

SynopticStation

StormsDate

18.01.2549.619.439.616.9Abadan

181.5855.620.935.115.7Ahvaz

221.2648.419.132.414.8Mahshar

181.2648.419.138.416.6Omidyeh

181.3453.620.44017Dezful

1.34

30Mar1998

Averag

e

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Table (1) the Moisture and Wind Maximization Factors for Some Selected Storms

Maximpersisting

wind (k

Moisturefactor

Precipitablewater(mm)

Maximumpersisting 12hr dew point in 1000 mb

level (100 year return period)()

Precipitablewater(mm)

Maximum persisting12hr dew points in 1000mb level (in the storm

time) ()

SynopticStation

StormsDate

18.01.262.022.052.020.1Abadan

211.3857.521.141.617.4Ahvaz

151.4358.021.240.417.1Bushehr

181.1856.520.94819Mahshar

171.5764.423.440.817.2Omidyeh

201.216121.850.419.6BushehrCoastal

211.564.422.442.817.7Aghajari

23-26Nov1994

1.35Average

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2.1.3. Wind maximization

Wind maximization is most commonly used in orographic regions. (WMO.332) the wind maximization

ratio is simply the ratio is simply the maximum average wind speed for some specific duration and

critical direction obtained from a long record of observations, e.g. 50 y, to the observed maximum

average wind speed for the same duration in the storm being maximized.

Maximum values of wind speed are obtained from maximum persisting 12h. wihd. In order to estimate

high wind speed used 50-y return period.

2.2. Statistical Method

Statistical Method for estimating probable maximum precipitation is used wherever sufficient

precipitation data are available, and particularly useful for making quick estimate, or where othermeteorological data, such as dew point and wind records, are lacking. The statistical method is used

mostly for making quick estimate for basins of no more than about 1000 km2, but has been used for

much larger areas (WMO, 332). This method requires data for annual maximum precipitation series in

the region for required storm durations. The statistical method developed by Hershfield (1961) and

modified (1965) is based on the general frequency equation 2 (chow 1961)

Xt = Xn+ KSn (2)

Where Xt is the precipitation for return period t; Xn and Sn are respectively the mean and standard

deviation of a series of n annual maximum: and K is a common statistical variable, which varies with the

different frequency distributions fitting extreme value hydrologic data. This method requires some

adjustment, such as adjustment Of mean of annual series (Xn), standard deviation of annual series (Sn)

for maximum observed precipitation. Such as adjustment of mean (Xn) and standard deviation (Sn) of

annual series for maximum observed precipitation and also adjustment of mean and standard deviation

of annual series for length of record.

Table (3)estimated probable maximum precipitation value

based on statistical (Hershfield) and physically methods

Station Statistical (Hershfilds) methods

Physically

(maximization of

storms) methodAbadan 236 92.5

Ahvaz 371 114.7

Mahshahr 362 137.3

Brojerd 277 98

Bushehr 637 145.3

Dezful 418 151.7

Masjed

soleiman516 144

Omidieh 407 93.9

yasoj 356 134.9

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y = -5E-06x6

+ 0.0004x5

- 0.0118x4

+ 0.1789x3

- 1.3036x2

+ 3.2302x + 21.247

R2

= 0.8364

15

16

17

18

19

20

21

22

23

24

25

26

1-Oct.

10 20

1-Nov. 10 20

1-Dec.

10 20

1-Jan. 10 20

1-Feb. 10 20

1-Mar.

10 20

1-Apr.

10 20

1-May 10 20

TIME(DAY) a

DEWPOINT(C)

y = -8E-06x6

+ 0.0007x5

- 0.0199x4

+ 0.2907x3

- 1.9959x2

+ 4.2433x + 27.461

R2

= 0.9711

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

1-Oct.

10 20

1-Nov.

10 20

1-Dec.

10 20

1-Jan.

10 20

1-Feb.

10 20

1-Mar.

10 20

1-Apr.

10 20

1-May 10 20

TIME(DAY) b

DEWPOINT(C)

y = -4E-06x6

+ 0.0004x5

- 0.0113x4

+ 0.1667x3

- 1.0181x2

+ 0.4534x + 31.376

R2 = 0.9162

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

3031

32

1-Oct.

10 20

1-Nov. 10 20

1-Dec.

10 20

1-Jan.

10 20

1-Feb.

10 20

1-Mar.

10 20

1-Apr. 10 20

1-May 10 20

TIME(DAY) C

DEWPOINT(C)

y = -4E-06x6

+ 0.0003x5

- 0.01x4

+ 0.1469x3

- 0.9727x2

+ 1.23x + 27.867

R2

= 0.9859

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

1-Oct.

10 20

1-Nov. 10 20

1-Dec.

10 20

1-Jan.

10 20

1-Feb.

10 20

1-Mar.

10 20

1-Apr. 10 20

1-May 10 20

TIME(DAY) d

DEWPOINT(C)

Figure (3): Maximum dew point envelope curves (a) in the Dezful station(b) Abadan (c) Ahvaz and (d) Bushehr station

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ConclusionsIn this study two techniques used for estimating PMP which incloude statistical and physically methods.Statistical method based on the transposition and maximization of historical precipitation (annual maximumprecipitation series) and physically method based on the maximization of the physical factors (dew pointand wind) that the precipitation evolution. Using statistical method of estimating PMP rather than physicallymethod showed larger values. Also application of the physically method will allow for the provision of PMPestimates for catchments in the southwest Of Iran from 1-5000 km2 in size and duration of 24,48,72,96,120and 144 hours, whereas statistical method used mostly for basins about 1000km2. We found that PMPestimates by statistical method are well comparable with values of obtained by the physical method fordifferent durations. Results also shows that limited transposition of statistical methods gives higherestimates, in comparisons with physical method.

References

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Loukas, A and quick, M.C.A. (1996) Spatial and temporal distribution of storm precipitation in

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Kennedy, M.R. (1982) The estimation of probable maximum precipitation in Australia- past and

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