Soil and Water Management
Rice Irrigation - Methods
Sprinkler Irrigated Rice •
Land Grading •
Establishing Levees •
Water Delivery to
Fields •
Multiple Inlet
Irrigation
Sprinkler Irrigated
Rice
Sprinkler irrigation of rice is very limited at this writing. Research and
experience show that the best potential is either on clay or sandy loam soils
that are relatively free of johnsongrass. Many silt loam soils tend to crust
which causes excessive runoff and inadequate infiltration in the soil. This can
lead to drought stress or excessive irrigation which generally results in
decreased yields and increased pumping costs. Rutting and sticking of the center
pivot is also a potential problem. There is also a possibility that certain
disease problems could be increased when the foliage is wetted at the frequency
associated with sprinkler irrigation. Sprinkler irrigation should be used on an
experimental basis only, and the following recommendations should be considered:
- Don’t attempt on silt loam soils that tend to crust or seal.
- Use residual herbicide program.
- Be certain sufficient water is available during reproductive growth
(after joint movement).
- Be prepared to use phenoxy herbicides at midseason.
- Plant rice varieties with blast resistance.
Land Grading
Precision land grading is desirable, but not absolutely necessary. If you are
considering precision grading, make certain that cut areas won’t expose a
subsoil with undesirable characteristics. The Natural Resource Conservation
Service has county soil descriptions that can be helpful. Also, taking several
deep (> 6 inch depth) soil cores or samples may be beneficial if a problem soil
is suspected. The application of poultry litter has also shown good results for
improving rice yields on cut soil areas.
When a field is precision graded, it is recommended that a slope of no less
than 0.05 percent (0.05 foot per 100 feet) should be provided in at least one
direction. A slope of 0.1 percent (0.1 foot per 100 feet) is the general
recommendation because it provides good drainage and is often easier to
construct and maintain than flatter slopes. It is also recommended to consider
putting a field to grade in only one direction (i.e., zero cross slope) if it
doesn’t require a significant amount of extra dirt work. Building a permanent
pad or elevated road on one or more sides of a field should also be considered
in the grading plan. Settling often occurs in the deeper fill areas following a
grading job. If possible, touch up these areas before planting or provide field
drain furrows for improved drainage. The land grading design should consider the
type of drain outlets and the number required for the field. If possible, it is
best to provide an outlet point for every 20 acres.
It is not usually desirable to precision grade a field to zero slope
(zero-grade) in all directions unless continuous rice production is planned.
Rotation crops will usually perform better on zero-grade fields that are not
over 50 acres in size. It is also critical that the perimeter ditch around the
field have unrestricted drainage at its outlet(s). Another consideration for the
rotation crop would be to plant on a slightly raised bed but still install a
network of drain furrows in the field.
Yearly preplant field leveling or smoothing is essential for seedbed
preparation, surface drainage and maintaining optimum flood depths. A landplane
or float should be used to remove reverse grades, fill "potholes" and smooth out
old levees, rows or ruts in a field. Rice can germinate under either soil or
water, but not both. Therefore, maintaining a field surface that provides
good drainage is important for stand establishment; controlling weeds, diseases
and insects; maintaining desired flood depths; and providing a dry field for
harvesting.
Establishing Levees
An accurate levee survey is important to assure proper control of water. All
surveying instruments should be properly adjusted and checked for accuracy. Be
careful not to exceed the operating range or distance of the equipment. A levee
elevation difference of no more than 0.2 foot is generally recommended. This
difference is increased on steeper fields when narrow distances between levees
present a problem for combine operation. Premarking levees on clay soils and
establishing levees as soon as conditions allow can reduce water loss from levee
seepage. Levee gates should be installed early in case flushing is necessary and
also to provide outlets to avoid levee washouts in case of a heavy rain. One
gate per levee is usually adequate. Two gates may be necessary in small loop
levees near the water source and in larger bays (> 10 acres) to assure adequate
water control.
Water Delivery to Fields
Ditches and canals are sometimes used for water delivery to fields. There is
a certain amount of water loss associated with seepage and evaporation from
ditches. In addition, canals and ditches require continuous maintenance.
Replacing ditches and canals with either surface or underground pipe when
possible is desirable. Installing pipe not only eliminates seepage and
evaporation losses but provides more accurate water control and may return land
back to production.
Flexible irrigation tubing may be used to replace ditches and canals. The
tubing is designed for low pressure and comes in various thicknesses that have
different pressure capacities. If water will flow in the ditch or irrigation
canal, then the tubing should be applicable to the situation. The minimum
thickness recommended for this application is 9 mL. This can be an alternative
when installing underground pipe is not affordable.
Multiple Inlet Irrigation
The basic concept of multiple inlets is to proportion the irrigation water
evenly over the whole field at one time. The proportioning is accomplished by
placing irrigation tubing across each paddy (area between levees) and releasing
water into each paddy at the same time through holes or gates in the tubing.
Tubing can be placed along the side of the field or through the field depending
on the location of the irrigation source. This can be done on fields with
straight levees and also on fields that have crooked levees (see Figure 2).
