Prepared by:-

MR WAN AFNIZAN BIN WAN MOHAMED

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I

INTRODUCTION

CLASSIFICATION OF PRECIPITATION

FORMATION OF PRECIPITATON

PRECIPITATION TYPES

NON RECORDING GAUGES

RECORDING GAUGES (PLUVIOGRAPH)

Weighing gauge

Tipping bucket

Float type

MEASUREMENT OF PRECIPITATION

LOCATION OF INSTALLATION RAIN GAUGE

MISSING DATA

POINT PRECIPITATION

GAUGE CONSISTENCY

MEAN AREAL PRECIPITATION

What is

Point Precipitation

MISSING DATA

Arithmatic Mean Method

Normal Ratio Method

Quadrant Method

MISSING DATA

Precipitation measured at rain gauge.

Three method used to estimate missing point precipitation :-

GAUGE CONSISTENCY

When the catch at rain gauges is inconsistent.

Adjustment is necessary fifififi provide consistent record.

What method is employed ??? fifififi Use DOUBLE MASS CURVE.

DMC fifififi technique to detect changes in data-collection procedures / condition at location.

Changes because :-

Instrumentation

Observation procedures

Surrounding condition

How DOUBLE MASS

CURVE is constructed

GAUGE CONSISTENCY

Plot Accumulation of total precipitation over time at TEST STATION VS Accumulation of total precipitation over time at BASE SATIONS

GAUGE CONSISTENCY

DMC fifififi approximately a straight line if both test & base stations consistent.

However if not fifififi will be break point (suggest possible change at test station).

So, if that the case fifififi the early or later record needs to be adjusted.

Procedures :-

1. Add the annual precipitation of base stations

2. Cummulate the sums of Step 1 fifififi (Value at x-axis).

3. Cummulate the annual precipitation for test station (changed station) fifififi (Value at y-axis)

4. Plot GRAPH value from STEP 3 VS. STEP 2.

5. Compute slope of ADJUSTED (Ma) & ORIGINAL (Mo)

GAUGE CONSISTENCY

Formula to calculate Mo & Ma :-

6. Calculate the adjusted precipitation at changed station (either early or later using the following formula ;-

GAUGE CONSISTENCY

23

23o XX

YYM

----

----====

12

12a XX

YYM

--------

====

====

o

aoa M

MPP

Note:-Mo = Original SlopeMa = Adjusted Slope

Note:-Po = Original precipitation

(Station X)Pa = Adjusted precipitation

(Station X)

!"###$ %&()&*(!+,- ./ #, ,00%1%%00-23 40 0 % &()&56 0## -

$%&$%&$%&$%&

Step !!

1. Based on the data given, ADD the annual precipitation of base stations.

2. Cummulate the sums of Step 1 fifififi (Value at x-axis).

3. Cummulate the annual precipitation for test station (changed station) fifififi (Value at y-axis)

Value at X- axisValue at Y- axis

4. Plot GRAPH value from STEP 3 VS. STEP 2

4. From Graph compute slope of ADJUSTED (Ma)& ORIGINAL (Mo)

23

23o XX

YYM

----

----==== 19.0

7742785148679

Mo ====--------

====

12

12a XX

YYM

--------

==== 26.015477433148

Ma ====--------

====

4. Calculate the adjusted precipitation at changed station (5 years early)

====

o

aoa M

MPP

mm 02.45 19.026.0

9.32P1926 ====

====

mm 5.38 19.026.0

1.28P1927 ====

====

mm 90.45 19.026.0

5.33P1928 ====

====

mm 55.40 19.026.0

6.29P1929 ====

====

MEAN AREAL PRECIPITATION

Arithmatic Mean Method

Thiessen Polygon Method

Isohyetal Method

Why??? fifififi for engineering application (represent precipitation over a defined area).

THREE method used to estimate MEAN AREAL precipitation :-

ARITHMATIC MEAN METHOD

Only stations within the topographic basin are considered.

Formula used :-

MEAN AREAL PRECIPITATION

nP

P i

====

Which :-

P = Average precipitation depth (mm)Pi = Precipitation depth at each station within the

basin (mm)n = Total station within the basin

THIESSEN POLYGON METHOD

Advantage fifififi quick if multiple storm (uses fixed sub-areas)

PROCEDURES :-

MEAN AREAL PRECIPITATION

1. CONNECT each precipitation STATION with STRAIGHT lines

MEAN AREAL PRECIPITATION

2. CONSTRUCT PERPENDICULAR bisectors of the connecting lines and forming POLYGONSwith these bisectors.

THIESSEN POLYGON METHOD

MEAN AREAL PRECIPITATION

3. Determine the AREA of the POLYGON

4. Calculate AVERAGE PRECIPITATION using this formula :

THIESSEN POLYGON METHOD

====APA

P ii

Which :-

P = Average precipitation depth (mm)Pi = Precipitation depth at each station (mm)A i = Sub area at each station (Polygon area) A = Total area

!+,#%0!0%0 -

$%$%$%$%

Step !!

====APA

P ii

1. Based the data given, compute AVERAGE PRECIPITATION using Theissen formula:-

2. For easy calculation construct table :-

3. Form the calculated table :-

====APA

P ii

mm 8.121 568

69210P ========

MEAN AREAL PRECIPITATION

ISOHYETAL METHOD

Based on interpolation between gauges. Resembles the calculation of contours in

surveying and mapping. PROCEDURES:-

1. PLOT the rain gauge LOCATIONS on map & record RAINFALL AMOUNTS .

2. PERFORM INTERPOLATION between gauges fifififiSelect suitable increments.

MEAN AREAL PRECIPITATION

ISOHYETAL METHOD

3. CONNECT identical depth from EACH interpolation fifififi ISOHYETS LINES .

MEAN AREAL PRECIPITATION

ISOHYETAL METHOD

4. COMPUTE the MEAN AREAL PRECIPITATIONusing the following formula :-.

====APA

P ii

Which :-

P = Average precipitation depth (mm)Pi = Average precipitation between 2 isohyets

lines (mm)A i = Sub area between 2 isohyets lines A = Total area

00/ %0%0!0,00+ 0% -

$%($%($%($%(

Step !!

====APA

P ii

1. Based the data given, compute AVERAGE PRECIPITATION using the following formula :-

2. For easy calculation construct table :-

3. Form the calculated table :-

====APA

P ii

cm 3.39 428

16830P ========

THANK YOU