secondary validation of water level data (1)

HYDROLOGY PROJECTTechnical Assistance

SECONDARY VALIDATION OF WATER LEVEL DATA (1)

• PRIMARY VALIDATION:– BASED ON KNOWLEDGE OF INSTRUMENTATION

AND METHODS OF MEASUREMENT WITH ASSOCIATED ERRORS

• SECONDARY VALIDATION:– EMPHASIS ON COMPARISONS WITH

NEIGHBOURING STATIONS AND/OR RELATED PHENOMENA TO:

* IDENTIFY SUSPECT VALUES

* IDENTIFY AND QUANTIFY SHIFTS (TIME-SHIFTS AND SHIFT IN REFERENCE LEVEL)

– REQUIREMENT IS SIGNIFICANT SPATIAL CORRELATION

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SECONDARY VALIDATION OF WATER LEVEL DATA (2)

• ACTIVITY INCLUDES:– COMPARISON BETWEEN WATER LEVEL SERIES AT

SUCCESSIVE POINTS ON THE SAME RIVER* TIME SERIES

* RELATION CURVES

– COMPARISON WITH OTHER SOURCES IN CASE OF DOUBT ON STATION AT FAULT:

* E.G. COMPARISON BETWEEN INCIDENT RAINFALL AND LEVEL HYDROGRAPHS

– FOR ROUTINE MONTHLY VALIDATION:* PLOT SHOULD INCLUDE TIME SERIES OF AT LEAST THE

PREVIOUS MONTH

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SCRUTINY OF MULTIPLE HYDROGRAPH PLOTS (1)

• FIRST STEP:– A HARD COPY IS MADE OF THE WATER LEVEL

DATA FOR REFERENCE PURPOSES

• SECOND STEP: – GRAPHICAL INSPECTION OF COMPARATIVE PLOTS

OF TIME SERIES. THIS PROVIDES A RAPID AND EFFECTIVE MEANS TO DETECT ANOMALIES:

* INDIVIDUAL OUTLIERS

* FABRICATION

* SHIFTS (TIME AND REFERENCE LEVEL)

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SCRUTINY OF MULTIPLE HYDROGRAPH PLOTS (2)

• ERROR TYPES:– PEAKS ARE OBSERVED AT ONE STATION BUT NOT

AT ITS NEIGHBOUR

– LAG TIME BETWEEN STATIONS IS WIDELY DIFFERENT FROM THE NORM

– SUDDEN SHIFTS AT A STATION OFTEN VISIBLE HYDROGRPAH TAILS OF DIFFERENT PERIODS

• RESEMBLANCE OF HYDROGRAPHS AFFECTED BY:– INFLOW/OUTFLOW BETWEEN STATIONS

– FLOW/STAGE REGULATION

– LAG BETWEEN STATIONS VARYING WITH RIVER FLOW

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EXAMPLE OF SCRUTINY OF TIME SERIES PLOTS

• TWO STATIONS ON WATRAK RIVER (TRIBUTARY OF SABARMATI) ARE CONSIDERED:– MAHEMDABAD (SSW-GUJARAT), AND

– NSB00I7 (CWC)

• STATIONS ARE DISTANCED SOME 33 KM APART

• LATERAL INFLOW BETWEEN THE STATIONS IS SMALL COMPARED TO RIVER FLOW

• U/S AS WELL AS D/S ADDITIONAL HYDROMETRIC STATIONS ARE BEING OPERATED

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LISTING OF WATER LEVEL DATA (REFERENCE)

Hourly data of series code MAHEMDABAD ZA Year 1998, month 9

Day 0 1 2 3 4 5 6 7 8 9 10 11

1 .09 .09 .09 .09 .09 .09 .09 .09 .09 .09 .09 .09 2 .02 .02 .02 .02 .02 .02 .02 .02 .02 .02 .02 .02 3 .02 .02 .02 .02 .02 .02 .02 .02 .02 .02 .02 .02 4 -.01 -.01 -.01 -.03 -.03 -.03 -.03 -.04 -.04 -.04 -.05 -.05 5 -.06 -.06 -.06 -.06 -.06 -.06 -.05 -.05 -.05 -.05 -.05 -.05

6 -.07 -.07 -.07 -.07 -.07 -.07 -.07 -.07 -.08 -.08 -.08 -.08 7 -.08 -.08 -.08 -.08 -.08 -.08 -.08 -.09 -.09 -.09 -.09 -.09 8 -.10 -.10 -.10 -.10 -.10 -.11 -.11 -.11 -.11 -.11 -.12 -.12 9 -.12 -.12 -.12 -.12 .00 .33 .35 .38 .39 .40 .40 .40 10 .33 .75 .80 .80 .77 .79 .83 .84 .85 1.10 1.10 1.11

