In this manner, the water, even below the drainage level,
is changed with each addition at the surface. In a barrel filled with
coarse pebbles, the water of saturation would maintain a nearly level
surface; if the material were more compact and retentive, a true level
would be attained only after a considerable time. Toward the end of the
flow, the water would stand highest at the points furthest distant from
the outlet. So, in the land, after a drenching rain, the water is first
removed to the full depth, near the line of the drain, and that midway
between two drains settles much more slowly, meeting more resistance from
below, and, for a long time, will remain some inches higher than the floor
of the drain. The usual condition of the soil, (except in very dry
weather,) would be somewhat as represented in the accompanying cut, (Fig.
12.)
[Illustration: Fig. 12 - LINE OF SATURATION BETWEEN DRAINS.]
Fig. 12 - LINE OF SATURATION BETWEEN DRAINS.
_YY are the draings. The curved line b is the line of saturation, which
has descended from a, and is approaching c._
To provide for this deviation of the line of saturation, in practice,
drains are placed deeper than would be necessary if the water sunk at once
to the level of the drain floor, the depth of the drains being increased
with the increasing distance between them.
Theoretically, every drop of water which falls on a field should sink
straight down to the level of the drains, and force a drop of water below
that level to rise into the drain and flow off.
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