Gauge Measurement Errors
limited influence on rain-intensityfigures, but areimportant for rain accumulation measurements at the daily, monthly or longer time scales. These are commonlycategorized as wind-induced, wetting, splashing, and evaporation errors, all of them occurring within or atthe top of the water collector implemented to convey rainfall from a standardized orifice into the measuring device.
On the other hand, mechanical errors due to the inherent characteristics of the counting device, althoughless important in terms of accumulated rainfall, have a strong influence on the measurement of the rainintensity, with an increasing impact as the rain rate increases. In particular, the measurement of rain intensityis traditionally performed by means of tipping-bucket rain gauges, the most popular and widespread typeof rain gauge actually employed worldwide. This is known to underestimate rainfall at higher intensitiesbecause of the rainwater amount that is lost during the tipping movement of the bucket.
The bias, estimated on average at about 10–15% for rain rates higher than 200 mm $h^{-1}$, is strongly specificto the single rain gauge, depending on the manufacturer, the date of production and the type of wear
The bias induced by systematic mechanical errors of tipping-bucket rain gauges is usually neglected inhydrological practice, based on the assumption that it has little influence on the total recorded rainfall depth.We have demonstrated that, since the error increases with rainfall intensity, the assumption is not acceptablefor the assessment of design rainfall in urban-scale applications.
Simplified method to eorreet rainfall measurements from tipping bucket rain gauges
Tipping bucket rain gauges are commonly used in rainfall measurement campaigns. The main disadvantage of these devices is that they underestimate rainfall volumes at the higher rainfall intensities, due to the loss of water during the tipping action of the device.
Stampoulis, D. and E. N. Anagnostou (2012). “Evaluation of Global Satellite Rainfall Products over Continental Europe.” Journal of Hydrometeorology 13(2): 588-603.
The convective nature of rainfall will increase gauge-interpolation uncertainties as the standard deviation increased. Specifically, both satellite techniques (TRMM, CHMRPH) underestimate rainfall over higher elevations, especially during the cold season, and their performance is subject to seasonal changes.
Pollock, M., et al. (2010). “Accurate Rainfall Measurement: The Neglected Achiles Heel of Hydro-Meteorology.”
Wind as an essential factor in tropical cyclones would substantially affect the performance of rain gauges, with the relative bias ranging from 5 percent to 80 percent according to the speed.