Accuracy Assessment of Water Survey Canada Flow Measurement Station on the Blindman River for Low-Flow Conditions (0.14 m³/s)

The Water Survey of Canada (WSC) hydrometric station on the Blindman River near Blackfalds (Station 05CC001) provides critical flow monitoring data that informs water management decisions. This report examines the accuracy of measurements at this station during low-flow conditions, specifically around 0.14 m³/s, and analyzes the factors affecting measurement reliability in this flow range.

Station Characteristics and Context

The Blindman River hydrometric station (05CC001) is located near Blackfalds at coordinates 52°21'25" N, 113°47'42" W2. This station monitors a watershed with a gross drainage area of 1,800 km² and an effective drainage area of 1,460 km²2. The river has a main channel length of approximately 125.9 km with a relatively gentle slope of 0.00112. The Blindman River is a tributary of the Red Deer River, joining it near the City of Red Deer3.

The minimum instream objective flow for the Blindman River is established at 0.156 m³/s, below which no water abstractions are permitted according to Alberta Environment and Protected Areas regulations6. This threshold is very close to the 0.14 m³/s flow rate examined in this assessment, making accuracy at this level particularly important for regulatory compliance and ecological protection.

General Accuracy of Hydrometric Measurements

Standard Measurement Uncertainty

The Water Survey of Canada reports that the uncertainty of estimated daily mean discharge is generally around 5% at the 95% confidence interval7. However, this represents an ideal scenario and does not account for specific challenges associated with low-flow conditions or site-specific factors.

Low-Flow Measurement Challenges

For low-flow conditions, measurement accuracy is typically reduced compared to normal flow ranges. Research indicates that "uncertainties on estimated flows based on rating curves can be significant, especially for high and low flow regimes"10. This increased uncertainty stems from several factors specific to low-flow conditions:

Rating Curve Extrapolation: Low flow extrapolation is particularly challenging and "in most cases, low flow extrapolation is not very accurate"8. When extending rating curves to very low flows, the relationship between stage and discharge often becomes less predictable.
Zero-Flow Inconsistencies: If the existing trend in a rating curve is extended to the zero discharge point, "the curve will rarely pass through the zero stage point"8. This discrepancy creates systematic errors in the lowest flow ranges.

Measurement Methods and Associated Uncertainties

The accuracy of flow measurements is directly dependent on stage (water level) measurement quality. As noted by the WSC, "the accuracy of stage data directly impacts discharge data"9. Several stage measurement technologies are employed at hydrometric stations, each with distinct accuracy implications:

Stage Measurement Technologies

Submersible Pressure Sensors:
- Advantages: Relatively inexpensive and easy to install
- Limitations: "Accuracy decreases with increasing effective stage," and sensors can be damaged when encased in ice9
- Low-flow implications: While the accuracy issue with "increasing" stage doesn't directly address low-flow conditions, these sensors can be affected by sediment buildup or other interferences at low water levels
Non-submersible Pressure Sensors (Bubbler Systems):
- Advantages: Only the orifice line is in the water, reducing risk of sensor damage
- Limitations: "Orifice can routinely be blocked by sediment, algae, calcium or other material," and "accuracy decreases as effective stage increases"9
- Low-flow implications: Blockages are more likely during low-flow conditions, potentially affecting measurement accuracy
Float Systems with Shaft Encoders:
- Advantages: Reduced fluctuations from wind and waves
- Limitations: Requires a stilling well, which can be problematic during low-flow conditions if sediment accumulates9

Specific Considerations for Blindman River Station 05CC001

The Blindman River watershed has several characteristics that may influence measurement accuracy during low-flow conditions:

Agricultural Activities: The watershed contains "more than 40 feedlots/intensive livestock operations"16, which can affect water quality and potentially measurement equipment during low-flow periods.
Oil and Gas Development: With "an average well density of 1.92 wells/km²"16 in the watershed, industrial activities may influence both water quantity and quality, particularly during low-flow periods.
Linear Infrastructure: The watershed contains numerous road crossings, pipelines, and other infrastructure that can influence flow patterns, particularly during low flows16.
Stage-Discharge Relationship: During a review of the rating curve for a nearby station on the Red Deer River, researchers noted that "simulated rating curve, and the WSC rating curve [required an] elevation shift [to be] corrected"19, suggesting potential calibration issues that might also exist at the Blindman River station.

Expected Accuracy for 0.14 m³/s Measurements

Based on the available information, the accuracy of measurements at the 0.14 m³/s flow level at station 05CC001 can be estimated as follows:

Standard Uncertainty Range: While the general uncertainty is cited as 5% at the 95% confidence interval7, for low flows near 0.14 m³/s, the actual uncertainty is likely significantly higher.
Comparative Analysis: For stations with similar characteristics, stage-dependent uncertainties "increase significantly at low stages reaching values larger than 20% for some [rating curves]"10. This suggests that measurements around 0.14 m³/s could have uncertainties in the 15-25% range.
Regulatory Implications: Given that the minimum instream objective for the Blindman River is 0.156 m³/s6, the measurement accuracy at 0.14 m³/s is particularly important for determining compliance with water withdrawal restrictions.

Conclusion

The accuracy of Water Survey Canada's flow measurements at station 05CC001 on the Blindman River for low-flow conditions around 0.14 m³/s is subject to higher uncertainty than measurements at moderate flow levels. While the general uncertainty for daily mean discharge is reported as 5% at the 95% confidence interval, low-flow measurements likely experience significantly higher uncertainty, potentially in the 15-25% range.

This increased uncertainty stems from challenges in rating curve extrapolation at low flows, potential equipment limitations, and environmental factors specific to the Blindman River watershed. For water management purposes, especially when enforcing the minimum instream objective of 0.156 m³/s, these uncertainty ranges should be considered when making regulatory decisions based on measurements near this threshold.

Further studies specifically targeting low-flow measurement accuracy at this station would be valuable, particularly given the regulatory importance of accurate measurements in this flow range for managing water allocations and protecting ecological functions in the Blindman River system.

Citations:

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