they contained as the altitude increased. But until the clouds rise
above the hill there is an obvious countervailing tendency to compression,
and in steep slopes this may reduce or entirely prevent precipitation
until the summit is reached, when a fall of pressure with
commotion must occur. Very high mountain ranges usually consist
of many ridges, among which rain-clouds are entangled in their
ascent, and in such cases precipitation towards the windward side of
the main range, though on the leeward sides of the minor ridges of
which it is formed, may occur to so large an extent that before the
summit is reached the clouds are exhausted or nearly so, and in this
case the total precipitation is less on the leeward than on the windward
side of the main range; but in the moderate heights of the
United Kingdom it more commonly happens from the causes explained
that precipitation is prevented or greatly retarded until the
summit of the ridge is reached. The following cause also contributes
to the latter effect. Imagine eleven raindrops A to K to fall simultaneously
and equi-distantly from the horizontal plane A M. A strong
wind is urging the drops from left to right. The drops A and K may
be readily conceived to be equally diverted by the wind, and to fall
near the tops of the two hills respectively. Not so drop C, for directly
the summit is passed the wind necessarily widens out vertically and,
having a greater space to fill, loses forward velocity.
It may even
eddy backwards, as indicated by the curved arrows, and it is no
uncommon thing, in walking up a steep hill in the contrary direction
to the flight of the clouds, to find that the rain is coming from
behind. Much the same tendency exists with respect to all drops
between B and E, but at F the wind has begun to accommodate itself
to the new regime and to assume more regular forward motion, and
as J is approached, where vertical contraction of the passage through
which the wind must pass takes place, there is an increasing tendency
to lift the raindrops beyond their proper limits. The general effect
is that the rain falling from between G and K is spread over a greater
area of the earth G′K′ than that falling from the equal space between
B and F, which reaches the ground within the smaller area B′F′.
From this cause also, therefore, the leeward side of the valley receives
more rain than the windward side.
In the United Kingdom
the prevailing winds are from the south-west, and some misapprehension
has been caused by the bare, but perfectly correct, statement
that the general slope towards the western coast is wetter than
that towards the eastern. Over the whole width of the country from
coast to coast, or of the Welsh mountain ranges only, this is so;
but it is nevertheless true that the leeward side of an individual valley
or range of hills generally receives more rain than the windward side.
Successive abstraction of raindrops as the rain-clouds pass over
ridge after ridge causes a gradually diminishing precipitation, but
this is generally insufficient to reverse the local conditions, which
tend to the contrary effect in individual ranges.
The neglect of
these facts has led to many errors in estimating the mean rainfall on
watershed areas from the fall observed at gauges in particular parts
of those areas.
In the simplest case of a single mountain valley to be used for the supply of an impounding reservoir, the rainfall should be known at five points, three being in the axis of the valley, of which one is near the point of intersection of that axis with the boundary of the watershed. Then, in order to connect with these the effect of the right- and left-hand slopes, there should be at least one gauge on each side about the middle height, and approximately in a line perpendicular to the axis of the valley passing through the central gauge. The relative depths recorded in the several gauges depend mainly upon the direction of the valley and steepness of the bounding hills. The gauge in the bottom of the valley farthest from the source will in a wide valley generally record the least rainfall, and one of those on the south-west side, the highest. Much will depend upon the judicious placing of the gauges. Each gauge should have for 10 or 15 yds. around it an uninterrupted plane fairly representing the general level or inclination, as the case may be, of the ground for a much larger distance around it. The earliest records of such gauges should be carefully examined, and if any apparently anomalous result is obtained, the cause should be traced, and when not found in the gauge itself, or in its treatment, other gauges should be used to check it. The central gauge is useful for correcting and checking the others, but in such a perfectly simple case as the straight valley above assumed it may be omitted in calculating the results, and if the other four gauges are properly placed, the arithmetical mean of their results will probably not differ widely from the true mean for the valley. But such records carried on for a year or many years would afford no knowledge of the worst conditions that could arise in longer periods, were it not for the existence of much older gauges not far distant and subject to somewhat similar conditions. The nearer such long-period gauges are to the local gauges the more likely are their records to rise and fall in the same proportion. The work of the late Mr James Glaisher, F.R.S., of the late Mr G. J. Symons, F.R.S., of the Meteorological Office and of the Royal Meteorological Society, has resulted in the establishment of a vast number of rain-gauges in different parts of the United Kingdom, and it is generally, though not always, found that the mean rainfall over a long period can be determined, for an area upon which the actual fall is known only for a short period, by assigning to the missing years of the short-period gauges, rainfalls bearing the same proportion to those of corresponding years in the long-period gauges, that the rainfalls of the known years in the short-period gauges bear to those of corresponding years in the long-period gauges. In making such comparisons, it is always desirable, if possible, to select as standards long-period gauges which are so situated that the short-period district lies between them. Where suitably placed long-period gauges exist, and where care has been exercised in ascertaining the authenticity of their records and in making the comparisons, the short records of the local gauges may be thus carried back into the long periods with nearly correct results.
