User Guide
11
7. Advantages and Limitations of a Tipping Bucket
Gauge
Gauges which operate on the tipping bucket principle provide a digital output,
which simplifies connection to a datalogger. The pulses returned during rainfall
may be counted over any time interval desired allowing accurate determination of
the rainfall rate (this variable, sometimes called ‘intensity’, is frequently used in
soil erosion studies and is relevant to some aspects of crop pathology).
When the ARG100 is used primarily to measure rainfall rates, the resolution of
the gauge may be too coarse to detect the significant changes in rate which nor-
mally occur from minute to minute in moderately heavy rain. In this case the
gauge may be adjusted to improve the resolution slightly (see Section 6.1);
alternatively, the bucket volume may be reduced by adding plasticine (or possibly
candle wax). It is generally much more satisfactory and convenient to improve the
resolution by increasing the collecting area of the gauge. This can be achieved by
attaching a funnel with a diameter greater than 25.5 cm over the instrument and
avoids the need for internal modifications.
A tipping bucket gauge responds to discrete quanta of rainfall, and the accuracy
and reproducibility of this quantum are determined not only by factors such as
friction in the bearings, etc. but also by the rate of fill of the buckets. When the
rainfall rate is high, a bucket may
start
to tip when the necessary volume of water
has been collected, but while the bucket is moving away from the funnel outlet, an
extra volume will have been collected and lost through spillage. The resulting
degradation in accuracy is of the order of 4% at rainfall rates of 25 mm/hr and 8%
at 133 mm/hr for most gauges (Parkin et al, 1982). This is important when results
from gauges of different designs are compared. These errors worsen when gauge
sensitivity is increased. It follows that gauge design is always a compromise
between the need for good resolution and good overall accuracy in rainfall totals.
References
HMSO (1956)
Handbook of Meteorological Instruments
, Part 1, Met.0. 577.
HMSO (1982)
Observers Handbook
, Met.0. 933.
Painter, R.D. (1976) in
Methods of Plant Ecology
pp 369-410. Ed. by
S.B.Chapman, Blackwell Scientific Press, Oxford
Parkin, D.A., King, W.D. and Shaw, D.E. (1982)
An automatic raingauge
network for a cloud seeding experiment
J.Appl.Meteorol. p 228.
Rodda, J.C., (1967)
The rainfall measurement problem
Proc. IAHS Gen. Ass.
Bern, IAHS Pub. No. 78, 215-231
