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A generalized correction equation for large tipping-bucket flow meters for use in hydrological applications

Shimizu, Takanori, Kobayashi, Masahiro, Iida, Shin'ichi, Levia, Delphis F.
Journal of hydrology 2018 v.563 pp. 1051-1056
Cryptomeria japonica, buckets, equations, flowmeters, forest stands, stemflow, water flow
Large tipping-bucket flow meters (TBFs, one tip > 200 ml) have been employed by hydrologists to quantify various water fluxes in a variety of contexts. The over-arching goal of this study is to develop a generalized correction equation for various TBFs. Based on our testing, we recommend the following to minimize TBF error: (1) periodic checking of the static calibration volume (c) since c is a gauge-specific value which has been found to vary after field deployment; and (2) for dynamic calibration, the use of our newly derived generalized correction equation when the tipping rate is less than 0.2 Hz. In equation form, the generalized correction equation for common TBFs with flat triangular buckets is: V = −0.75Q2 + 0.72Q + 1 (R2 = 0.843; p <0.0001), where V = v/c and v is the water volume for one tip under dynamic conditions, and Q = q/c [s−1] and q is the water flow rate into the TBF. From our field test in a Japanese cedar forest stand, using stemflow (SF) as an example, we found that use of the generalized correction equation was successful in eliminating the 2–3% error in SF amounts. Moreover, we found that the generalized correction equation performed nearly as well as gauge-specific derived correction equations. Thus, our generalized correction equation is applicable to correct flow estimates of TBFs when one does not have time and/or laboratory set-up for the laborious task of testing individual TBFs themselves. Although our correction procedure may not completely eliminate all error, we recommend use of the generalized correction equation for TBFs to improve the accuracy of water flux calculations in hydrologic studies.