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CORRECTION AND COMPLETION OF WATER LEVEL DATA (1)

A continuous process of validating and correcting incorrect or missing water level data using interpolation or relations. Suspect values will be inspected and corrected if necessary or flagged if acceptable.

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CORRECTION AND COMPLETION OF WATER LEVEL DATA (1)

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  1. CORRECTION AND COMPLETION OF WATER LEVEL DATA (1) • WHERE: • DIVISIONAL OFFICES • WHEN: • AS A CONTINUOUS PROCESS WITH VALIDATION • WHAT: • INCORRECT AND MISSING VALUES WILL BE REPLACED USING INTERPOLATION OR RELATIONS • SUSPECT (FLAGGED) VALUES WILL BE INSPECTED AND CORRECTED (IF NECESSARY) OR FLAG REMOVED IF FOUND ACCEPTABLE OHS - 1

  2. CORRECTION AND COMPLETION OF WATER LEVEL DATA (2) • NOTE: • VALUES ESTIMATED FROM OTHER GAUGES ARE INHERENTLY LESS RELIABLE THAN VALUES PROPERLY MEASURED • HENCE IF STANDARD ERROR IN RELATION CURVE IS SUBSTANTIAL, THEN: • EITHER DOUBTFULL VALUES ARE RETAINED WITH A FLAG, OR • VALUE IN RECORD IS SET TO ‘MISSING’ OHS - 2

  3. CONDITIONS FOR CORRECTION AS LEVEL • LEVEL RECORD COMPLETE BUT RECORDER OUT OF ADJUSTMENT AND CHECK OBSERVATION IS AVAILABLE • RECORD CORRECT BUT SHIFTED IN TIME • PRIMARY RECORD (dwlr-record) IS MISSING BUT ALTERNATIVE AVAILABLE AT THE SAME SITE • PART MISSING BUT DURATION IS SHORT DURING A PERIOD OF LOW FLOW OR RECESSION OHS - 3

  4. CORRECTION USING RIVER LEVEL OR DISCHARGE ? • CORRECTIONS AND COMPLETION AS WATER LEVEL: • SMALL GAP: INTERPOLATION ACCEPTABLE • STANDARD ERROR OF STAGE-RELATION CURVE SMALL • CORRECTION AND COMPLETION AS DISCHARGE: • STANDARD ERROR OF STAGE-RELATION CURVE TOO LARGE • ONE OR BOTH STATIONS ARE AFFECTED BY BACKWATER • RAINFALL-RUNOFF MODEL REQUIRED FOR COMPLETION OHS - 4

  5. COMPARISON OF STAFF GAUGE AND AWLR/DWLR RECORD (1) • IF MORE MEASUREMENTS OF THE SAME VARIABLE ARE MADE AT A STATION, ONE RECORD MAY BE USED TO CORRECT OR REPLACE THE OTHER WHEN MISSING • ERROR CLASSIFICATION: • OBSERVER ERRORS • RECORDER TIMING ERRORS • PEN LEVEL ERRORS • ERRORS ARISING FROM STILLING WELL AND INTAKE PROBLEMS • MISCELLANEOUS INSTRUMENT FAILURES OHS - 5

  6. COMPARISON OF STAFF GAUGE AND AWLR/DWLR RECORDS (2) • OBSERVER ERRORS: • IDENTIFICATION: • DISPLAY OF STAFF GAUGE AND AUTO-GRAPHIC OR DIGITAL RECORD IN ONE GRAPH • DISPLAY OF DIFFERENCES • TRANSCRIPTION ERRORS (E.G. 6.57 FOR 5.67) • CORRECTION: • REPLACEMENT BY CONCURRENT MEASUREMENT AT THE RECORDING GAUGE OHS - 6

