Irrigation requires a relatively high investment in equipment, fuel, maintenance and labor, but offers a significant potential for increasing net farm income. Frequency and timing of water application have a major impact on yields and operating costs.
To schedule irrigation for most efficient use of water and to optimize production, it is desirable to frequently determine the soil water conditions throughout the root zone of the crop being grown. A number of methods for doing his have been developed and used with varying degrees of success, but the two which have proven most practical for field use are methods using tensiometers and electrical resistance meters. In comparison to investment in irrigation equipment, these instruments are relatively inexpensive. A third method using a neutron probe is being used in some areas. The neutron probe is very expensive compared to tensiometers and therefore not feasible for use on many farms. When properly used and coupled with grower experience any measuring devices can improve the irrigator's chances of success. The following discussion covers the working principles of methods and their use.
To use either variation you must know your soil
type and the available water holding capacity of the
soil. This may be obtained for your soil from the soil
conservation guide from your local Soil Conservation
Service. Next you determine the zone you are trying
to manage. This zone will vary according to the
effective rooting depth of the particular crop. Usually
24 inches (2 feet) is the most that can be managed
with irrigation in Southeastern soils. The effective
rooting depth of some crops maybe somewhat less.
Determine the total water you have available to
manage in this zone. It is desirable to try to manage
only a percentage of this total water, usually 50
percent. As water is removed daily (ET), these
amounts are subtracted from the adjusted water
available column.
When the water available approaches a zero balance it is time to irrigate. The amount to add depends on the soil type, but will usually be the same as the 50 percent value calculated earlier plus an added amount to account for application efficiencies less than 100%. (Typical application efficiencies for sprinkler irrigation equipment vary from 75 percent to 90 percent.) Water-use curves for various crops in Georgia are included at the end of this publication along, with pan evaporation coefficients (depending on which method you are using).
EXAMPLE: Tifton Soil Series. Assuming the upper 24 inches is the rooting depth (hardpans may change this), the total available water is 2.2 inches (from Georgia Irrigation Guide). Assume a 65-day-old corn crop.
Step 1. From the crop curve (see Figure 5), this corresponds to a daily use rate of.32 inches per day.
Step 2. Determine irrigation by setting lower limit for water balance. For this example, use 50 percent as the limit. Then l.l inches of water will need to be replaced.
Step 3. Determine amount of irrigation to apply by dividing amount replaced by irrigation efficiency. Using 75 percent as the irrigation efficiency, the amount of irrigation to apply is:
1.1 = 1.47 inches or 1.5 inches. .75
Step 4. Determine frequency of irrigation by dividing amount replaced by water use per day. For this example:
frequency = 1.1 = 3.44 = 3.5 days. .32
Step 5. Therefore it is necessary to apply 1.5 inches every 3.5 days to maintain 50 percent available water on Corn that is 65 days old.
Note: this same procedure can be used for other crops as long as you have the crop water use curve. (See figures 3 through 11 for additional curves.)
EXAMPLE: Tifton Soil Series. Assuming the upper 24 inches is the rooting depth (hardpans may change this), the total available water is 2.2 inches (from Georgia Irrigation Guide). Assume a 65-day-old corn crop.
Step 1. Local pan evaporation data (available from local weather station) reports daily pan evaporation rates of 0.26 inches per day.
Step 2. The crop coefficient for 65 day old corn (see Figure 13) is 1.06.
Step 3. Determine daily water removal by multiplying daily evaporation by crop coefficient.
0.26 x 1.06 = 0.28 inches
Step 4. Determine irrigation by setting lower limit. For this example use 50 percent. Then 2.2 inches x 50 percent or l.l inches will need to be replaced.
Step 5. Determine amount of irrigation to apply by dividing amount replaced by irrigation efficiency. Using 75 percent as the irrigation efficiency, the amount of irrigation to apply is:
1.1 = 1.47 inches or 1.5 inches. .75
Step 6. Determine frequency of irrigation by dividing amount replaced by water use per day. For this example 1.1 inches divided by 0.28 inches = 3.93 or 4 days.
Step 7. For this example it is necessary to apply 1.5 inches every 4 days to maintain a balance with pan evaporation.
Note: This same procedure can be be used for other crops
as long as you have the crop coefficient curve. Figures 12-18 have additional crop coefficient curves.
A tensiometer is a sealed, water-filled tube with a porous ceramic tip on the lower end and a vacuum gauge on the upper end. The tube is installed in the soil with the ceramic tip placed at the desired root zone depth and with the gauge above ground. In dry soil, water is drawn out of the instrument, reducing the water volume in the tube and creating a partial vacuum which is registered on the gauge. The drier the soil, the higher the reading. When the soil receives water through rainfall or irrigation the action is reversed. The vacuum inside the tube draws water from the soil back into the instrument which in turn results in lower gauge readings.
