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  Planning a Furrow Irrigation
In This Advisory
Purpose - The purpose of this advisory is to...
  • Present a planning process for furrow irrigations.
  • Provide a simple-to-use calculator for estimating the depth of water applied during furrow irrigations.
  • Describe how to estimate soil moisture depletions using the look and feel of a soil sample.
  • Discuss the more common suggestions for improving furrow irrigation performance.
  • Describe the construction and use of a soil probe
INTRODUCTION
Irrigation has often been called "both an art and a science".   No where is that more true than in furrow irrigation.  This Advisory provides some simple steps that can be followed to pre-plan a furrow irrigation.  This type of planning can prevent large errors in irrigation set management.  However, as noted in Steps 5 and 6 below, it is all important that the irrigator be able to react to the first set.  It must be noted whether the water is advancing as expected or whether runoff is more or less than expected.  A soil probe should be used to judge the rate at which water is infiltrating.

Science has developed rational concepts and equations for helping to design and manage irrigation systems.  However, with furrow irrigation it will always be important to have an irrigator that knows both the art and the science of irrigation.
Steps in Planning a Furrow Irrigation
Step 1.   Estimate the SOIL MOISTURE DEPLETION in the EFFECTIVE ROOT ZONE.  Using an auger and the Merriam "Feel" chart (Table 1 below) is fast, flexible, and inexpensive.  Using a neutron probe is expensive, less flexible (constrained to the sampling site), but more accurate.  A water budget irrigation scheduling system (refer to the advisory for a discussion) will provide guidance as it will provide an estimate of total crop evapotranspiration (water use) since the last irrigation. - the EFFECTIVE ROOT ZONE may or may not be the total depth of crop roots (deeper or shallower). It is the depth of soil in which you want to control moisture contents. - Soil will hold only so much water, the FIELD CAPACITY. The difference between the current soil moisture and the soil's field capacity is called the SOIL MOISTURE DEPLETION, SMD. Soaking in more water than the SMD will result in some deep percolation below the effective root zone. At some irrigations you may want to do this to maintain a salt balance. In a drought year you want to reduce deep percolation as much as possible.

Step 2.   Do some pre-planning to check the initial operating parameters, furrow flow and set length.  This will identify "limits" on the depth of water to be applied versus the soil moisture depletion.  Here are some examples that can be used with the calculator that appears below.

IMPORTANT!! Note that the RUNOFF percentage is runoff that is used on another irrigation set, another field, or another farm.  It is not runoff that is reapplied to the current set.

