A rain gauge network, when properly sited and maintained, is a valuable tool for water, infrastructure, and emergency management teams, providing timely rainfall information at point locations. But precipitation can be highly variable, even over short distances, making it difficult to fully assess what is happening between sensors. The heaviest and most intense rainfall could easily go unobserved if you are simply relying on a gauge network alone.
Radar, on the other hand, detects precipitation at ranges exceeding 200 kilometers away from its location and can create a spatially complete, high-resolution profile of the approaching storm. However, it is prone to errors in rainfall estimation. By leveraging the strengths of both systems, you can use ground-truth rainfall observations from rain gauges to calibrate the radar data and produce a rainfall dataset that is geographically comprehensive and more accurate than either sensor could produce alone.
That approach is called Gauge-Adjusted Radar Rainfall (GARR).Moving forward, we’ll explore…
A rain gauge’s biggest strength at a single location can also be considered its biggest weakness when area-wide rainfall information is needed: it provides accurate data but only for a specific site. When you build a gauge network across a given area, you gain localized information about how particular locations experience rainfall events both incrementally and cumulatively. But it provides little to no insight as to what is going on between those gauges, and when you try to spatially interpolate network data to fill in the coverage gaps, the results are usually not accurate.
For years, people assumed the answer was to install more rain gauges. And while it is true that network density can be beneficial, more measurement stations may not be the most practical solution for every team or situation. In many cases, it is more powerful and cost-effective to combine information from rain gauges with another data source - radar - which provides the complete “rainfall pattern” view that is missing from gauge network data.
Radars can detect and estimate precipitation from more than 200 kilometers away, offering a high-resolution spatial picture of a rainfall event over a wide area. They are an excellent way to visualize where rain has fallen and to track where it might be headed.
With that said, rainfall amounts derived from radars are just an estimation. They are very often biased either too high or low because translating remotely sensed radar reflectivity to rainfall is inexact. Proximity to the radar location will also impact how accurately the system is profiling the rainfall occurring at ground level. Due to these potential inaccuracies, radar data alone does not provide fully reliable rainfall information that can be confidently used for critical decision making.
However, a well-sited rain gauge network can help correct the radar-derived rainfall by providing trustworthy “ground-truth” rainfall information. These crucial rain gauge measurements are used to calibrate or gauge-adjust the spatially complete rainfall picture provided by the radar, allowing you to determine accurate precipitation totals between gauges.
By using ground-level data from rain gauges to calibrate a radar’s spatially complete rainfall estimations, GARR can offer a comprehensive, high-resolution, and accurate rainfall dataset in near-real time. You can see an example of GARR in action for yourself here:
With GARR, emergency and water management teams have better insight into an unfolding rainfall event and can pinpoint exactly where and when action is needed. This improved visibility helps prioritize responses, reduces risks, and empowers teams to act with confidence when it matters most.
GARR data isn’t just for real-time flood awareness either. Post-analysis GARR data, or examining rainfall events after they have occurred, can help public works and stormwater/wastewater teams better understand potential rainfall impacts as they plan new projects or serve as a reliable input dataset for hydrologic/hydraulic modeling. Detailed GARR reports can aid regulatory reporting and even guide a rain gauge maintenance program. In any context where water matters, GARR helps tell a far more precise, powerful rainfall story.
The Northeast Ohio Regional Sewer District (NEORSD) is on a mission to provide progressive sewage and stormwater management across their 374-square-mile service area and community of more than a million people. A major part of that is understanding how rainfall events translate into community impacts so they can deploy teams and communicate with the people they serve when dangerous conditions develop.
NEORSD had a network of just 40 rain gauges spread across their large service area and needed to increase their awareness during potential flood events so no community members fell between the gaps. Rather than buying, installing, and maintaining additional hardware in the field, they decided to add Gauge-Adjusted Radar Rainfall.
GARR proved its value to NEORSD during Tropical Storm Debbie in August of 2024. Rain was light across the vast majority of their service territory throughout the storm, but in localized areas, Debbie brought the full brunt of a 1,000-year rainfall event. Thanks to GARR, the sewer district was aware of the variation in intensity across their region, enabling them to provide warnings to local residents and redirect their resources toward the areas where they’d be needed most.
If you’re in stormwater, wastewater, floodplain, or emergency management, Gauge-Adjusted Radar Rainfall data might best way to increase operational intelligence without creating any new maintenance responsibilities for your team. To schedule a conversation with a GARR specialist, click here.