TRANSACTIONS OF SECTION G. 4.93 
so heavy, the water surface in the lake will not rise very much, nor will it greatly 
sink at the end of a long drought. Where there is no moderating lake, a river 
fed from a glacier has a precious source of supply. The hotter the weather, the 
more rapidly will the ice melt, and this is just when irrigation is most wanted, 
Elsewhere, if crops are to be raised and the rain cannot be counted on, nor 
well irrigation be practised, water storage becomes necessary, and it is with the 
help of water storage that in most countries irrigation is carried on. 
Water Storage. 
To one who has not given the subject attention surprise is often expressed at 
the large volume of water that has to be stored to water an acre of land. In the 
case of rice irrigation in India, it is found that the storage of a million cubic 
feet does not suffice tor more than from 6 to 8 acres. For the irrigation of 
wheat about one-third this quantity is enough. It would never pay to excavate 
on a level plain a hollow large enough to hold a million cubic feet of water, It 
is invariably done by throwing a dam across the bed of a river or a valley and 
ponding up the water behind it. Many points have here to be considered: The 
length of dam necessary, its height, the material of which it is to be constructed, 
the area and the value of the land that must be submerged, the area of the land 
that may be watered. The limits of the height of a dam are from about 150 
to 15 feet. If the slope of the valley is great, it may be that the volume which 
can be ponded up with adam of even 150 feet is inconsiderable, and the cost may 
be prohibitory. On the other hand, if the country is very flat, it may be that 
a dam of only 20 feet high may require to be of quite an inordinate length, 
and compensation for the area of land to be submerged may become a very large 
item in the estimate. I have known of districts so flat that in order to irrigate 
an acre more than an acre must be drowned. This looks ridiculous, but is not 
really so, for the yield of an irrigated acre may be eight or ten times that of an 
unirrigated one; and after the storage reservoir has been emptied it is often 
possible to raise a good crop on the saturated bed. 
The advantage of a deep reservoir is, however, very great, for the evaporation 
is in proportion to the area of the surface, and if two reservoirs contain the same 
volume of water, and the depth of one is double that of the other, the loss by 
evaporation from the shallow one will be double that of the deep one. In India, 
from time immemorial, it has been the practice to store water for irrigation, and 
there are many thousands of reservoirs, from the great artificial lakes holding as 
much as 5,000 or 6,000 millions of cubic feet, down to the humble village 
tank holding nota million. There are few of which the dam exceeds 80 feet 
in height, and such are nearly always built of masonry or concrete. For these 
it is absolutely necessary to have sound rock foundations. If the dam is to 
be of earth, the quality of the soil must be carefully seen to, and there should be 
a central core of puddle resting on rock and rising to the maximum height ot 
water surface. If the dam is of masonry, there may perhaps be no harm done 
should the water spill over the top. If it is of earth, this must never happen, and 
a waste weir must be provided, if possible cut out of rock or built of the best 
masonry, and large enough to discharge the greatest possible flood. More acci- 
dents occur to reservoirs through the want of sufficient waste weirs or their faulty 
construction than from any other cause. 
As important as the waste weir are the outlet sluices through which the water 
is conveyed for the irrigation of the fields. If possible they should be arranged 
to serve at the same time as scouring sluices to carry off the deposit that accumu- 
lates at the bottom of the reservoir. For, unless provided with very powerful 
scouring sluices, sooner or later the bed of the reservoir will become silted up, 
and the space available for water storage will keep diminishing. As this happens 
in India, it is usual to go on raising the embankment (for it does not pay to dig 
out the deposit), und so the life of a reservoir may be prolonged for many years. 
Ultimately it is abandoned, as it is cheaper to make a new reservoir altogether 
than to dig out the old one, 
