Tipping-bucket rain gauges: resolution, error sources, siting and calibration
How a tipping-bucket gauge turns rain into pulses, what limits its accuracy at both ends of the intensity range, why the gauge samples almost nothing of the catchment, and how to check one.
Rainfall is the input to every flood, and the tipping-bucket gauge is the workhorse that measures it at remote hydromet stations. It is cheap, needs little power and gives an electrical pulse that any logger can count. It also has well-documented weaknesses that decide how far its numbers can be trusted. This article explains the mechanism, puts numbers on each error, and sets out how to site, calibrate and check one.
How it works
Rain falls into a funnel of defined area and runs into a small two-compartment bucket balanced on a pivot. When one side has collected a set volume, the bucket tips, empties, and brings the other side under the funnel. Each tip closes a switch, and the logger counts one pulse. The amount of rain per tip, the resolution r (for example 0.25 mm), follows from the funnel area A and the bucket volume V:
V = A · r → r = V / AFor a funnel 200 mm in diameter, A = 314 cm², and a 0.25 mm tip is 7.85 mL. For an 8-inch (203 mm) funnel, A = 324 cm² and the tip is 8.1 mL. That small volume is the thing being counted, and everything that follows is about how well it is counted.
The counting problem: quantisation
Because rainfall arrives in discrete 0.25 mm steps, any total or intensity you compute has a step size. The time between tips gives an instantaneous intensity:
I (mm/h) = 900 / Δt (s) for r = 0.25 mm| Rainfall intensity | Time between tips |
|---|---|
| 2 mm/h | 450 s |
| 10 mm/h | 90 s |
| 25 mm/h | 36 s |
| 50 mm/h | 18 s |
| 100 mm/h | 9 s |
| 200 mm/h | 4.5 s |
If instead you count tips in a fixed window, the smallest non-zero intensity that window can report is r divided by the window length:
| Window | Smallest non-zero intensity |
|---|---|
| 1 minute | 15 mm/h |
| 5 minutes | 3 mm/h |
| 15 minutes | 1 mm/h |
| 1 hour | 0.25 mm/h |
A 1-minute record from a 0.25 mm gauge can only say 0, 15, 30 or 45 mm/h, so it is a poor basis for light-rain intensity, and a good basis for detecting heavy rain quickly. For warnings, a threshold stated as depth in a window converts directly into tips. For example, 0.25 inches in 15 minutes (a figure used in some automated flood-warning programmes) is 6.35 mm, which is 25 tips.
Error at high intensity: tips take time
A bucket does not tip instantly. While it is moving, water continues to fall into it, and that water can be lost or counted on the wrong side. The WMO calibration training material states the effect plainly: at a rainfall rate of 120 mm/h with a 0.2 mm resolution, the loss can be 5 % if the tip duration is 300 ms [1]. The loss is the fraction of time the bucket is tipping:
loss ≈ t_tip · (tips per second) = t_tip · I / (3600 · r)For 0.25 mm per tip this is t_tip · I / 900:
| Intensity | Loss if tip takes 0.2 s | Loss if tip takes 0.3 s |
|---|---|---|
| 25 mm/h | 0.6 % | 0.8 % |
| 50 mm/h | 1.1 % | 1.7 % |
| 100 mm/h | 2.2 % | 3.3 % |
| 200 mm/h | 4.4 % | 6.7 % |
| 300 mm/h | 6.7 % | 10 % |
The same relation explains a typical specification such as "±5 % at 50 mm/h": it is the accuracy at a stated intensity, and a laboratory result. Because the error is systematic and increases with intensity, it can be corrected. The WMO calibration approach is to test the gauge at several intensities, since tipping behaviour depends mainly on rain intensity, and apply an intensity-dependent correction [1]. Field work on gauge accuracy proposes the same remedy [2].
Error from wind: the gauge disturbs what it measures
Every gauge that stands above the ground deflects the wind over its rim. Air accelerates over the funnel, the trajectories of the drops bend, and some drops that would have fallen in miss it. This wind-induced undercatch is the largest systematic error for catching gauges. It depends on wind speed at the orifice, the size of the drops, the rainfall intensity and the aerodynamic shape of the gauge, so it varies by site and season [2].
