IRRIGATION 
hours will cover one acre two feet deep, 
equal to two acre-feet. 
Approximate Estimate of Water 
It requires an engineer with proper 
instruments to obtain an accurate esti- 
mate of the amount of water flowing in 
a ditch, but an approximate estimate 
may be made as follows: Secure the 
cross section by measuring. the depth in 
a number of places at given distances 
across the stream, adding them together 
and dividing by the number of measure- 
ments taken. This will give the average 
depth, which should be multiplied by the 
width of the stream at the surface. This 
will give the number of square feet in 
the cross-section of the stream. Then 
measure off on the ditch bank any dis- 
tance, say 20 feet parallel with the cur- 
rent. Throw a stick into the center of 
the stream where the water is running 
at its greatest velocity and far enough 
above the first measurement so that the 
stick will have time to acquire the same 
velocity as the stream before it reaches 
the point of the first measurement. Take 
the time in seconds required for the stick 
to float the distance measured. This will 
give the velocity of the mid-current. 
1171 
Eighty-three per cent of this will give 
the average mean velocity of the whole 
stream. Multiplying the number of 
square feet in the cross-section by the 
velocity of the stream, in seconds, will 
give the number of cubic feet per second 
of time. 
In the case of small ditches reduce all 
measurements to inches. On account of 
the filamentous or thread-like character 
of water there is a tendency to compres- 
sion as it passes through an orifice. After 
the discharge it continues to converge 
so that the section of the water after 
leaving the orifice is smaller than the 
orifice from which it discharges. This is 
because the inertia of the water opposes 
any change in direction, and the converg- 
ence continues for a distance of about 
half the diameter of the orifice. 
Laterals 
A lateral is a ditch taken from the 
sides of the main ditch. 
A sub-lateral is taken from the side 
of a lateral. 
In taking water from the main chan- 
nel, leading it out in different direc- 
tions over the lands, many laterals and 
Fig. 1. 
Showing Method of Conducting Water Around a Low Spot at the Corner of an Orchard. 