Rainfall is proverbially uncertain; but it would appear from the most trustworthy records that at any given place the total rainfall during any period of 50 years will be within 1 or 2% of the total rainfall at the same place during any other period of 50 years, while the records of any period of 25 years will generally be found to fall within 312% of the mean of 50 years. It is equally satisfactory to know that there is a nearly constant ratio on any given area (exceeding perhaps 1000 acres) between the true mean annual rainfall, the rainfall of the driest year, the two driest consecutive years and any other groups of driest consecutive years. Thus in any period of 50 years the driest year (not at an individual gauge but upon such an area) will be about 63% of the mean for the 50 years.
That in the two driest consecutive years will be about 75% of the mean for the 50 years.
That in the three driest consecutive years will be about 80% of the mean for the 50 years.
That in the four driest consecutive years will be about 83% of the mean for the 50 years.
That in the five driest consecutive years will be about 85% of the mean for the 50 years.
That in the six driest consecutive years will be about 8612% of the mean for the 50 years.
Apart altogether from the variations of actual rainfall produced by irregular surface levels, the very small area of a single rain-gauge is subject to much greater variations in short periods than can possibly occur over larger areas. If, therefore, instead of regarding only the mean rainfall of several gauges over a series of years, we compare the relative falls in short intervals of time among gauges yielding the same general averages, the discrepancies prove to be very great, and it follows that the maximum possible intensity of discharge from different areas rapidly increases as the size of the watershed decreases. Extreme cases of local discharge are due to the phenomena known in America as “cloud-bursts,” which occasionally occur in Great Britain and result in discharges, the intensities of which have rarely been recorded by rain-gauges. The periods of such discharges are so short, their positions so isolated and the areas affected so small, that we have little or no exact knowledge concerning them, though their disastrous results are well known. They do not directly affect the question of supply, but may very seriously affect the works from which that supply is given.
Where in this article the term “evaporation” is used alone, it is to be understood to include absorption by vegetation. Of the total quantity of rainfall a very variable proportion is rapidly absorbed or re-evaporated. Thus in the western mountain districts of Great Britain, Evaporation and absorption. largely composed of nearly impermeable rocks more or less covered with pasture and moorland, the water evaporated and absorbed by vegetation is from 13 to 15 in. out of a rainfall of 80 in., or from 16 to 19%, and is nearly constant down to about 60 in., where the proportion of loss is therefore from 22 to 25%. The Severn down to Worcester, draining 1,256,000 acres of generally flatter land largely of the same lithological character, gave in the dry season from the 1st of July 1887 to the 30th of June 1888 a loss of 17·93 in. upon a rainfall of 27·34 in. or about 66%; while in the wet season, 1st of July 1882 to the 30th of June 1883, the loss was 21·09 in. upon a rainfall of 43·26 in., or only 49%. Upon the Thames basin down to Teddington, having an area of 2,353,000 acres, the loss in the dry season from the 1st of July 1890 to the 30th of June 1891 was 17·22 in. out of a rainfall of 21·62 in., or 79%; while in the wet season, 1st of July 1888 to the 30th of June 1889, it was 18·96 out of 29·22 in., or only 65%. In the eastern counties the rainfall is lower and the evaporation approximately the same as upon the Thames area, so that the percentage of loss is greater. But these are merely broad examples and averages of many still greater variations over smaller areas. They show generally that, as the rainfall increases on any given area evaporation increases, but not in the same proportion. Again, the loss from a given rainfall depends greatly upon the previous season. An inch falling in a single day on a saturated mountain area will nearly all reach the rivers, but if it falls during a drought seven-eighths may be lost so far as the period of the drought