The amount of vacuum reflected by the gauge is a direct measure of soil water tension or soil suction. The standard unit of measurement of soil water tension, or soil suction, is the "bar." The bar is a unit of pressure (or vacuum) in the metric system and is approximately equivalent to one atmosphere or 14.5 lbs./sq. in. Most tensiometer gauges are calibrated in hundredths of a bar (called centibars) and graduated from zero to 100. In these units of calibration a tensiometer can operate in a range of 0 to 80 centibars.
Plant roots must overcome the soil suction or the attraction that soil particles have for water in the soil in order to withdraw and use this water. The measurement of soil suction is a direct indication of the amount of work the plant roots must do to get water from the soil. The tensiometer measures soil suction directly without calibration for soil type, salinity or temperature.
Try to select representative areas of the field for tensiometer stations. However do not place them in low spots as they are not representative. It may be best to limit the total number of stations until you gain experience. After a trial period it will be easier to determine the total number needed.
Always try to place tensiometers in accessible locations so that the operator can get to them relatively easily.
Generally, you can select a location where
tensiometers will not be in the way of field
operations. Mark stations so you can avoid them with
only minor inconvenience to equipment operators. To
protect against accidental striking by tools or
machinery, drive stakes near the instruments with
colored flags or tape attached. You can also cover
them with a box, a tile, a steel pipe or similar
protective device (provided water movement within
the soil is not impeded).
For most crops and soils, two depths per station are recommended. Set one with the tip at a depth between 1/4 and 1/2 of the root zone and the other at a depth of about three-fourths of the active root zone. Use the shallow tensiometer for scheduling the "start" of the irrigation cycle. Use the deep tensiometer to evaluate if the proper "amount or depth" of water was applied. When soil water is known at these depths, an accurate estimate can be made of water conditions throughout the root zone.
One method recommends inserting the ceramic tip into a prepared hole so that the walls of the tip are in close contact with undisturbed soil and roots. Prepare the hole by driving a steel rod or pipe of the same diameter as the instrument tube to the desired depth. Carefully remove the rod and push the tensiometer to the bottom of the hole. Press soil around the tensiometer at the surface and pile it slightly so water will not collect and seep down along the tube of the tensiometer.
Another method often used for installing tensiometers with equally good results is to bore the hole with a soil auger (1 1/4 inches) to the desired depth. Next make a slurry in the bottom of the hole with screened soil, place the tensiometer and backfill with screened soil, tamping the soil firmly around the tube with a 1/2-inch dowel.
The tensiometer may need occasional refilling with water. The best time to add water is after an irrigation when the vacuum is low. After refilling, the vacuum pump may be used to remove air bubbles.
The following general guidelines to interpreting gauge readings may be used under most conditions:
Readings 0-5 - This range indicates a nearly saturated soil and often occurs for one or two days following a rain or irrigation. Plant roots may suffer from lack of oxygen if readings in this range persist.
Readings 5-20 - This range indicates field capacity. Discontinue irrigation in this range to prevent waste of water by percolation and also to prevent leaching of nutrients below the root zone.
Readings 20-60 - This is the usual range for starting irrigation. Most field plants having root systems 18 inches deep or more will not suffer until readings reach the 40 to 50 range. Starting irrigations in this range insure maintaining readily available soil water at all times. It also provides a safety factor to compensate for practical problems such as delayed irrigation, or inability to obtain uniform distribution of water to all portions of the field.
Readings 70 and Higher - This is the stress range for most soils and crops. Deeper rooted crops in medium textured soils may not show signs of stress before readings reach 70. A reading of 70 does not necessarily indicate that all available water is used up, but that readily available water is below that required for maximum growth. For readings above 70, tensiometers are likely to break tension (The vacuum is destroyed.) especially in coarser textured soils.
For irrigation systems that require several days to cover a given field (such as a center pivot), it will be necessary to anticipate how high the tension will go before the system reaches a given location in the field. In these cases it will be necessary to start the irrigation system at lower tensiometer readings so that some sections of the field do not get too dry before the system gets there. For instance, it is not unusual to start a center pivot at tensiometer readings of 15 or 20. This is especially true in the sandier soils which have a relatively low water holding capacity.
The blocks, which have stainless steel electrodes imbedded in them, are installed permanently at desired locations and depths in the soil. Insulated wires from each block are brought above the soil surface where they can be plugged into a portable meter for reading.
Install the blocks soon after planting to allow time for plant roots to grow around the blocks and to assure a positive contact between blocks and soil. Any separation between blocks and surrounding soil will lead to inaccurate readings.