11 .33 .95 .90 .86 .83 .80 .85 .70 .65 .61 .58 .56 12 .33 .31 .30 .30 .29 .28 .27 .27 .26 .25 .25 .24 13 .15 .14 .14 .14 .12 .12 .10 .10 .10 .09 .09 .09 14 .03 .03 .03 .03 .03 .03 .03 .03 .03 .03 .02 .02 15 .00 .00 .00 .10 .23 .28 .30 .35 .37 .30 .28 .25

16 .50 .48 .47 .46 .45 .40 .38 .36 .45 .60 .80 .85 17 .74 .72 .70 .68 .65 .60 .60 .55 .55 .55 .55 .55 18 5.30 5.15 5.00 4.90 4.60 4.35 4.00 3.80 3.75 3.35 3.25 3.20 19 2.05 2.00 1.95 1.90 1.85 1.80 1.80 1.75 1.70 1.68 1.66 1.65 20 1.68 1.65 1.62 1.60 1.57 1.55 1.55 1.50 1.45 1.42 1.40 1.40

21 1.80 1.70 1.67 1.60 1.57 1.50 1.47 1.47 1.45 1.42 1.39 1.37 22 1.19 1.19 1.18 1.17 1.17 1.15 1.15 1.10 1.10 1.09 1.08 1.07 23 1.56 1.55 1.54 1.53 1.47 1.40 1.34 1.31 1.28 1.24 1.20 1.21 24 1.01 .98 .95 .94 .89 .88 .88 .87 .87 .87 .87 .86 25 3.00 2.80 2.75 2.70 2.50 2.40 2.30 2.25 2.20 2.17 2.13 2.10

26 1.61 1.58 1.55 1.52 1.50 1.47 1.44 1.43 1.39 1.37 1.35 1.32 27 1.15 1.15 1.14 1.14 1.13 1.13 1.12 1.12 1.09 1.08 1.07 1.06 28 1.25 1.43 1.48 1.51 1.53 1.51 1.49 1.47 1.43 1.39 1.35 1.25 29 .95 .94 .92 .90 .89 .88 .87 .86 .85 .85 .84 .84 30 .87 .86 .83 .81 .81 .81 .80 .80 .80 .74 .71 1.25

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Water level time series (2) (2 months)

ZA_MAHEMDABAD ZS_NSB00I7

Time31/10/9824/10/9817/10/9810/10/9803/10/9826/09/9819/09/9812/09/9805/09/98

Wat

er le

vel (

m+G

Z)6

5.5

5

4.5

4

3.5

3

2.5

2

1.5

1

0.5

0

-0.5

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Water level time series (2) (detail)


Time07/10/9806/10/9805/10/9804/10/9803/10/9802/10/9801/10/9830/09/9829/09/9828/09/9827/09/98

Wat

er le

vel (

M+G

Z)2

1.8

1.6

1.4

1.2

1

0.8

0.6

0.4

0.2

0

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DATA VALIDATION

• OBSERVATIONS:– OCCURRENCE OF PEAKS TOO REGULAR TO BE

NATURAL, LIKELY DUE TO RELEASES FROM RESERVOIR(S)

– TIME SERIES PLOTS SHOW TWO TYPES OF ERRORS:* SPURIOUS ERRORS DUE TO READING OR ENTRY ERRORS

* ERRORS IN THE OCCURRENCE OF THE PEAKS

– ADDITIONAL INFORMATION REQUIRED FROM U/S OR D/S STATIONS TO EVALUATE STATION(S) AT FAULT

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Water level time series (4) (2 months)

ZA_AMBALIYARA ZA_DABHA ZA_MAHEMDABAD ZS_NSB00I7

Time31/10/9824/10/9817/10/9810/10/9803/10/9826/09/9819/09/9812/09/9805/09/98

Wat

er le

vel (

m+G

Z)8

7.5

7

6.5

6

5.5

5

4.5

4

3.5

3

2.5

2

1.5

1

0.5

0

-0.5

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Time07/10/9806/10/9805/10/9804/10/9803/10/9802/10/9801/10/9830/09/9829/09/9828/09/9827/09/98

Wat

er le

vel (

m+G

Z)2

1.8

1.6

1.4

1.2

1

0.8

0.6

0.4

0.2

0

-0.2

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Time07/10/9806/10/9805/10/9804/10/9803/10/9802/10/9801/10/9830/09/9829/09/9828/09/9827/09/98

