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Difference between model and field data during average/low flow conditions

In the following tables are some results of field tests compared to model predictions at average/low flow conditions in table 1 and then at high flow conditions in table 2. In the 1st table one can see that the hydraulic grade during average/low flow conditions differs from results of the model prediction but on average they do not differ as much as the ones form the high flow conditions.  The values in red are values that seem to be bad data since the hydraulic grade is higher than that of the reservoir. From one of the articles in the calibration tips it is explained that in the low flow scenario model predictions should match field data. But my question then is if the difference during the average/ low flow scenario is significant enough or are they small enough to where one could take away that the model and field data are matching well enough?

Table 1

Hydraulic grade of reservoir (average /low flow conditions)

44.91

Hydraulic grade from model prediction (average /low flow conditions)

Hydraulic grade from measurement (average /low flow conditions)

Percentage difference

44.81

46.829

-4.51%

44.81

43.686

2.51%

44.81

43.524

2.87%

44.81

43.639

2.61%

44.81

44.353

1.02%

44.81

42.516

5.12%

44.81

46.67

-4.15%

Table 2

Hydraulic grade of reservoir (High flow conditions)

44.6

Hydraulic grade from model prediction (High flow conditions)

Hydraulic grade from measurement (High flow conditions)

Percentage difference

42.54

39.444

7.28%

43.51

36.033

17.18%

42.7

26.856

37.11%

43.51

38.378

11.79%

42.68

37.278

12.66%

42.55

23.091

45.73%

44.07

44.612

-1.23%

Parents
  • Hello Christen,

    You may want to confirm the conditions in the system at the time of the measurement. Unless there is a pump adding head to the system or some other source in the system with a higher hydraulic grade, you should not see a hydraulic grade higher than the reservoir. This may indicate that the boundary conditions are different from the settings in the model at the time the measurements were taken.

    Correct boundary conditions (tank levels, pump status, etc) are important to model calibration. This is alluded to in the calibration tips wiki.

    Regards,

    Scott

  • I think where the Hydraulic grade is higher then the reservoir could be related to wrong elevations i do not think it is the reservoir level since the rest of the data seems to be pretty close during normal/low flow conditions. So I more so wanted to know about when one can consider that the field data is matching the model. So in the low/avergae day condition aside from the bad data (in red) does the percentage difference seem reasonable.

  • Usually when the agreement gets worse as you move away from the source, the problem lies in system hydraulics.  The pipe roughness may be much higher in the system than in the model, or there may be one or more closed/throttle valves or you have some incorrect diameters. You need to determine WHY there is disagreement.

    A good check of your data is to plot measured values of HGL on a map, You can do this manually or using a user data extension and annotate the map.

    With a typical good quality pressure gauge, the accuracy of the pressure may be +/- 2 ft. The elevation data may be +/- 5 or 10 ft depending on the source. Showing values like 39.444 doesn't make much sense. Just report 39 ft.

  • Just to clarify the hydraulic grade in this case is in meters. However it seems to me if I reduce my results to round numbers my field data seems to agree pretty well under average flow conditions except for the red highlighted field data points which seem to be bad data. Would Tom agree? And i did not use guages i used pressure loggers.

  • Hello Christen,

    I cannot speak to the difference between pressure loggers and pressure gauges, though other users here may be able to offer their opinions. 

    Aside from the advice that Tom offers above, if you suspect that some of the data may be inaccurate, it may be worth trying to do another test to confirm that the information in accurate. If you believe the data is accurate, reviewing some of the items Tom mentions (closed or throttled valves, incorrect diameters or elevations, etc.) would be worth checking.

    Regards,

    Scott

Reply
  • Hello Christen,

    I cannot speak to the difference between pressure loggers and pressure gauges, though other users here may be able to offer their opinions. 

    Aside from the advice that Tom offers above, if you suspect that some of the data may be inaccurate, it may be worth trying to do another test to confirm that the information in accurate. If you believe the data is accurate, reviewing some of the items Tom mentions (closed or throttled valves, incorrect diameters or elevations, etc.) would be worth checking.

    Regards,

    Scott

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