Verification and calibration of tipping-bucket rain gauges used in the state of São Paulo for intense rain situations

Abstract

The importance of rain data for several purposes, including planning and design in Civil and Environmental Engineering, Architecture, Agriculture, Aeronautics, Tourism and other human activities, has risen as land occupation grows and natural resources become rarer. An increasing number of different models of rain gauges is available in the market, particularly some used in São Paulo State both for the purposes of regular data assembling for the State Hydrological Data Basis and real time monitoring for the protection of population and urban infra-structure. Many of these devices, particularly in the second category, adopt a tipping-bucket design - well accepted as those more suitable for automatic and telemetric monitoring. The importance of reliability on the precision of such gauges has been emphasized in the literature, particularly when measuring intense rain, through practical demonstration in a controlled, laboratory environment. Depending on each gauge design, problems related to: device cleaning; regular checking; fluid dynamical effects during the filling and the emptying phases of the bucket and the duration of each of these two phases have brought different authors to recommend calibration for the correct interpretation of intense rain measurements - with special attention to these points - and, eventually, changes or simple improvement in design. This paper proposes practical calibration rules demonstrated to considerably increase rain data reliability. Such rules are based on laboratory experiments with two different types of rain gauges performed at Mauá Institute of Technology - School of Engineering, in São Caetano do Sul, São Paulo State, Brazil. The results reinforce the need for defining similar procedures for all types of similar gauges.