Wat

er le

vel (

m+G

Z)2

1.8

1.6

1.4

1.2

1

0.8

0.6

0.4

0.2

0

-0.2

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Time07/10/9806/10/9805/10/9804/10/9803/10/9802/10/9801/10/9830/09/9829/09/9828/09/9827/09/98

Wat

er le

vel (

m+G

Z)2

1.8

1.6

1.4

1.2

1

0.8

0.6

0.4

0.2

0

-0.2

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DATA VALIDATION

• ACTION AFTER EACH STEP:– FLAG ANOMALIES ON THE GRAPHS AND IN THE

TABULAR OUTPUT OF THE SERIES

– ADJUST ANOMALIES WHEN SOURCE AND SIZE OF ERROR IS EXACTLY KNOWN

– CORRECTION SHOULD ONLY BE CARRIED OUT WHEN NO BETTER OPTIONS ARE AVAILABLE (TO ALLOW MORE POWERFUL APPLICATION OF SUBSEQUENT TESTS)

– IF IN DOUBT OR CORRECTION INCLUDES HIGH UNCERTAINTY AWAIT VALIDATION AS DISCHARGE

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COMBINED HYDROGRAPH AND RAINFALL PLOTS

• OBJECTIVES:– TO ASSESS TIMING ERRORS

– TO OBTAIN EVIDENCE OF LATERAL INFLOW

• OFTEN:– RISE IN RIVER STAGE PRECEDED BY A RAINFALL

EVENT IN THE BASIN

• EXCEPTIONS (E.G):– NOT EVERY STORM CAUSES RUNOFF

– SPILL FROM RESERVOIRS ALSO CAUSE RISING LEVELS

– ONE OR BOTH SITES AFFECTED BY BACKWATER

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Time Series

ZA_AMBALIYARA PH_RAHIOL PH_VADAGAM

Time09/08/9408/08/9407/08/9406/08/9405/08/9404/08/9403/08/9402/08/9401/08/9431/07/9430/07/94

Wat

er le

vel (

m+G

Z)7

6.5

6

5.5

5

4.5

4

3.5

3

2.5

2

1.5

1

0.5

0

-0.5

-1

Rainfall (m

m)

32

30

28

26

24

22

20

18

16

14

12

10

8

6

4

2

0

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RELATION CURVES (1)

• FUNCTIONAL RELATIONSHIP BETWEEN TWO SERIES:

Y(t) = f(X(t+t1)

• APPLICATION CONDITIONS TO ARRIVE AT ONE-TO-ONE RELATION:– STATIONS LOCATED ALONG THE SAME RIVER

– NO MAJOR TRIBUTARY JOINS IN BETWEEN

– NO BACKWATER EFFECTS

– TIME OF TRAVEL OF FLOOD WAVE BETWEEN STATIONS IS TAKEN INTO CONSIDERATION

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RELATION CURVES (2)

• APPLICATION:– TO IDENTIFY INDIVIDUAL OUTLIERS

– TO IDENTIFY SHIFTS IN THE RELATIONSHIP BETWEEN TWO STATIONS, WHICH MAY BE CAUSED BY:

* SHIFT IN GAUGE SETTING

* CHANGE IN THE CONTROL SECTION(S)

– TO FILL IN MISSING DATA

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Water level time series (2)


Time30/07/9729/07/9728/07/9727/07/97

Wat

er le

vel (

m+G

Z)12

11

10

9

8

7

6

5

4

3

2

1

0

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Scatter plot (no time shift)

NSB00I7 - H(t)1211109876543210

Mah

emda

bad

- H(t)

10

9

8

7

6

5

4

3

2

1

0

-1

-2

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RELATION CURVES (3)

• TO ELIMINATE THE LOOPING IN THE SCATTER PLOT A TIME SHIFT HAS TO BE APPLIED TO ACCOUNT FOR TRAVEL TIME OF THE FLOOD WAVE

• TRAVEL TIME IS DERIVED FROM:– HYDRAULIC CALCULATION OF FLOOD WAVE

CELERITY

– CROSS-CORRELATION ANALYSIS

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RELATION CURVES (4)

• FLOOD WAVE CELERITY(c) AND TRAVEL TIME (t1)– L is distance between stations

– As and Ar are total and conveying area

– Bs and Br are total and conveying widthc = dQ/dAs c.t1 = L so: t1 = L/c

Q = KmArR2/3S1/2 KmBrh5/3S1/2

dQ/dAs = dQ/(Bsdh)

hence: c = 5/3Br/BsKmh2/3S1/2 = 5/3.Br/Bs.u

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RELATION CURVES (5)

