TECHNICAL NOTES

  ---------------------------------- ------------------------------------
  **US DEPARTMENT OF AGRICULTURE**   **NATURAL RESOURCES CONSERVATION
                                     SERVICE**

  **AGRONOMY - 62**                  **Albuquerque, NM**
  ---------------------------------- ------------------------------------

### July, 2001

**FARM IRRIGATION RATING SYSTEM (FIRS)**

**For New Mexico**

-   **INTRODUCTION**

The Farm Irrigation Rating System (FIRS) is a computer assisted
evaluation tool that can be used to plan and rate irrigation
improvements for irrigation systems. FIRS is used for planning onfarm
irrigation improvements with the farmer to achieve a specific level of
water management. It is **not** meant to be used in place of onfarm
evaluations

In the past, NRCS, NM used a mixture of older system evaluation programs
to establish the existing system efficiency. The maximum potential
efficiency of the new system was used for the after condition. This was
incorrect. FIRS will be used to make a more realistic estimate of the
after condition with a given set of management practices applied. FIRS
will give a lower after efficiency and more consistency between offices
and individual planners.

FIRS was developed using criteria from Farm Irrigation Rating Index
(Hedlund and Koluvek). The method has been tested against field trials
on intensively managed basins in Welhon-Mohawk, Arizona; the Glasgow
area in Montana; flood irrigated mountain meadows in Colorado; the
Uintah Basin in Utah; and the Grand Valley in Colorado.

The DOS version of the program was developed by David Taylor (NRCS) of
Casper, Wyoming, 11/1/00. FIRS in NM, is an MS Excel spreadsheet based
on the Wyoming (DOS) version of the program. A new version is being
developed by the Water and Climate Center in Portland, OR. It is
expected the MS Excel version of the system (covered by this Technote)
will be used until a new version is ready. The spreadsheet and this note
can be downloaded from:
<http://www.nm.nrcs.usda.gov/techserv/TechNotes/agro.htm>. The MS Excel
file name is FIRS-NM.xls for the spreadsheet, and ag61 FIRS.doc for the
technical note.

-   **FIRS ELEMENTS**

FIRS provides a uniform and simple evaluation method to analyze onfarm
irrigation water use. It provides good documentation of the effects of
changes in the system and management.

The index is the **product** (factors multiplied together) of three
elements:

-   Potential efficiency

-   Management efficiency

-   System efficiency

All factors listed in the table sheet (tab at the bottom of the first
sheet) of the worksheet are selected by left clicking in the cell
needing the value and selecting the appropriate value.

-   **GENERAL JOB DATA (Basic data)**

Prior to selecting the potential efficiency, basic data for the farm
and/or field must be entered. **Client, Planner, Crop Grown** (put down
list)**, Soil Name and Texture, Location, Farm ID, Net Consumptive Use
(CU)** of water for the crop in inches, **Field number**, and **Acres**
must be entered at the top of the sheet. The **CU** for a given crop is
listed in the FOTG in Section 1 Volume 4 (Irrigation Guide part 1).
Present or Future condition must be indicated by a check mark. Surface
or Sprinkler/Drip must be indicated by a check mark.

-   **IRRIGATION SYSTEM POTENTIAL EFFICIENCY ELEMENT**

The irrigation system potential efficiency for an optimally performing
system for the site-specific physical layout can be determined from the
irrigation guide or National Engineering Handbooks (NEH-I). Potential
efficiency is a measure of that optimum efficiency for that method of
irrigation. All factors must be perfect for this efficiency to be
attained. New Mexico NRCS has set these system efficiencies for various
systems. They are listed in **Table 1**.

  -------------------------- ---------------- ----------------------- ----------------
  **Table 1-NM Potential                                              
  System Efficiency**                                                 

  **Type**                   **Efficiency**   **Type**                **Efficiency**

  **(name)**                 **(%)**          **(name)**              **(%)**

  Border, contour levee,     70               Furrow, graded          70
  field crop                                                          

  Border, ditch              60               Furrow, level           80

  Border, graded             70               Furrow, surge           80

  Border, guide              60               Linear move             85

  Border, level or basin     80               Sprinkler, biggun or    55
                                              boom                    

  Center Pivot, (low pres.   80               Sprinkler, handline or  65
  drops)                                      wheelline               

  Center Pivot, (over-pipe   70               Sprinkler, solid set    75
  impact)                                     (overhead)              

  Center Pivot, LEPA (drag   90               Sprinkler, solid set    75
  hose)                                       (under tree)            

  Flood, contoured ditch     60               Traveling big gun       60

  Flood, controlled          60               Trickle, continuous     90
                                              tape                    

  Flood, uncontrolled        45               Trickle, micro-spray    85

  Furrow, contour            70               Trickle, pt source      90
                                              emitter                 

  Furrow, corrugation        70                                       
  -------------------------- ---------------- ----------------------- ----------------

*Note: The table was compiled by Dave Fisher and Mike Sporcic from the
alpha windows version of FIRS, WY-FIRS, the NEH, NM Irr. Guide, and WA
Irr. Guide on June 8, 2001.*

**TYPES OF SYSTEMS**

There are six types of irrigation systems commonly used today, border,
furrow, flood, sprinkler, sub-irrigation, and trickle.

-   **Borders**

**Level or basin borders**, as the name implies, are developed on land
that is nearly level. Generally basins are as wide as long, and level
border are much longer than wide. In both cases, the topography is
nearly level. Most are laser leveled.

