Water level measurement is a fundamental aspect of water resource management and environmental monitoring in aquatic ecosystems. Monitoring the water level in natural waters such as lakes, rivers, reservoirs, oceans, groundwater, and other water bodies is essential to researchers, resource managers, infrastructure planners, port authorities, and other environmental professionals. Climate change concerns have also amplified the importance of water level monitoring, particularly in coastal and flood or drought-prone areas.
Water level is normally reported in units such as feet or meters relative to a reference point. The reference point may be a marker such as a staff gauge, geodetic datum, top of a well, or other local benchmark like mean sea level (MSL).
Why Measure Water Level?
The study of water level and the movement of water, commonly referred to as hydrology, has been of great interest and importance throughout human history. Even ancient civilizations used rudimentary techniques like nilometers to monitor river levels for agriculture and flood prediction.
In the modern era of large-scale infrastructure projects, intensive agricultural practices, growing global population, and shifting climate patterns, water level measurement and sustainable water use are more important than ever.
In natural waters, measurement of water level is crucial for a wide range of applications:
- Flood Prediction and Management: Accurate water level data help predict and manage flood events by providing early warnings and enabling the implementation of flood control measures.
- Water Resource Management: Water level monitoring is essential for assessing the availability and distribution of water resources in both groundwater and surface waters for drinking water supply, irrigation, and other uses.
- Environmental Monitoring: Monitoring water levels in natural ecosystems helps researchers understand the ecosystem health and functions of aquatic habitats, especially in wetlands, rivers, lakes, and coastal areas.
- Infrastructure Planning: Engineers use water level data to design and manage infrastructure projects such as bridges, dams, and stormwater management systems to ensure they are properly sized for the hydraulic conditions they will experience.
- Navigation: Boating, shipping, and recreational activities rely on accurate water level information to ensure safe navigation, particularly in rivers, ports, and shallow coastal waters.
- Climate and Weather Studies: Water level measurements contribute to climate research and weather forecasting, as they are related to meteorological phenomena like storm surges and long-term sea-level rise.
How is Water Level Measured?
Water level measurement can be accomplished using various methods and sensor technologies, depending on the application and environmental conditions. Remote sensing systems utilize either non-contact or contact sensor options.
Non-contact sensing methods typically involve a sensor mounted from a structure above the water with the instrument directed at the surface of the water. A minimum distance above the maximum expected water height is normally required. Non-contact technologies include ultrasonic and radar level sensors.
Ultrasonic water level sensors transmit bursts of high-frequency, ultrasonic sound waves above the range of human hearing (greater than 20 kHz). The ultrasonic waves are sent from the sensor through the air to the water surface, partially reflecting the waves back to the sensor. By measuring the time from transmission to return, the sensor can measure the distance to the water surface.
Ultrasonic sensors may work well over short distances but can be affected by environmental conditions such as temperature, humidity, and wind. They may incorporate temperature compensation and other algorithms to improve data quality.
Radar water level sensors emit radar waves and measure the return time of signals reflecting off of the water surface.
Radar sensors also measure signal reflection time but emit microwave signals typically in the GHz range. In comparison with ultrasonic waves, electromagnetic radar waves are less affected by environmental conditions. As a result, they can provide better accuracy in many conditions without advanced data processing, though some sensors still include algorithms for data filtering, for example, to account for water surface turbulence.
Vented pressure transducers are exposed to atmospheric pressure changes by means of a vent tube.
Contact water level sensors include a variety of technologies where at least part of the system physically contacts the water. This includes staff gauges, sounders, float switches, shaft encoders, and bubblers, though one of the most common sensor types for remote, automated monitoring is the submersible pressure transducer.
Pressure transducers for water level measurement consist of a rugged housing with a pressure-sensing element mounted behind a thin membrane or diaphragm. Pressure exerted onto the diaphragm changes the resistance of the internal electronics, which is used to calculate the pressure and corresponding water level.
Sensors fall into two categories: vented and non-vented (absolute) pressure sensors. Vented pressure transducers negate the effects of atmospheric pressure by incorporating a vent tube that allows air to enter into the sensor housing behind the diaphragm. Since the sensor is open to the air, changes in measured pressure correspond only to changes in water level.
Absolute pressure transducers are completely sealed and measure both atmospheric and water pressure.
