Geoscience Reference
In-Depth Information
There is a considerable scientific literature study-
ing the accuracy and errors involved in measuring
rainfall. It needs to be borne in mind that a rain
gauge represents a very small point measurement
(or sample) from a much larger area that is covered
by the rainfall. Any errors in measurement will be
amplified hugely because the rain gauge collection
area represents such a small sample size. Because
of this amplification it is extremely important that
the design of a rain gauge negates any errors and
inaccuracies.
The four main sources of error in measuring
rainfall that need consideration in designing a
method for the accurate measurement of rainfall are:
Wetting loss
As the water trickles down the funnel it is inevitable
that some water will stay on the surface of the funnel
and can be lost to evaporation or not measured
in the collection tank. This is often referred to as a
wetting loss
. These losses will not be large but may
be significant, particularly if the rain is falling as
a series of small events on a warm day. In order to
lessen this loss it is necessary to have steep sides
on the funnel and to have a non-stick surface.
The standard UK Meteorological Office rain gauge
is made of copper to create a non-stick surface,
although many modern rain gauges are made of
non-adhesive plastics.
1
Losses due to evaporation
2
Losses due to wetting of the gauge
Rain splash
3
Over-measurement due to splash from the
surrounding area
The perfect rain gauge should measure the amount
of rainfall that would have fallen on a surface if the
gauge was not there. This suggests that the ideal
situation for a rain gauge is flush with the surface.
A difficulty arises, however, as a surface-level gauge
is likely to over-measure the catch due to rain land-
ing adjacent to the gauge and splashing into it. If
there was an equal amount of splash going out of
the gauge then the problem might not be so severe,
but the sloping sides of the funnel (to reduce
evaporative losses) mean that there will be very little
splash-out. In extreme situations it is even possible
that the rain gauge could be flooded by water
flowing over the surface or covered by snow. To
overcome the splash, flooding and snow coverage
problem the rain gauge can be raised up above the
ground (Figure 2.5) or placed in the middle of a
non-splash grid (see Figure 2.6).
4
Under-measurement due to turbulence around
the gauge.
Evaporation losses
A rain gauge can be any collector of rainfall with
a known collection area; however, it is important
that any rainfall that does collect is not lost again
through evaporation. In order to eliminate, or at
least lessen this loss, rain gauges are funnel shaped.
In this way the rainfall is collected over a reason-
ably large area and then any water collected is passed
through a narrow aperture to a collection tank
underneath. Because the collection tank has a
narrow top (i.e. the funnel mouth) there is very
little interchange of air with the atmosphere above
the gauge. As will be explained in Chapter 3, one
of the necessary requirements for evaporation is the
turbulent mixing of saturated air with drier air
above. By restricting this turbulent transfer there
is little evaporation that can take place. In addition
to this, the water awaiting measurement is kept out
of direct sunlight so that it will not be warmed;
hence there is a low evaporation loss.
Turbulence around a raised gauge
If a rain gauge is raised up above the ground (to
reduce splash) another problem is created due to air
turbulence around the gauge. The rain gauge
presents an obstacle to the wind and the consequent
aerodynamic interference leads to a reduced catch
(see Figure 2.7). The amount of loss is dependent on
