When a city is facing inflow and infiltration issues, expensive sewer system rehabilitation decisions often involve the use of rainfall data. In those cases, it’s important you’re doing everything you can to ensure the accuracy of that data.

Rainfall-derived I&I prediction can be greatly obstructed by imprecise precipitation data, often due to improper rain gauge density, inadequate gauge siting, equipment malfunction or data collection errors. By choosing the right rain gauge equipment, maintaining it per manufacturer standards, spacing gauges appropriately and establishing proper siting, you’re giving yourself the best possible chance of precise data.

Tipping-bucket accuracy

The most commonly used technology to measure rainfall in municipal sewer system studies is the standard tipping bucket, due to the simplicity of its installation, operation and maintenance.

It works by funneling rain into one of two carefully calibrated compartments on a seesaw pivot. When one compartment collects the calibrated amount of rainfall, the bucket tips to drain and the second compartment rises to catch water. When a bucket tips, a reed switch is triggered and that data is sent to a rainfall monitor.

However, the standard tipping bucket has a drawback. During the brief tipping motion, as one bucket drops and the other moves into position, rain falling into the funnel is not recorded. The heavier the storm, the more pronounced this drawback becomes.

Alternatives to the tipping-bucket gauge typically include weighing rain gauges and manual water-level rain gauges. The weighing gauges are the most accurate technology available but can be prohibitively expensive and also require the most maintenance, while water-level gauges can be less accurate. A third option is a variation of the standard tipping bucket that uses a siphon at the base of the funnel to control the rate of flow, allowing the system to account for rainfall that occurs during the bucket tip.

Deciding on the right technology for your application is largely a determination of how much rain you’re expecting. The World Meteorological Organization recommends a measurement uncertainty within plus or minus 5% for rainfall intensity. According to a 2020 white paper by ADS Environmental Services titled Rain Gauge Equipment Selection for the Urban Sewer Environment, the standard tipping bucket meets WMO guidelines for rainfall intensities up to 6 inches per hour. Meanwhile, the siphoning tipping bucket can operate within plus or minus 5% uncertainty up to 20 inches per hour.

By using the National Oceanic and Atmospheric Administration’s Precipitation Frequency Data Server at hdsc.nws.noaa.gov/hdsc/pfds, you can calculate the best tipping bucket for your needs. 

Gauge location and siting

Choosing the proper siting for a rain gauge is a balance between good connectivity to wireless networks (if needed), limiting easy public access to prevent tampering, maintenance accessibility, and consideration of local weather patterns or physical obstructions. For these reasons, pump stations, wastewater treatment plants or other municipal buildings are good locations for rain gauges.

Next, you’ll want to select a location for the gauge to receive rainfall unobstructed, as poor siting conditions can diminish the accuracy of the equipment. Because wind can cause the gauge to underreport actual rainfall (the WMO estimates between 2% and 10% undercatch in direct wind), ideal conditions are found in clearings where there’s a windbreak nearby.

“Although surrounding trees provide shelter from the wind, a rain gauge must not be too sheltered and should also be situated in an area free from rain interference, such as overhanging trees, nearby buildings or other obstructions,” states a second 2020 white paper by ADS titled Rain Gauge Siting Principles for the Urban Sewer Environment.

Guidance from WMO on this matter states that nearby obstacles or obstructions should be no closer than twice their height from the rain gauge. If you can’t find the perfect location, the ADS white paper recommends documenting the site conditions and accounting for any potential impacts. The white paper — available online at www.adsenv.com/white-papers — also includes instructions for a do-it-yourself siting device to estimate distances from windbreaks.

Finally, it’s also important to consider the height above grade at which you’re installing the rain gauge, as wind speeds increase higher up. While the rooftop of a municipal building might seem appealing, if there’s another secure or fenced location closer to the ground with a windbreak, the gauge would be better off there. If you have to install it on a roof, install it as close to the center as possible to avoid eddies of wind.

ADS recommends installing a rain gauge about 3 feet above grade, ideally. However, WMO offers an acceptable range between 1.5 and 5 feet.