• NOTE: – CELERITY AND HENCE TRAVEL TIME VARIES WITH

RIVER-CROSS-SECTION AND FLOW CONDITION

• TRAVEL TIME COMPUTATION FROM HYDRAULICS:

L=33 km, h= 5 m, S = 3.6x10-4 Km = 33, Bs = Br

u = Kmh2/3S1/2 = 33 x 52/3 x (3.6 x 10-4)1/2

u = 1.84 m/s, c = 1.67 u = 3.07 m/s = 11 km/hrt1 = L/c = 33/11 hr = 3 hr

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RELATION CURVES (6)

• TRAVEL TIME FROM CROSS-CORRELOGRAM:– CORRELATION COMPUTED FOR INTEGER VALUES

OF TIME-LAG T

– FIND TIME-LAG WHICH GIVES MAXIMUM LINEAR CORRELATION BETWEEN THE RECORDS AT THE TWO SITES; THIS TIME-LAG IS OPTIMISED.

RYX() = CYX()/(SYSX)

CYX()=1/N.(Yt-mY)(Xt+-mX)

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Scatter plot (shift = 2.8 hrs)

NSB00I7 - H(t-2.8)1211109876543210

Mah

emda

bad

- H(t)

10

9

8

7

6

5

4

3

2

1

0

-1

-2

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FITTING OF RELATION CURVE

• RELATION EQUATION OF POLYNOMIAL TYPE:

Yt = c0 + c1Xt+t1 + c2X2t+t1 + c3X3

t+t1 + …

• AT MAXIMUM 3 SEGMENTS PER RELATION TO FIT THE SCATTER PLOT AND TO ACCOUNT FOR DIFFERENT TIME SHIFTS t1 ARE ALLOWED IN HYMOS

• ADVISABLE NOT TO GO BEYOND SECOND ORDER POLYNOMIAL

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Relation curve fit (period 27-31/7/97)

NSB00I7 - H(t-2.8)1211109876543210

Mah

emda

bad

- H(t)

10

9

8

7

6

5

4

3

2

1

0

-1

-2

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RELATION CURVE (TABULAR OUTPUT)Relation curves Computed time shift interval 1: -2.87 Used time shift interval 1: -2.87 Error analysis of relation of MAHEMDABAD ZA with NSB00I7 ZS Period from: 1997 7 27 to 1997 7 30 Equations: interval boundaries coefficients 1 .000 -.113974E+01 12.000 .150284E+01 -.550978E-01 Date Obs. Est. Diff. Rel.diff 1997 7 27 0 1 .34 .31 .03 10.39 1997 7 27 1 1 .37 .31 .06 20.13 1997 7 27 2 1 .37 .31 .06 20.13 1997 7 27 3 1 .37 .31 .06 18.78 1997 7 27 4 1 .37 .34 .03 9.04 1997 7 27 5 1 .37 .37 .00 .78 1997 7 27 6 1 .37 .39 -.02 -5.04 . . . . . . . . . 1997 7 30 4 1 2.20 2.15 .05 2.41 1997 7 30 5 1 2.15 2.14 .01 .67 Summary of errors Interval Number of data Standard error 1 96 .30 Overall error analysis Total number of data: 96 Overall standard error: .30

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COMPARISON OF RELATION CURVES

• TWO WAYS TO CHECK CONSISTENCY OF WATER LEVEL RELATION CURVES:– COMPARE THE CURVE FOR ONE PERIOD WITH THE

SCATTER PLOT FOR A PREVIOUS OR FOLLOWING PERIOD

– COMPARE THE RELATION CURVES FOR DIFFERENT PERIODS

• THE TIME SHIFT APLLIED EITHER IN THE SCATTER PLOTS AND/OR IN THE RELATION CURVES SHOULD APPROXIMATELY BE THE SAME FOR A SPECIFIC WATER LEVEL RANGE.

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Water level time series (2)


Time04/08/9703/08/9702/08/9701/08/97

Wat

er le

vel (

m+G

Z)10

9

8

7

6

5

4

3

2

1

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Relation curve fit (period 1-5/8/'97)

NSB00I7 - H(t-3)10987654321

Mah

emda

bad

- H(t)

8

7

6

5

4

3

2

1

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Comparison of Relation Curves (July vs August '97)

First relation Second relation

NSB00I7 - H(t-3)11109876543210

Mah

emda

bad

- H(t)

9

8

7

6

5

4

3

2

1

0

-1

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Comparison of Relation Curves (July vs August '97)

First relation Second relation

NSB00I7 - H(t-3)11109876543210

Mah

emda

bad

- H(t)

9

8

7

6

5

4

3

2

1

0

-1

Shift due to resetting of gauge

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secondary validation of water level data (1)

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