  7. COMPARISON OF STAFF GAUGE AND AWLR/DWLR RECORDS (3) • RECORDER TIMING ERRORS: • IDENTIFICATION: • FAST OR SLOW RUNNING CLOCK CAN BE IDENTIFIED BY COMPARISON WITH STAFF GAUGE READING • CORRECTION: • WHEN DIGITAL RECORD IS PRODUCED BY DIGITISER FROM ANALOGUE RECORD: CONTRACTION OR EXPANSION USING DIGITISER • WHEN DIGITAL RECORD IS EXTRACTED MANUALLY: ADD RECORDS (WHEN CLOCK IS FAST) OR ELIMINATE RECORDS (WHEN CLOCK IS SLOW) (CLOCK 4 HOURS FAST IN 8 DAYS: ONE DATA POINT IS REMOVED AT 2 DAY INTERVALS) OHS - 7

  8. COMPARISON OF STAFF GAUGE AND AWLR/DWLR RECORDS (4) • PEN LEVEL ERRORS: • IDENTIFICATION: • GRADUALLY DRIFTING PEN DETECTED BY DEVIATION FROM STAFF GAUGE OBSERVATIONS • CORRECTION: • ERROR X OBSERVED AT TIME t = i + k WHICH HAS COMMENCED k INTERVALS BEFORE: X corr,j = X meas,j - [(j - i)/k]X for j = i, i+1, …., i + k OHS - 8

  9. COMPARISON OF STAFF GAUGE AND AWLR/DWLR RECORDS (5) • ERRORS FROM STILLING WELL AND INTAKE PROBLEMS: • INDENTIFICATION: • TRUNCATED HYDROGRAPHS • LAGGED RECORDS • FLAT TRACES • CORRECTION: • IF FLUCTUATIONS ARE SMALL: REPLACEMENT BY STAFF GAUGE RECORD • OF FLUCTUATIONS ARE LARGE: USE OF RELATION CURVE IS PREFERRED OHS - 9

  10. COMPARISON OF STAFF GAUGE AND AWLR/DWLR RECORDS (6) • MISCELLANEOUS INSTRUMENT FAILURES: (LIKE: SILTING OF STILLING WELL, BLOCKING OF INTAKES, HUNG FLOAT, ETC.) • IDENTIFICATION: • STEPPED TRACES • FLAT TRACES • CORRECTION: • INTERPOLATING A SMOOTH CURVE ON THE HYDROGRAPH PLOT OHS - 10

  11. OHS - 11

  12. OHS - 12

  13. FILLING IN MISSING DATA • SHORT GAPS: • SIMPLE LINEAR INTERPOLATION WHEN LEVEL DIFFERENCE AT BEGINNING AND END OF GAP IS SMALL • GAPS IN RECESSION CURVE: • EITHER INFILL AS DISCHARGE, OR • AS WATER LEVEL BUT USING THE EXPONENTIAL DECAY IN DISCHARGE AND Q-H RELATION • LONGER GAPS: • USE OF RELATION CURVE: • STANDARD ERROR SHOULD BE SUFFICIENTLY SMALL • ADJUSTMENT MAY BE REQUIRED TO GET PROPER CONNECTION AT BEGINNING AND END OF GAP OHS - 13

  14. OHS - 14

  15. FILLING IN MISSING DATA • GAP IN RECESSION CURVE: • RECESSION CURVE: Qt2 = Qt1 exp{- (t2-t1)} • STAGE-DISCHARGE RELATION Q = c(h + a)b • COMBINING: ht2 = (ht1 + a)[exp{-(t2-t1)/b}]- a OHS - 15

  16. USE OF RELATION CURVE WITH ADJACENT STATION • FILLING IN MISSING RECORDS • APPROPRIATE TECHNIQUE IF THE STANDARD ERROR IN THE RELATION IS SMALL • IDENTIFYING AND CORRECTING MISREADINGS • IDENTIFYING AND CORRECTING SHIFT IN: • GAUGE ZERO, THEN CORRECTION ONLY REQUIRED IF: • DISCHARGE RATING CURVE WILL NOT BE CHANGED • LEVEL INFORMATION ITSELF IS OF IMPORTANCE (COASTAL ZONE) • CHANGE IN CROSS-SECTION • ESTABLISH NEW DISCHARGE RATING CURVE OHS - 16

  17. OHS - 17

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