Chapter 11: Sprinkle Irrigation - NRCS Irrigation ToolBox Home Page

gation guides. In the absence of these, Chapter 1
can be used to gain an insight into the computation
process.
WaterbHolding Capacity
Soils of various textures have varying abilities to
retain water. Except in the case of required periodic
leaching, any irrigation beyond the field capacity of
the soil is an economic loss. Table 11-1, which was
taken from Chapter 1, gives typical ranges of available
water-holding capacities of soils of different
textures (field capacity minus permanent wilting
point) and is presented here for convenience. If local
data are not available, the listed averages may be
used as a guide.
The total amount of soil water available for plant
use in any soil is the sum of the available waterholding
capacities of all horizons occupied by plant
roots,
Table 11-1,-Range in available water-holding capacity
of soils of different texture
Inches of water per
foot of depth
Range Average
1. Very coarse texture-very
coarse sands 0.40 to 0.75 0.6
2. Coarse texture-coarse
sands, fine sands, and
loamy sands 0.75 to 1.28 1.0
3, Moderately coarse
texture-sandy
loams 1.25 to 1.75 1.5
4. Medium texture-very fine
sandy loams, loams, and
silt loams 1.50 to 2.30 2.0
5. Moderately fine textureclay
loams, silty clay
loams, and sandy clay
loams 1.75 to 2.50 2.2
6. Fine texture-sandy clays,
silty clays, and clays 1.60 to 2.50 2.3
7, Peats and mucks 2.00 to 3.00 2.5
Chapter 1, Section 15, Soil-Plant- Water Relationships,
Root Depth
Typical plant f der root and total root depth are
given in many references; however, the actual
depths of rooting of the various crops are affected
by soil conditions and should be checked at the site.
Where local data are not available and there are no
expected root restrictions, table 11-2 can be used
E~EJ a guide to estimating the effective root depths of
various crops,
The values given are averages selected from several
references. They represent the depth at which
crops will get most of their needed water when they
are grown in a deep, well-drained soil that is adequately
irrigated.
Application Depth and Frequency
For periodic-move, and low-frequency continuousmove
systems such as traveling sprinklers, it is
desirable to irrigate as infrequently as practical to
reduce labor costs. A general rule of thumb for
crops in arid and semiarid regions is that the soil
moisture deficit (SMD) within the root zone should
not fall below 50 percent of the total availablewater-holding
capacity, This is a managementallowed
deficit, MAD = 50%. It is also desirable to
bring the moisture level back to field capacity with
each irrigation; therefore, the duration of each irrigation
is identical.
In humid regions it is necessary to allow for rains
during the irrigation period; however, the 50 percent
limitation on soil moisture depletion should be
followed for design purposes.
Local soil conditions, soil management, water
management, and economic considerations determine
the amount of water used in irrigating and the
rate of water application. The standard design approach
has been to determine the amount of water
needed to fill the entire root zone to field capacity
and, then, to apply at one application a larger
amount to account for evaporation, leaching, and
efficiency of application. The traditional approach
to the frequency of application has been to assume
MAD = 50%, then take the number of inches of
water in the root zone reservoir that can be extracted
and, using the daily consumptive use rate of
the plant, determine how long this supply will last.
Such an approach is useful only as a guide to irrigation
requirements because many factors affect the
amount of irrigation water and the timing of applications
for optimal design and operation of a system.