Answer:
Water Measurement Units
There are two conditions under which water is measured—water at rest and water in motion. Water at rest is measured in units of volume. Water in motion is measured in units of flow— unit of volume for a convenient time unit. It is important that the difference between a unit of volume and a unit of flow be kept in mind.
Volume Units
Water at rest; i.e., ponds, lakes, reservoirs, and in the soil, is measured in units of volume — gallon, cubic foot, acre-inch, and acre-foot.
Cubic Foot – The volume of water that would be held in a container one foot wide by one foot long by one foot deep.
Acre-lnch – The volume of water that would cover one acre (43,560 square feet) one inch deep.
Acre-Foot – The volume of water that would cover one acre one foot deep.
Flow Units
Water in motion; i.e., flowing in streams, canals, pipelines, and ditches, is measured in units of volume per unit of time—gallons per minute (gpm), cubic feet per second (cfs), acre-inches per hour and acre feet per day. Cubic feet per second, sometimes written second-feet (sec. ft. or cusec) is most commonly used for measuring flow of irrigation water moving by gravity from streams and reservoirs. Gallons per minute is most commonly used for measuring flow from pumps.
Cubic foot per second – The quantity of water equivalent to a stream one foot wide by one foot deep flowing with a velocity of one foot per second.
Gallon per minute – The quantity of water equivalent to a stream which will fill a gallon measure once each minute.
A flow of one cfs is approximately equal to either 450 gpm, one acre-inch per hour, or two acre-feet per day (24 hours).
Conversion Factors
The following equivalents are useful for converting from one unit to another and for calculating volumes from flow units.
Volume Units
One gallon
= 231 cubic inches
= 0.13368 cubic foot weighs approximately 8.33 pounds
One cubic foot
= 1,728 cubic inches
= 7.481 gallons (7.5 for ordinary calculations) weights 62.4 pounds (62.5 for ordinary calculations)
One acre-inch
= 3,630 cubic feet
= 27,154 gallons (27,200 for ordinary calculations)
= 1/12 acre-foot weighs approximately 113.1 tons
One acre-foot
= 43,560 cubic feet
= 325,851 gallons
= 12 acre-inches weighs approximately 1,357 tons
Flow Units
One gallon per minute
= 0.00223 (approximately 1/450 ) cubic foot per second
= 0.00221 acre-inch per hour
= 0.00442 acre-foot per (24 hour) day
= 1 acre-inch in 452.6 hours (450 for ordinary calculations)
= 1 acre-foot in 226.3 days
One cubic foot per second
= 448.83 gallons per minute (450 for ordinary calculations)
= 1 acre-inch in 1 hour and 30 seconds (1 hour for ordinary calculations)
= 1 acre-foot in 12 hours and 6 minutes (12 hours for ordinary calculations)
= 1.984 acre-feet per (24 hours) day (2 acre-feet for ordinary calculations)
Million gallons per day (mgd)
= 694.4 gallons per minute (695 for ordinary calculations)
= 1.547 cubic feet per second (1.5 for ordinary calculations)
Conversion Table.
To convert from: To: Multiply by:
Cubic-ft Gallons 7.5
Acre-in Cubic-ft 3,630
Acre-ft Cubic-ft 43,560
Acre-ft Acre-in 12
cfs gpm 450
Acre-in per hr cfs 1
Acre-ft per hr cfs 12

Figure 1. Conversion from gpm to Acre-in for different water application durations.

