Environmental Engineering Reference
In-Depth Information
have been experiments in measuring snow depth photogrammetrically, with aerial or
satellite photography. Where the snowfall is substantial the depth can be obtained fairly
accurately, but without ground observations the water equivalent of the snow is unknown.
Whichever approach is used, measurements of the water equivalent of snowfall always
entail problems and probable inaccuracies. We have just to accept that, apart from a few
areas of intensive observations, we do not know the precise input of water to the ground
surface by snow.
HAIL
Hail measurement is even more difficult. Hailstones possess considerable kinetic energy
and many will bounce out of a conventional gauge, causing underestimation of their
water contribution. The size distribution of hailstones can be obtained from a hail pad
which measures the degree of impaction made by the stones. If pads are left out for
known times, the amount of ice and water equivalent can be found. Fortunately hail is
normally insignificant as a precipitation input to the hydrological cycle, so it is normally
recorded in terms of the number of days with hail.
FOG-DRIP AND DEWFALL
The water content of fog-drip and dew is small, so special measurement techniques have
to be used. Fog-drip falls to the surface after contact with leaves or trees, so trough-
shaped rain gauges have been designed to increase the sampling area and make
measurements more accurate. In principle they work like an ordinary gauge.
The most commonly used instrument for dewfall is an accurate weighing device. The
dewdrops collect on hygroscopic plates which are attached to a balancing system to
weigh the amount of water collected. All methods suffer from the basic uncertainty about
how accurately the gauges collect dew, compared with natural surfaces. Fortunately,
water quantities are minute, so that even large errors are insignificant in relation to the
total precipitation input.
TEMPORAL VARIATIONS OF PRECIPITATION
SHORT-TERM VARIABILITY
The variations of rainfall over time are of vital importance to engineers, environmental
scientists and hydrologists. For example, decisions about bridge size, storm sewer
construction, river quality changes and even flood protection measures must be taken by
experts on the basis of the expected inputs of precipitation. For this type of decision a
single total is not very informative. We need to know not only how much rain is likely to
fall but over what period of time. Twenty-five millimetres of rainfall in a day may not be
significant, but if that amount fell in an hour, or even less, there could be drastic
consequences. Surface run-off might occur, soil erosion might be initiated, streams might
start to swell and flooding might result as happened in Lewes, East Sussex, in October
2000. Clearly, the intensity of precipitation is extremely important.
