|Figure 2.13 Rain gauges. A simple rain gauge
consists of a straight-sided can in which the
depth of water accumulated each day can be
measured with a dipstick. An improved design
incorporates a funnel, to reduce evaporation, and
a calibrated collection bottle. A rain gauge should
be set firmly in the soil away from overhanging
trees etc. and the rim should be 300 mm above
ground to prevent water flowing or bouncing
from surrounding ground.
The term precipitation covers all the ways in which water reaches the
ground as rain, snow and hail. It is usually measured with a rain gauge
(see Figure 2.13).
Simple rain gauges are based on straight-sided cans set in the ground
with a dipstick used to determine the depth of water collected. Accurate
readings to provide daily totals are achieved with a design that
maximizes collection, but minimizes evaporation losses by intercepting
the precipitation water in a funnel. This leads to a tapered measuring
glass calibrated to 0.1 mm. These gauges are positioned away from
anything that affects the local airflow e.g. buildings, trees and shrubs.
They are set in the ground but with the rim above it to prevent water
running in from the surroundings.
Recording rain gauges
are available which also give more details of the
pattern of rainfall within twenty-four hour periods. The 'tipping bucket’
type has two open containers on a see-saw mechanism so arranged that
as one bucket is filled, it tips and this is recorded on a continuous chart;
meanwhile the other bucket is moved into position to continue collection.