In absolute pressure transducers, the sensor housing is completely sealed, and there is no vent tube open to the atmosphere. Instead, a vacuum created within the housing ensures that the sensor responds to both changes in water pressure and atmospheric pressure. For accurate water level measurement, a barometer reading is needed for data correction by subtracting barometric (atmospheric) pressure from the total pressure measured by the transducer.
The primary advantage offered by vented pressure sensors over absolute sensors is that they give a direct water pressure measurement that does not need to be corrected using a separate barometric pressure measurement. This simplifies the measurement and can improve accuracy because the error range is limited to a single sensor (rather than cumulative error between the pressure transducer and barometric pressure readings).
However, vented sensor designs are vulnerable to physical obstruction of the vent tube. Any blockage of the tube, whether caused by debris, moisture, or bending of the cable, will affect measurement accuracy. Excessive moisture intrusion can also damage sensor electronics, so it is common to install a desiccant kit on the vent tube to absorb moisture.
How to Select a Water Level Sensor?
Selecting a water level sensor for a particular application begins with deciding which sensor technology is most appropriate for the measurement site.
Non-contact sensors generally have less maintenance needs, but they require a stable mounting structure that will not introduce errors from movement or vibration. Furthermore, the sensor must have a clear, unobstructed view of the water surface at all times with sufficient blanking distance but not exceeding the sensor’s measurement range.
Submersible sensors can offer more flexibility in deployment, but they also require a stable mounting structure so that the sensor does not move in the water column. If the measurement system does not include a barometer, and a local barometric pressure reading is not available, a vented pressure transducer may be favorable. However, sensor maintenance should be considered.
Measurement range and accuracy are other important considerations for sensor selection. Generally speaking, both contact and non-contact sensor types will achieve better accuracy with a smaller measurement range. However, the sensor range must be sufficient to cover the full range of expected water levels, especially with submerged pressure transducers, which can sustain damage from exposure to pressure beyond the sensor’s tolerable range.
What to Consider When Preparing a Water Level Sensor?
For non-contact instruments, consider the measurement characteristics, the water surface to be measured, environmental conditions, calibration requirements, and potential interference from multiple radars.
Unlike submersible sensors, which measure water pressure, non-contact sensors measure the distance to the water surface. Thus, to give a water level reading, correlation to a reference point such as a staff gauge must be performed.
Non-contact level sensors measure distance to the surface and must be correlated to a reference datum.
Although many sensors include algorithms to improve accuracy, a smooth, flat water surface will typically yield the most accurate results. When possible, avoid areas prone to disturbances from vegetation, rocks, sediment loads, or environmental conditions like wind, rain, snow, and intense fog.
Many non-contact sensors have automated calibration procedures to ensure that the sensor is performing in accordance with specifications. In some cases, the calibration procedure must be performed periodically or in the event of troubleshooting. Consult manufacturer user guides for recommendations.
If an application will require multiple radar sensors placed within close proximity to one another, consider the possibility of interference between the sensors, and stagger measurement times if there are any concerns. Extraneous wideband radiation sources can also theoretically cause interference if present in the vicinity of the measurement site.
For submersible pressure transducers, consider the depth of installation, the characteristics of the water body, calibration and offsets, and potential impacts of water composition.
Submersible sensors must remain submerged at all times for correct measurement. This means that they must be securely placed below the minimum water level expected in the water body they are measuring. Water bodies with large fluctuations will have relatively deep placements when water levels are high. The sensor measurement range must be chosen accordingly.
Most pressure transducers are factory-calibrated to set their measurement range and do not require user calibration other than the optional setting of a zero-point in some cases. For example, vented transducers may allow for the sensor to be zeroed to the current atmospheric pressure value with the sensor in air just prior to deployment.
Absolute pressure sensors typically do not require zeroing because the factory calibration procedure sets them relative to a perfect vacuum. However, the water level and atmospheric pressure at the time of installation should be measured and noted, regardless of any adjustments made to the sensor readings.
Since water level readings are often reported relative to a specific reference point such as a lakebed, river bottom, sea level, or other site-specific datum, many sensors or post-processing programs allow for data to be adjusted with an offset. This accounts for the sensor’s physical placement relative to the reference point.
An offset can be applied to sensor readings to correlate the level to a reference datum.
