Environmental Engineering Reference
In-Depth Information
precipitation of 1600 mm, the coefficient of variation would be 12·5 per cent, but with the
same standard deviation and a mean of only 400 mm the coefficient of variation would
rise to 50 per cent. This is a useful measure, since it gives an indication of the importance
of the variability. In Britain coefficients of variation of annual rainfall range between 10
per cent in north-west Scotland and about 20 per cent in south-east England.
SPATIAL VARIATIONS OF PRECIPITATION
We all know that annual rainfall totals vary from one part of the world to another, even
when altitude is allowed for. Locally, however, it seems likely that annual totals will be
fairly consistent. It is also clear that, in the short term, quite marked differences in rainfall
may occur within short distances, depending upon the route taken by a particular storm or
cyclone; indeed, it is possible for it to be raining on one side of the street and dry on the
other.
In order to study spatial variation on a local scale, we need a dense network of
recording gauges, for otherwise individual storms may be missed as they pass between
the rain gauges. One such investigation was carried out in Illinois, where fifty recording
rain gauges were set up in an area of 1400 km
2
of flat rural land. The experiment was
maintained for five years, measuring individual storms, and for thirteen years for monthly
and seasonal analyses. Comparisons were made by correlating rainfall at a gauge at the
centre of the area with all other gauges. Correlation is a statistical measure which
provides an index of the strength of the linear relationship between two variables; a value
of +1·0 indicates a perfect positive linear relationship, a value of −1·0 shows a perfect
negative linear relationship, and a value of 0·0 shows no relationship (Figure 5.9).
For the shortest time period studied (one minute) the degree of correlation fell rapidly
with distance from the
Figure 5.9
Scatter plots demonstrating different correlation
coefficients
.