Example A:
FLOW/FURROW = 20 GPM per furrow
HOURS/SET = 12 hours per set
LENGTH = 40 feet
FURROW WIDTH = 1320 feet
RUNOFF = 0%
Entering these numbers into the calculator results in a equavalent depth of 5.26 inches on this set.  The important questions are...
  • what happens to that 5.26 inches? Does some of it run off? If so, how much?
  • how evenly is the remaining water distributed in the furrow? Does 6 inches soak in at the top of the furrow and only 3 inches at the bottom?
  • does the 5.26 inches, minus any reused tailwater, to be applied match up with my estimated soil moisture depletion? Am I applying 5.26 inches of water when I only need 2 inches?
Example B:
FLOW = 20 GPM per furrow
HOURS = 12 hours per set
WIDTH = 40 inch spacing - but it is planned to water every other furrow so the WIDTH would be doubled to equal 80 inches
LENGTH = 1320 feet
RUNOFF = 0%
Assuming that you are looking at the first seasonal irrigation and you only need to put in 2 inches, example B demonstrates why advisors recommend irrigating every other row (the change being that WIDTH went from 40 inches to 80 inches).
IMPORTANT!! In the calculator below, you don't have to double the entry for WIDTH if irrigating every other furrow, just check the answer YES or NO when prompted at 6. for watering Every Other Furrow
Example C:
FLOW/FURROW = 20 GPM per furrow
HOURS/SET = 12 hours per set
FURROW WIDTH = 40 inch spacing (watering every furrow)
FURROW LENGTH = 1320 feet
RUNOFF = 25%
Entering these numbers into the calculator results in a equavalent depth of 3.94 inches on this set.  In addition to the questions asked in Example A above, one would expect that the runoff is either being used on another irrigation set (which would contribute to good on-field irrigation efficiency), or another field on the farm  (which would contribute to high on-farm efficiency), or on another farm (which would contribute to high district/basin irrigation efficiency)
Step 3.   With an initial set of operating parameters chosen, consider the expected DISTRIBUTION UNIFORMITY (DU).  You must have good DU before you can have good irrigation efficiency. DU in furrows depends on three factors, down-row uniformity, cross-row uniformity, and general soil variability. DOWN-ROW UNIFORMITY - the water advance must be fast enough to give even soak times at all points in the furrow.  Water has to advance faster in lighter soils than in heavier.  Starting rule-of-thumbs are to get water down in 1/4 - 1/3 (3-4 hours of a 12 hour set) of the total set time in light soils and 1/3 - 1/2 (4-6 hours of a 12 hour set) of the total set time in heavier soils.  Some cracking clays have been operated at ratios as high as 2/3 (18 hours of a 24 hour set). CROSS-ROW UNIFORMITY - due to differential compaction from tractor traffic.  Another advantage of irrigating every-other row can be that you are either in all wheel rows or all non-wheel rows (depending on your tool bar setups).  Note that you don't necessarily want even water advance in all furrows.  You want the SAME DEPTH OF WATER infiltrated in each furrow.  This may mean using a higher flow for a shorter time in non-wheel rows.
GENERAL SOIL VARIABILITY - sometime you just have to live with it. If the streaking is up/down the field, maybe plan different set times and flows in the different soils.  If primarily cross the field, surge irrigation may help since surge acts to reduce infiltration rates quickly.

Step 4.   What about RUNOFF (tailwater)? Good distribution uniformity requires a fast water advance.  The faster water gets to the bottom of a furrow, the faster you have to deal with tailwater. You can either run it off the farm, cutback the furrow flow, or use a tailwater return system. RUN IT OFF THE FARM - not a viable option for most farmers anymore. CUTBACK - requires more labor and will result in several different sets running at any one time
TAILWATER RETURN - requires capital investment and annual operating costs for the pump but simplifies management. The economics include a comparison of how much water (lost tailwater) is costing, how much deep percolation is costing (external costs to you or the irrigation district), and the alternative costs of cutbacks.

Step 5.   MOST IMPORTANT!!!  You must react to the first set!   Use a soil probe (what is it?) to judge how fast water is soaking into the ground. Keep track of how long it is taking for water to reach the end of the furrow. Change the operating parameters (set time, flow per furrow) as needed. Night irrigators cost more money but again, it's a drought year.

Step 6.   React to the total irrigation.  Use the soil probe 2 or 3 days after the irrigation to judge how far water penetrated at the top and bottom of the furrow.  Judge crop response and water penetration to see how close your initial estimate of SMD was to actual.  Compare your water district's calculated deliveries, or pump flow meter if using your own water well or river pump,   to your estimates of SMD and the calculations from Step 2 using the actual furrow flows and set lengths. IMPORTANT!!!  Don't make radical changes in the whole field. If you have never used every-other row irrigation, take a set or two and try it. Compare crop responses. Use the soil probe in both areas to see where water penetrated. Keep track of district deliveries so you know how much water went in to each set.

Calculating an Equavilent Depth of Water Applied per Irrigation
The calculator below can be used to estimate the average depth of water that will be applied to a furrow irrigation set.
How to Use:
1. Enter the data.  Note the units.  Also note that the RUNOFF % is runoff that is used on another irrigation set, another field, or another farm.  The Distribution Uniformity is usually based on the average depth infiltrated in the that 1/4 of the field with the lowest infiltration.