Multiple inlets provide the potential for improved water management
in the following areas:
1. Can flood field quicker – increased fertilizer and herbicide
efficiency;
2. Reduces pumping time during season;
3. Reduces pumping cost;
4. Reduces amount of water pumped;
5. Reduces runoff from field;
6. Reduces irrigation labor;
7. Reduction in cold water effect;
8. Avoids risk of washing out levees from over-pumping top
levees;
9. Reduces problems associated with scum and algae buildup in
levee spills (gates).
A quicker flood time is usually achieved when it is possible to put the water
in at different points down the field. Once a flood is established with multiple
inlets, the levee gates can be set above the desired flood depth so that more
rainfall can be held on the field. Inlets to individual levees can provide more
precise water control for situations such as when there are one or more levees
that seem to dry out faster. The flow to these levees can be increased
independently of the other levees to avoid excessive pumping on the rest of the
field. Multiple inlet irrigation is possibly more easily managed on precision
graded fields that have uniform acreage between the levees, but it can still
offer improved water management on ungraded fields.
Information from field demonstrations and producer experiences indicates that
the average reduction in labor is 30 percent and the average reduction in
pumping is 25 percent. Some producers that are using multiple inlets on sandier
fields feel that it has reduced pumping by 50 percent. This is very possible
under certain conditions, but most producers will experience about a 25 percent
savings and in some situations there may be a minimal reduction in pumping
savings. Producers who aren’t experiencing a significant reduction in pumping
still use it for the other benefits it offers, especially the irrigation labor
savings.
A few potential disadvantages or problems that can occur with multiple inlets
are:
1. Cost of riser bonnets (universal hydrants) and irrigation
tubing;
2. Initial installation of irrigation tubing and initial
adjustment of the inlets (holes or gates);
3. Floating, moving and twisting of irrigation tubing early in
the season;
4. Working around or over irrigation tubing with field equipment
(i.e., spraying levees);
5. Animal damage to tubing – especially coyote;
6. Removal and disposal of tubing
Discussions about these problems/ disadvantages with producers using multiple
inlets indicate that most are willing to deal with these problems because of the
advantages offered.
To lay out multiple inlets for a field, a grower needs a good estimate or
measurement of the pumping capacity at the field and the field acreage. The
pumping capacity in gpm (gallons per minute) is divided by the field acreage to
get the ratio of gpm per acre. The estimated or measured acreage in each paddy
is then multiplied by this ratio to determine the amount of water to proportion
into each paddy. Following is an example of this process:
Pumping capacity – 1,500 gpm
Field Acreage – 100 acres
1,500 gpm divided by 100 acres = 15 gpm/acre
4 acre paddy: 15 gpm per acre x 4 acres = 60 gpm
7 acre paddy: 15 gpm per acre x 7 acres = 105 gpm
The required flow to each paddy is provided either through several punched
holes or a few adjustable gates. Either will work, but the adjustable gate seems
to be easier to manage. The most common adjustable gate has a 2.5 inch opening
that can be shut completely off or left open to flow approximately 75 gpm.
The 9 to 10 mL tubing is recommended for multiple inlet irrigation and the
suggested sizes for different flow rates are as follows:
12 inch – less than 1,200 gpm;
15 inch – 1,200 to 2,200 gpm; and
18 inch – greater than 2,200 gpm.
The tubing is usually placed over the levees or along the side of the field
on the permanent pad. In both cases it is recommended that the tubing be placed
in a shallow trench when it is installed. In some fields where an outside levee
is pulled, it is possible to place the tubing in the borrow ditch on the inside
of this levee.
When crossing the levees, the tubing should go as straight over as possible
without any angle in order to avoid twisting of the tubing. On firm levees, some
of the levee top should be knocked off into the borrow ditch to provide a smooth
ramp across the ditch so the tubing won’t tend to kink. The tops of fresh or
sandy levees should not be knocked off. They will usually settle enough from the
weight of the water and tubing, so, some soil should be shoveled from the field
into the borrow ditch. On sandier levees it may be necessary to put a plastic
spill under the tubing at the levee crossing to better avoid levee wash out.
If the tubing is laid further out in the field, a short pipe might need to be
placed under the tubing at the low side of each levee pad, as a culvert, to
assure water can flow under the tubing. When placed out in the field there is
more tendency for the tubing to float and move and this can cause the tubing to
twist at the levee. Some type of stake can be placed on both sides of the tubing
to keep it from moving. Once the rice has some size it will help keep the tubing
from moving.
When the tubing is laid on the permanent pad it is critical that it be placed
on the flat area in a shallow ditch to avoid rolling or twisting. In this
application, the water will tend to flow to the low end of the tubing. It may be
necessary to make some humps under the tubing with mounded soil, pipe, buckets,
barrels, etc., as it goes down the slope to help hold the water back to the high
side of the field. This can also be accomplished by using some type of rope or
strap around the tubing to squeeze or choke down on the tubing in order to
restrict the water flow in the tubing. It is also recommended that small holes
be punched in the air pockets that form in the tubing once it is laid. These
holes can be punched with pencil or ink pen points, wire flags, toothpicks, etc.
The idea is to punch a small hole rather than cut the tubing, so caution has to
be taken if a pocket knife is used.
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