Performance of Off-Level Tipping-Bucket Rain Gauges: A Laboratory Assessment

The performance and accuracy of tipping-bucket rain gauges (TBRGs) have been widely examined. However, whilst for proper operation TBRGs must be carefully levelled, to date there appears to be no published analysis of the errors that can arise in tilted gauges. Although tilt could exist in any direction with respect to the rotation axis of the buckets, the greatest effect occurs for tilt orthogonal to the rotation axis. Here, potential errors associated with tilt in this direction are analysed experimentally for the first time. Tests on a TBRG were made with the gauge carefully levelled, when inclined at inclinations of 1°, 2°, 3°, 4°, and 5°, and at several pumped flow rates equivalent to rainfall rates in the range 9.6–114.6 mm h−1. Results confirm that in a tilted TBRG, the buckets tip with unequal volumes of water. This results in the gauge requiring a progressively larger mean depth of rainfall to trigger tips as tilt increases, because the bucket tilted up requires more water to tip than the TBRG calibration would suggest, whilst the bucket tilted down requires less. Consequently, a tilted TBRG reports too few tips, and so under-reports the rainfall depth. The increase in the mean tipped volume is ~1% for a tilt of 1° and can exceed 8% for a tilt of 5°. The kinematic error associated with the TBRG mechanism is also shown to persist in tilted TBRGs, such that the potential aggregate error from both sources in a field installation may seriously degrade the quality of rainfall data.