2. Press CALCULATE

3. Compare the Equivalent Depth Applied to the estimate of the actual soil moisture depletion.  Note that the Minimum Infiltrated Depth is the estimate for infiltrated water at the point of lowest infiltration in the field, or to whatever basis you enter the DU.

4. IMPORTANT!! - 5% is subtracted automatically from both the Equivalent Depth Applied and the Minimum Infiltrated Depth to account for evaporation of water while it is flowing in the furrow.
1. Flow into the Furrow:  gallons/minute
2. Hours per Set:  hours
3. Furrow Spacing:  inches
4. Furrow Length:  feet
5. % Runoff:  %
6. Irrigating Every Other Furrow?  No   Yes
7. Distribution Uniformity:  %
Average Equivalent Depth Applied per Set =  inches
Minimum Depth Infiltrated (per entered DU) =  inches

Estimating Soil Moisture Depletion from the Look and Feel of a Soil Sample
The chart below is a compendium of years of experience by the Soil Conservation Service, Bureau of Reclamation, University Extension, and private industry.    It is used to gauge soil moisture depletions by observing the look and feel of soil samples.  Many consultants would prefer to work with a sample from a coring tool rather than a screw auger or bucket auger.

Moisture
Deficiency
in/ft
Coarse
(loamy sand)
Sandy
(sandy loam)
Medium
(loam)
Fine
(clay loam)
(field capacity) (field capacity) (field capacity) (field capacity)
.0 Leaves wet outline on hand when squeezed. Appears very dark, leaves wet outline on hand, makes a short ribbon. Appears very dark, leaves wet outline on hand, will ribbon out about one inch. Appears very dark, leaves slight moisture on hand when squeezed, will ribbon out about two inches.
.2 Appears moist, makes a weak ball. Quite dark color, makes a hard ball. Dark color, forma a plastic ball, slicks when rubbed.
.4 Appears slightly moist, sticks together slightly. Dark color, will slick and ribbons easily.
.6 Fairly dark color, makes a good ball. Quite dark, forms a hard ball. Quite dark, will make a thick ribbon, may slick when rubbed.
.8 Dry, loose, flows through fingers. (wilting point) Slightly dark color, makes a weak ball. Fairly dark, forms a good ball.
1.0 Lightly colored by moisture, will not ball. Fairly dark, makes a good ball.
1.2 Very slight color due to moisture. (wilting point) Slightly dark, forms a weak ball. Will ball, small clods will flatten out rather than crumble.
1.4 Lightly colored, small clods crumble fairly easily. Slightly dark, clods crumble.
1.6 Slight color due to moisture, small clods are hard. (wilting point)
1.8 Some darkness due to unavailable moisture, hard & cracked clods (wilting point)
2.0 --- --- --- ---

Table 1 - Factors for estimating soil moisture deficit from the look and feel of soil samples


Common Suggestions for Improving Furrow Irrigations
The suggestions below are the most common seen for improving furrow irrigation.  Which one (or ones) you might use depends on your individual situation.   Note that whenever looking to improve an irrigation sytsem or management of a system, always look to measure and improve distribution uniformity (DU) first.    Water must infiltrate as evenly as possible across a field.  Then, try to improve control over the total infiltration and reduce or reuse surface runoff.  This all assumes that you have a reasonably accurate estimate of how much water is actually needed.

DU in furrows has three parts:
DOWN-ROW UNIFORMITY - the water advance must be fast enough to give even soak times at all points in the furrow.  Water has to advance faster in lighter soils than in heavier.  Starting rule-of-thumbs are to get water down in 1/4 - 1/3 (3-4 hours of a 12 hour set) of the total set time in light soils and 1/3 - 1/2 (4-6 hours of a 12 hour set) of the total set time in heavier soils.  Some cracking clays have been operated at ratios as high as 2/3 (18 hours of a 24 hour set). CROSS-ROW UNIFORMITY - due to differential compaction from tractor traffic.  Another advantage of irrigating every-other row can be that you are either in all wheel rows or all non-wheel rows (depending on your tool bar setups).  Note that you don't necessarily want even water advance in all furrows.  You want the SAME DEPTH OF WATER infiltrated in each furrow.  This may mean using a higher flow for a shorter time in non-wheel rows.