In addition to adjustments relative to a reference, water composition should also be considered as it relates to pressure readings. Water density is affected by both temperature and salinity. Density affects the pressure measured and, consequently, the accuracy of the water level readings. Many sensor types incorporate automatic compensation for temperature and salinity based on internal measurements or user inputs during configuration.
How to Deploy a Water Level Monitoring System?
Much like sensor selection, deployment methodology for water level monitoring systems depends on the sensor type.
Bridge decks or other structures are used to mount non-contact level sensors in a downward-looking orientation to the water surface.
Non-contact ultrasonic and radar sensors require an appropriate mounting structure for the instrument. Examples include bridge decks, culverts, canal walls, poles, piers, docks, and any other fixed or built structure that provides a sufficient placement for the sensor.
The sensor should be mounted precisely at a 90° angle towards the water surface unless further programming adjustments are made to account for the mounting angle. Avoid any obstructions to the measurement beam. Consult manufacturer specifications for the sensor’s beam angle and calculate how much clearance is required for the largest distance that will be measured.
Non-contact sensors must avoid obstructions of the measurement signals and maintain a minimum blanking distance to the water surface.
Similarly, the specifications will give a minimum measurement distance, sometimes called a blanking distance. Ensure that the sensor is mounted at least this distance or greater from the maximum expected water level, and avoid submersion during flooding that can damage the sensor.
Stability of the mounting is critical. Any vibration from wind, passing traffic, or other sources can affect measurement accuracy. If necessary, add damping to the mounting mechanism.
Non-contact systems are suitable for water level monitoring in rivers, lakes, reservoirs, and tide gauges, depending on specific site conditions.
Water level monitoring systems based on submersible pressure transducers are suitable for many of the same applications but are favored for stream gauge systems that may experience low-water conditions and for groundwater level measurement.
Submersible level sensors must be mounted below the minimum expected water level and well-protected from debris, high flow, and other potential disturbances.
They similarly require a robust and site-appropriate mounting strategy. However, in the case of submersible sensors, they must be securely mounted below the minimum water level.
For streams, rivers, ponds, and other surface waters, a perforated deployment pipe allowing the water level within the pipe to adjust to the changing conditions in the water body is a common strategy. Such deployment pipes can allow for the sensor to be extracted for periodic cleaning and maintenance. A stop bolt ensures that the sensor is returned to precisely the same position following maintenance.
One concern with deployments of this nature is potential debris loads and forces generated during high-flow events in rivers and streams. Heavy flows may damage deployment pipes, so selection of materials (steel, PVC, etc.), hardware for securing the pipe, and accessibility for maintenance and debris removal should be considered. Pointing the pipe in the downstream flow direction may also help to minimize such loads when site conditions allow for it.
For groundwater level, the sensor can simply be dropped down a well until it reaches the minimum water level. If a very long cable is needed to reach the water table, a supporting wireline should be used to avoid cable strain. A well cap can be used to protect the well, suspend the support line, and ensure that the sensor is replaced at the same depth following each maintenance visit.
Level sensors placed in wells must be located below the minimum water level and be placed at the exact same location following any maintenance.
Although less common because buoys move with changing water levels, currents, and waves, it is also possible to deploy a level measurement system from a floating buoy platform. This can be done using a sonar depth sensor mounted on the buoy below the surface of the water. Sonar sensors work similarly to ultrasonic sensors in that they emit sound waves and measure reflection time to determine distance, but they measure in water rather than in air.
Regardless of sensor selection, all remote water level monitoring systems utilize a data logger with wireless telemetry capabilities to log and transmit data in near real-time. Power can be supplied from an internal battery, solar-charged battery packs, or AC power adapter if available.
Conclusion
Water level is one of the most common hydrological measurements performed across all types of natural waters. Typical applications include stream gauge stations, reservoirs, groundwater wells, river and lake monitoring, and tide gauge stations. Understanding water levels and the movement of water is critical for flood prediction and warning, secure drinking water supply, climate change studies, and infrastructure planning, amongst other use cases.
Accurate water level monitoring is essential for effective water resource management and environmental protection. By selecting the appropriate sensor technology, coupled with proper deployment and maintenance, water managers can obtain reliable data in long-term monitoring applications. Such data enable informed decision-making to address the challenges posed by changing water levels and the need for sustainable water management.
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