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The Optimization and Calibration of a Rain Intensity Gauge

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  • 1 National Institute of Water and Atmospheric Research Ltd., Wellington, New Zealand
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Abstract

The high temporal variability of rainfall requires that measurements be taken at a high frequency for that variability to be recorded well, but most conventional gauges do not have this capability. The gauge described has a resolution estimated to be about 6 s and its measurements do capture the variability. Furthermore, the large datasets associated with high-frequency measurements can be avoided and the essential information retained by storing the data as breakpoints. These are a series of data pairs, each of which consists of the rain rate itself and the time when that rate commenced.

A gauge in which the collected rain is formed into a series of drips all of approximately the same known size was a practical choice. The design and calibration of this gauge, in which particular attention was paid to producing a robust instrument incorporating standard components wherever possible, is described. Long-term comparison in the field with a collocated tipping bucket gauge was used in a calibration scheme in which equality between the gauges was sought for both the long-term accumulation and the short-term rain rates. Although the gauge depends on the formation of equisized drips, it was found that drip size increased slowly with rain rate, and so two calibration parameters were required to convert the time interval between drips into the mean rain rate between the drips. After an initial aging period with inconsistent drip formation, the calibration was stable and the onset of the lowest rain rates (0.1 mm h−1) could be determined to within 1 or 2 min and streaming (i.e., the series of drips merging into a continuous stream) did not occur for rain rates less than 100 mm h−1. Some sample applications for the gauges are described: the extraction of breakpoints, the estimation of 1-min rain rates from Dine's tilting siphon data, and their use in a field experiment.

Corresponding author address: Dr. John Sansom, National Institute of Water and Atmospheric Research Ltd., P.O. Box 14-901, Kilbirnie, Wellington, New Zealand. Email: john.sansom@niwa.cri.nz

Abstract

The high temporal variability of rainfall requires that measurements be taken at a high frequency for that variability to be recorded well, but most conventional gauges do not have this capability. The gauge described has a resolution estimated to be about 6 s and its measurements do capture the variability. Furthermore, the large datasets associated with high-frequency measurements can be avoided and the essential information retained by storing the data as breakpoints. These are a series of data pairs, each of which consists of the rain rate itself and the time when that rate commenced.

A gauge in which the collected rain is formed into a series of drips all of approximately the same known size was a practical choice. The design and calibration of this gauge, in which particular attention was paid to producing a robust instrument incorporating standard components wherever possible, is described. Long-term comparison in the field with a collocated tipping bucket gauge was used in a calibration scheme in which equality between the gauges was sought for both the long-term accumulation and the short-term rain rates. Although the gauge depends on the formation of equisized drips, it was found that drip size increased slowly with rain rate, and so two calibration parameters were required to convert the time interval between drips into the mean rain rate between the drips. After an initial aging period with inconsistent drip formation, the calibration was stable and the onset of the lowest rain rates (0.1 mm h−1) could be determined to within 1 or 2 min and streaming (i.e., the series of drips merging into a continuous stream) did not occur for rain rates less than 100 mm h−1. Some sample applications for the gauges are described: the extraction of breakpoints, the estimation of 1-min rain rates from Dine's tilting siphon data, and their use in a field experiment.

Corresponding author address: Dr. John Sansom, National Institute of Water and Atmospheric Research Ltd., P.O. Box 14-901, Kilbirnie, Wellington, New Zealand. Email: john.sansom@niwa.cri.nz

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