A field comparison reported at the WMO instruments conference illustrates the size. Two tipping-bucket gauges with identical calibration stood side by side, one straight-sided and one aerodynamically shaped. Over a few months, the straight-sided gauge recorded about 15 % less rain, and about 20 % less in another period [2]. The same study also showed that wind speed at the gauge orifice differs from the wind speed at the standard 2 m height, which is why the correction cannot be applied from nearby weather data alone [2].
What reduces it:
- Low orifice height and a clear, level site reduce wind speed at the rim.
- An aerodynamic gauge body or a wind shield reduces distortion.
- Site-specific correction based on wind data and drop-size characteristics, where accuracy matters.
- Comparison with a reference gauge (for example a manual gauge set in a pit) over a season, to estimate the site's own undercatch.
Evaporation, wetting and blockage
- Wetting loss. Some water clings to the funnel and bucket. It is small per event and matters mostly for light rain.
- Evaporation. Water standing in the bucket in hot weather evaporates before it tips, which affects drizzle.
- Blockage. Leaves, insect nests, bird droppings and silt in the funnel or the filter screen reduce or stop the flow. The classic symptom is a station that reports zero while its neighbours report rain.
- Levelling and pivot. A gauge that is not level tips at the wrong volume, and a dirty pivot or a bucket with deposits changes the tip volume. Both are systematic.
Siting
A gauge represents the rain at its own funnel, so the site has to let the rain reach it:
- Open, level ground, away from buildings, trees and walls. Siting guidance asks for a clear distance of several times the height of nearby obstacles, and the exact rule is in the WMO Guide to Instruments and Methods of Observation (CIMO Guide) [3].
- Standard orifice height so records are comparable.
- Firm, vibration-free mounting, because vibration can make a bucket tip early.
- Protection from animals and vandals without shielding the orifice.
- Access for maintenance, since a gauge that cannot be reached cleanly will not be cleaned.
Calibration and checks
In the laboratory, calibration is done by delivering known volumes of water at controlled rates through the funnel and counting tips, at several intensities. The calibration process is compared against reference standards traceable to national measurement standards, and the results give the tip volume and the intensity-dependent error [1].
In the field, a simple volumetric check should be part of every visit:
- Level the gauge and check the pivot is clean.
- Pour a measured volume of water into the funnel slowly and steadily. For a 0.25 mm gauge, 100 tips need about 785 to 810 mL, depending on funnel size.
- Compare the tips counted with those expected, and record the difference.
- Check that the logger count matches the visible tips, and that the timestamp is right.
On the data side, check each station against its neighbours. Zero rainfall where neighbours report heavy rain suggests a blockage, and a constant trickle of tips suggests a fault. Flag, do not delete.
What a point gauge says about a catchment
A funnel with an area of about 0.03 m² samples the rain falling on that spot. Compare that with the area it is asked to represent:
| Catchment | Ratio of gauge area to catchment area |
|---|---|
| 10 km² | 3 in a billion |
| 100 km² | 3 in ten billion |
Convective storms, the kind that cause flash floods, can be a few kilometres across, with rain that varies strongly from one place to another. A single gauge can miss a storm cell entirely, or sit in its core. Even a perfectly accurate gauge has a representativeness error that is often larger than any instrument error. The practical response is more gauges where fast catchments form, placed at elevations where storms develop, combined with other data such as radar or satellite estimates and upstream river levels. See RTDAS explained for network design.
Where ClimMaTech fits
Rain gauges are part of the RTDAS networks we plan with clients. A typical specification for a station gauge is an 8-inch (about 200 mm) collector with 0.25 mm per tip and an accuracy of ±5 % at 50 mm/h, and we pair it with the river level and velocity stations that tell you how the catchment actually responds, using the logger and DeviceCloud. See the RTDAS and hydromet telemetry solution.
References
- WMO Expert Team on Quality, Traceability and Calibration, Calibration of Precipitation Instruments, Part 2: Methods of measurement and rain gauge calibration, training material. etrp.wmo.int.
- M. Pollock, Accurate rainfall measurement, WMO Technical Conference on Meteorological and Environmental Instruments and Methods of Observation (TECO-2014). WMO.
- World Meteorological Organization, Guide to Instruments and Methods of Observation (WMO-No. 8, the CIMO Guide), Chapter 6, Measurement of precipitation. WMO library.
Worked values (tip volume, intensity, quantisation and tip loss) are computed from the stated relations and illustrate the size of each effect. Gauge specifications vary with make and model.