is concerned. In such a case most of the water is absorbed by the few upper inches of soil, only to be re-evaporated during the next few days, and the small proportion which sinks into the ground probably issues in springs many months later. Thus the actual yield of rainfall to the streams depends largely upon the mode of its time-distribution, and without a knowledge of this it is impossible to anticipate the yield of a particular rainfall. In estimating the evaporation to be deducted from the rainfall for the purpose of determining the flow into a reservoir, it is important to bear in mind that the loss from a constant water surface is nearly one and a half times as great as from the intermittently saturated land surface. Even neglecting the isolated and local discharges due to excessive and generally unrecorded rainfall, the variation in the discharge of all streams, and especially of mountain streams, is very great. We have seen that the average flow from mountain areas in Great Britain towards the end of a dry season does not exceed one-fifth of a cubic foot per second per 1000 acres. Adopting this general minimum as the unit, we find that the flow from such areas up to about 5000 acres, whose mean annual rainfall exceeds 50 in., may be expected occasionally to reach 300 cub. ft., or 1500 such units; while from similar areas of 20,000 or 30,000 acres with the same mean rainfall the discharge sometimes reaches 1200 or 1300 such units. It is well to compare these results with those obtained from much larger areas but with lower mean rainfall. The Thames at Teddington has been continuously gauged by the Thames Conservators since 1883, and the Severn at Worcester by the writer, on behalf of the corporation of Liverpool, during the 10 years 1881 to 1890 inclusive. The highest flood, common to the two periods, was that which occurred in the middle of February 1883. On that occasion the Thames records gave a discharge of 7·6 cub. ft. per second per 1000 acres, and the Severn records a discharge of 8·6 cub. ft. per second per 1000 acres, or 38 and 43 respectively of the above units; while in February 1881, before the Thames gaugings were commenced, the Severn had risen to 47 of such units, and subsequently in May 1886 rose to 50 such units, though the Thames about the same time only rose to 13. But in November 1894 the Thames rose to about 80 such units, and old records on the Severn bridges show that that river must on many occasions have risen to considerably over 100 units. In both these cases the natural maximum discharge is somewhat diminished by the storage produced by artificial canalization of the rivers.
These illustrations of the enormous variability of discharge
serve to explain what is popularly so little understood, namely,
the advantage which riparian owners, or other persons
interested in a given stream, may derive from works
constructed primarily for the purpose of diverting
Compensation water.the water of that stream—it may be to a totally
different watershed—for the purposes of a town supply. Under
modern legislation no such abstraction of water is usually
allowed, even if limited to times of flood, except on condition
of an augmentation of the natural dry-weather flow, and this
condition at once involves the construction of a reservoir. The
water supplied to the stream from such a reservoir is known
as “compensation water,” and is generally a first charge upon
the works. This water is usually given as a continuous and
uniform flow, but in special cases, for the convenience of mill-owners,
as an intermittent one.[1]
In the manufacturing districts
of Lancashire and Yorkshire it generally amounts to one-third
of the whole so-called “available supply.” In Wales it is usually
about one-fourth, and elsewhere still less; but in any case it
amounts to many times the above unit of one-fifth of a cubic
foot per second per 1000 acres. Thus the benefit to the fisheries
and to the riparian owners generally is beyond all question; but
the cost to the water authority of conferring that benefit is
also very great—commonly (according to the proportion of the
natural flow intended to be rendered uniform) 20 to 35% of
the whole expenditure upon the reservoir works. Down to the
middle of the 19th century, the proportioning of the size of a
Yield of stream
with reservoir.reservoir
to its work was a very rough operation.
There were few rainfall statistics, little was known
of the total loss by evaporation, and still less of its
distribution over the different periods of dry and
wet weather. Certain general principles have since been laid
down, and within the proper limits of their application have
proved excellent guides. In conformity with the above-mentioned
convention (by which compensation water is determined
as a certain fraction of the average flow during the three driest
consecutive years) the available supply or flow from a given
area is still understood to be the average annual rainfall during
those years, less the corresponding evaporation and absorption
by vegetation. But this is evidently only the case when the
reservoir impounding the water from such an area is of just
sufficient capacity to equalize that flow without possible exhaustion
in any one of the three summers. If the reservoir were
larger it might equalize the flow of the four or more driest
consecutive years, which would be somewhat greater than that
of the three; if smaller, we might only be able to count upon
the average of the flow of the two driest consecutive years, and
there are many reservoirs which will not yield continuously
the average flow of the stream even in the single driest year.
With further experience it has become obvious that very few
reservoirs are capable of equalizing the full flow of the three
consecutive driest years, and each engineer, in estimating the
yield of such reservoirs, has deducted from the quantity ascertained
on the assumption that they do so, a certain quantity
representing, according to his judgment, the overflow which in
one or more of such years might be lost from the reservoir.
The actual size of the reservoir which would certainly yield
the assumed supply throughout the driest periods has therefore
been largely a matter of judgment. Empirical rules have grown
up assigning to each district, according to its average rainfall,
a particular number of days’ supply, independently of any inflow,
as the contents of the reservoir necessary to secure a given yield
throughout the driest seasons. But any such generalizations
are dangerous and have frequently led to disappointment and
sometimes to needless expenditure. The exercise of sound
judgment in such matters will always be necessary, but it is
nevertheless important to formulate, so far as possible, the
conditions upon which that judgment should be based. Thus
in order to determine truly the continuously available discharge
of any stream, it is necessary to know not only the mean flow
of the stream, as represented by the rainfall less the evaporation,
but also the least favourable distribution of that flow throughout
any year.
The most trying time-distribution of which the author has had experience in the United Kingdom, or which he has been able to discover from a comparison of rainfalls upon nearly impermeable areas exceeding 1000 acres, is graphically represented by the thick irregular line in the left-hand half of fig. 3, where the total flow for the driest year measures 100 on the vertical percentage scale; the horizontal time scale being divided into calendar months.
The diagram applies to ordinary areas suitable for reservoir construction and in which the minimum flow of the stream reaches about one-fifth of a cubic foot per second per 1000 acres. Correspondingly, the straight line a a represents uniformly distributed supply, also cumulatively recorded, of the same quantity of water over the same period. But, apart from the diurnal fluctuations of consumption which may be equalized by local “service reservoirs,” uniform distribution of supply throughout twelve months is rarely what we require; and to represent the demand in most towns correctly, we should increase the angle of this line to the horizontal during the summer and diminish it during the winter months, as indicated by the dotted lines b b. The most notable features of this particular diagram are as follows: Up to the end of 59 days (to the 28th February) the rate of flow is shown, by the greater steepness of the thick line, to be greater than the mean for the year, and the surplus water—about 11% of the flow during the year—must be stored; but during the 184 days between this and the end of the 243rd day (31st August) the rate of flow is generally below the mean, while from that day to the end of the year it is again for the most part above the mean. Now, in order that a reservoir may enable the varying flow, represented cumulatively by the irregular line, to be discharged in a continuous and uniform flow to satisfy a demand represented
- ↑ The volume of compensation water is usually fixed as a given fraction of the so-called “available supply” (which by a convention that has served its purpose well, is understood to be the average flow of the stream during the three consecutive driest years).
