Irrigation—Principles & Practices
Part 1 – 208 | Unit 1.5
C. Units of Water Measurement
- Definition of terms
a) Acre inch: The equivalent volume of water application that would cover one acre of
land one inch deep in water. Example: On average, approximately one inch of water is
lost through evaporation and plant transpiration each week from May 15th–October 15
along the central coast of California (see Appendix 2, Units of Water Measurement)
b) Acre foot: The equivalent volume of water application that would cover one acre of land
one foot deep in water
c) Gallons per minute (GPM): The number of gallons being delivered through an irrigation
system in one minute
d) One hundred Cubic Feet (CCF): Term commonly used by municipal water providers as a
means of water measurement based on volume. 1 CCF equals 748 gallons.
e) Pounds per square inch (PSI): Water pressure in irrigation systems is measured in PSI.
Determining your irrigation system’s specific PSI is important in irrigation planning.
f) Distribution Uniformity (DU): A measure of how uniformly water is applied to the area
being irrigated, expressed as a percentage. The higher the DU percentage, the more
uniform the application. See Appendix 3, Calculating Distribution Uniformity, for
additional information.
D. Overview of Garden vs. Field-Scale Irrigation (to be further discussed in Lecture 2,
Irrigation Scheduling and Delivery Systems)
- Features of garden- and field-scale cropping systems that influence irrigation
a) Gardens: Smaller, more diverse, hand cultivated
b) Garden irrigation water sources
i. Domestic wells with 2 to 5 horsepower submersible pumps (10 gal/minute average
output)
ii. Municipal water systems (for urban gardens)
c) Fields: Larger blocks of plantings, mechanically cultivated
d) Field irrigation water sources; note—minimum 10 gal/acre/minute recommended
i. Agricultural wells (10 horse power or larger electric or diesel pumps / 50 gallons per
minute minimum)
ii. Surface sources supplemental to well water (ponds, creeks)
iii. Water district deliveries from surface sources supplied through canals or pipe lines
- Similarities in irrigation scheduling and delivery systems between the two settings
a) Whether operating on a garden scale or field scale, irrigation managers need to make
decisions about water application rates and type of delivery based on crop needs, weed
management, disease potential, evapotranspiration (ET) rates, and harvest schedules
while using both labor and water wisely and efficiently.
- Differences in irrigation scheduling between the two settings
a) Garden scale: Typically use “soil moisture by feel” (qualitative) approach to determine
need for irrigation, as well as scheduling and reference to local ET rates (see Lecture 2
and Appendix 4, Estimating Soil Moisture by Feel)
b) Field scale: Typically use water budgeting (quantitative) approach along with
tensiometers or other moisture monitoring devices to determine need for irrigation (see
more at Lecture 2, as well as Supplement 2, Overview of “Water Budget” Approach for
Efficient Irrigation Management, and Supplement 3, Soil Moisture Sensing Instruments
Commonly Used for Irrigation Schedules)
Lecture 1: Irrigation—Concepts & Terminology