GENERAL SOIL VARIABILITY - sometime you just have to live with it. If the streaking is up/down the field, maybe plan different set times and flows in the different soils.  If primarily cross the field, surge irrigation may help since surge acts to reduce infiltration rates quickly.
With this short primer in mind, here are the more common suggestions for improving furrow irrigation:
Utilize an irrigation scheduling system so that you have reasonably accurate estimates of WHEN and HOW MUCH to irrigate. Reduce the length of the furrow - improves down-row uniformity by helping water to get to the end of furrow quicker in relation to the total time of irrigation.

Install a tailwater reuse system - improves overall irrigation efficiency by saving tailwater for reuse.

Increase the flow per furrow- improves down-row uniformity by helping water to get to the end of furrow quicker in relation to the total time of irrigation

Irrigate in every other furrow - helps to reduce over-application when either small irrigations are desired or the soil has a high infiltration rate.   Use the Calculator above with a set of parameters, varying only whether you are irrigatiing every furrow or every-other furrow.  Notice the change in the depth applied.
IMPORTANT!!   When irrigating every-other furrow keep in mind that the purpose of an irrigation is to wet the effective root zone.  Make sure water is spreading sufficiently laterally through the beds.  Use a Soil Probe to check this during and after an irrigation.
Use "torpedos" or some other means of compacting/smoothing furrows where tractor tires don't run - helps to improve distribution uniformity by evening the rate of advance between compacted and uncompacted furrows. Use cutback furrow flows - reduces the amount of surface runoff.

Regrade the field - you may want to increase or decrease the rate of water advance without changing the length of the furrows (this is a rather drastic change to make).

Utilize surge irrigation - surge irrigation is the practice of applying water to a set for a while, then switching the water to another set, then switching back and forth between the sets periodically.  This might occur from 3 times to as many as 10 depending on the situation, equipment, and experience.   Surging water acts to reduce the infiltration rate of the soil quickly so that differences between compacted and uncompacted furrows are minimized.  It can also help in very light soils as it will act to reduce over-applications.  It has been tried in many places, some successfully, others not so succesful.  Check with local irrigation experts.

The Soil Probe
A soil probe is simply a 4 -6 foot long piece of 3/8 steel with a rounded tip at one end and a handle at the other.  One configuration is shown in Figure 1 below.  

probe.gif (2872 bytes)


Figure 1 - One configuration of a soil probe
The soil probe is one of the handiest tools in your pick-up.   It has many uses:

It can be used to check when it is time to move and irrigation set.  Just press the tip into the soil at an appropriate spot in the field.  If the tip goes in easily to about 1/2 to 2/3 the depth of the effective root zone it is about time to move the set.  The area that the tip goes in is probably near saturation.  Excess water in this area will drain down to take the rest of the effective root zone to field capactiy. It can be used to check whether an irrigation was sufficient.   Wait a day or two after the irrigation to press the tip in.  Now it should go easily to the depth of the effective root zone- too shallow and the irrigation was insufficient, too deep and you over-irrigated. It can be used to check the uniformity of an irrigation.   One or two days after an irrigation, go to the most-watered part of the field and press the tip in and record how far you can easily push it in.  Then, go to the least-watered area and do the same thing.  The difference in the depths is an indication of the uniformity of the irrigation. You can also use the soil probe to check the lateral spread of water.  This may be especially handy when irrigating every-other furrow and there is a question of whether the beds are getting wette through sufficiently. It can be used to check the wetted soil volume under a drip irrigation system.  Press the probe in laterally from the emitter to see how far out and deep water spreads.