When the seeds are first planted, irrigation may
be needed to assure quick and uniform seed
germination. Visual inspection of the soil near the
seeds will indicate whether irrigation is needed. A
minimum of two blocks per station is recommended;
one shallow, one deep. Use the shallow block for
scheduling the "start" of the irrigation cycle. Use the
deep block to evaluate if the proper "amount or
depth" of water was applied. Table l gives
recommended depths for setting the blocks according
to soil depth or active root zone.
| Table 1. Recommended Depths for Placing Electrical Resistance Blocks According to Soil Depth or Active Root Zone | ||
| Soil Depth or Active Root Zone (inches) | Shallow Blocks (inches) | Deep Blocks (inches) |
| 18 | 8 | 12 |
| 24 | 12 | 18 |
| 36 | 12 | 24 |
Soil depth may be the limiting factor in determining the active root zone. Soils that have loamy sand or finer textured soil overlying an impermeable layer limit the potential root zone of deeper rooting crops. When soil depth is limiting, consider soil depth rather than active root zone when determining how deep to place the blocks.
The following generalizations apply to installation of all blocks:
Available soil water may be expressed either in percent of the total potential reserve or in terms of suction necessary to draw water from the soil particles. Such suction is referred to as negative pressure or tension, measured in bars.
Table 2 is a guide to interpreting meter readings as they relate to soil water tension for one type meter. Graduations on the meter scale will not be the same for all makes or models and will vary with soil type. Meter manufacturers, however, always provide instructions for interpreting readings.
Start irrigation at lower meter readings during hot
weather, high winds or low humidities, since loss of
water is accelerated by these conditions.
| Table 3. Electrical Resistance Meter Readings for Starting Irrigation | ||
| Meter Readings on Shallow Block* | ||
| Soil | Meter Reading | Bar Tension |
| Loamy sands; Sandy loams; Very fine sandy loams | 25 | 0.25 |
| Silt loams | 40 | 0.40 |
| Clay loams & Silty clay loams | 60 | 0.60 |
| * These are average readings and will vary depending on crop, rooting depth, type of irrigation system, meter and soil type. | ||
| Table 2. Interpretation of Readings on Electrical Resistance Meters as Related to Soil Water Tension | |||
| Bars Tension | Meter Readings* | Interpretation | |
| Nearly saturated | less than 0.05 | 0 to 5 | Near saturated soil. Occurs for a few hours following a rain or irrigation. |
| Field capacity | 0.10 to 0.20 | 5 to 20 | Field capacity. Irrigations discontinued in this range. |
| Irrigation range | 0.20 to 0.60 | 20 to 60 | Usual range for starting irrigation. Starting irrigation in this range insures maintaining readily available soil moisture at all times. |
| Dry | greater than 0.60 | more than 60 | Stress range for most soils and crops. Some soil moisture present but dangerously low for maximum plant growth and production. |
| * These readings will vary according to meter type and soil type. | |||
Computer models are being developed and updated each year. To use a computer model you obviously must have access to a computer on a regular basis. Simple models are available that do "checkbook" type record keeping. More sophisticated models are available that require weather data inputs (you can supply these inputs manually or some programs will record the data directly from an in-field weather station.) Models can also incorporate other management decisions such as whether to spray for insects and diseases. Models of the later type are called expert systems. Computer models can be very beneficial in decision making. However, the statement "Garbage in - Garbage out" is very applicable for computer models. Be certain that the model is applicable to your geographical area and that the input data you supply is valid or the results could be misleading.
You can obtain the full benefit of using
tensiometers, electrical meters or computer models by
recording soil moisture readings and plotting them on
a chart. Readings may be plotted directly in the field.
Use different colored pencils for different depth
tensiometers (or soil blocks) to make the chart easier
to read. The chart lines show what has happened in
the past. By projecting them ahead, you have an
advance indication of what you can expect in a few
days. This information is helpful in scheduling the
next irrigation and in measuring the effectiveness of
an irrigation -- what depth of penetration was
achieved and how soon the soil dried out. Most
manufacturers include charts with their instruments. If
not included, they can be easily made. Record rainfall
data along with the instrument reading to aid in
evaluating soil water conditions.
| Table 4. Comparison of Methods | ||||
| Expense | Balance Method | Tensiometers | Resistance Blocks | Computer Models |
| No expense | Moderate expense | Moderate expense | Expensive * | |
| Accuracy | Fair | Good | Good | Fair ** |
| Operate Automated Equipment | No | Yes | Yes | Yes |
| Repeatability | Fair | Good | Fair | Good |
| * Varies depending on if computer already exists in farming operation. | ||||
| ** Computer models are only as accurate as the data used for inputs (i.e., weather, rainfall, etc.). | ||||
The University of Georgia and Ft. Valley State College, the U.S. Department of Agriculture and counties of the state cooperating. The Cooperative Extension Service offers educational programs, assistance and materials to all people without regard to race, color, national origin, age, sex or disability.
An Equal Opportunity Employer/Affirmative Action Organization Committed to a Diverse Work Force
Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, The University of Georgia College of Agricultural and Environmental Sciences and the U.S. Department of Agriculture cooperating.
Gale A. Buchanan, Dean and Director
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