**Graded-border** irrigation is a form of controlled surface flooding.
The field to be irrigated is divided into strips by parallel dikes or
border ridges, and each strip is irrigated independently. The border
strips should have little or no cross slope but should have some slope
in the direction of irrigation. Each strip is irrigated by turning in a
stream of water at the upper end. The stream must be large enough to
spread over the entire width between the border ridges without
overtopping them. Usually the stream size should be such that the
desired volume of water is applied to the strip in a time equal to or
slightly less than that needed for the soil to absorb the net amount
required. Most are laser leveled.

**Guide borders** are primarily used to irrigate grass, legumes, and
grass-legume mixtures. Generally, guide borders are on steeper slopes
than graded borders. Guide border ridges are normally only 2-3 inches
high. Cross-slope is minimal, but there may be some cross slope. Stream
size is determined by the hydraulic characteristics of the site. Stream
size must be large enough to provide adequate spreading over the strip.
Water is allowed to run until a sufficient amount has infiltrated the
soil.

The **Contour-Levee** method is a modification of the contour-border or
basin method. Areas bounded by small contour levees and cross levees are
completely flooded. Water is applied at a rate considered to be in
excess of the intake rate of the soil spreads rapidly over the area, and
is allowed to remain until it has infiltrated the soil to the desired
depth. If the irrigation is for soil-moisture replenishment, the excess
water is then drained off immediately. If the irrigation is for weed
control on riceland, the water is impounded at a minimum depth of 3
inches and a maximum depth of 8 inches for several weeks.

-   **Furrows**

**Graded furrows** are small channels having a continuous, nearly
uniform slope in the direction of irrigation. They are used in
irrigating cultivated crops planted in rows. There are one or more
furrows between crop rows except for bedded crops, in which the furrows
are along each pair of rows. Size and shape of the furrows depend on the
stream size, slope, crop grown, equipment used, and spacing between crop
rows.

The **contour-furrow** method is similar to the graded-furrow method in
that irrigation water is applied by furrows, but the nearly level
furrows carry water across a sloping field rather than down slope. The
contour furrows are curved to fit the land surface. They have just
enough grade to carry the irrigation stream. Head ditches or pipelines
run downhill or slightly across the slope to feed the individual
furrows.

**Corrugation irrigation** is a partial surface flooding method.
Irrigation water is applied in small channels or corrugations evenly
spaced across the field. Water flowing in the corrugations soaks into
the soil and spreads laterally to irrigate the areas between
corrugations. The corrugations should be spaced to permit an adequate
lateral spread by the time the desired amount of water has infiltrated
the soil.

**Surge irrigation** is the same as graded furrow, except water is
applied at specific intervals during the set. Generally water is applied
through gated pipe and is regulated by a surge valve. The intervals are
setup to alternate water on and water off, so that the advance time is
reduced and a cut back to one half of the stream size can be used when
the water is advanced across the field.

-   **Flood**

In **controlled flooding,** the water applied to the surface is
controlled by dikes and ditches. Graded-border, level-border,
contour-levee, and contour-ditch methods are different types of
controlled-flooding irrigation. In general, flooding methods are used
for close-growing crops and furrow methods for row crops.

**Contour-ditch irrigation** is a form of controlled surface flooding.
Irrigation water is distributed from ditches running across the slope
approximately on the contour. Water is diverted from the ditches by
temporary dams. As the water rises, it is discharged through controlled
openings in the ditch bank, by siphon tubes, or over a uniformly graded
lower lip of the ditch. Water flows as an unconfined sheet down the
slope from one contour ditch to the next, and runoff is collected in
lower ditches for reuse. Water is applied to successive strips between
ditches until the field has been irrigated. The width covered by each
setting of the dams depends on the stream size available. A stream of 1
cubic foot per second usually covers a strip about 100 feet wide.
Topography, soil intake rate, and average net irrigation application
govern the spacing between contour ditches (80 to 300 feet).

-   **Sprinkler**

The **hand-move portable lateral** system is composed of either portable
or buried mainline pipe with valve outlets at various spacing. These
laterals are made of aluminum tubing with quick couplers and have either
center-mounted or end-mounted riser pipes with sprinkler heads. This
system is used on almost all crops and on all types of topography. A
disadvantage of the system is its high labor requirement. This system is
the basis from which all of the mechanized systems were developed.

**Wheel line** includes side roll and end-tow lateral systems. A
side-roll lateral system is similar to a hand-move system. The lateral
pipes are rigidly coupled, and a large wheel supports each pipe section.
The lateral line forms the axle for the wheels, and when it is twisted
the line rolls sideways. This unit is moved mechanically by a small gas
engine mounted at the center of the line, or by an outside power source
at one end of the line.

An **end-tow lateral** system is similar to hand-move laterals except
the system consists of rigidly coupled lateral pipe connected to a
mainline. The mainline should be buried and positioned in the center of
the field for convenient operation. Laterals are towed lengthwise over
the mainline from one side to the other. By draining the pipe through
automatic quick drain valves, a 20- to 30-horsepower tractor can easily
pull a quarter-mile 4-inch diameter lateral.

A **fixed-sprinkler (solid set)** system has enough lateral pipe and
sprinkler heads so that none of the laterals need to be moved. Thus to
irrigate the field the sprinklers only need to be cycled on and off. The
three main types of fixed systems are those with **solid-set portable
hand-move laterals**, **buried or permanent laterals**, and **sequencing
valve laterals**. Most fixed sprinkler systems have small sprinklers
spaced 30 to 80 ft apart, but some systems use small gun sprinklers
spaced 100 to 160 ft apart.

The **big gun, traveling sprinkler, or traveler,** is a high capacity
sprinkler fed with water through a flexible hose; it is mounted on a
self-powered chassis and travels along a straight line while watering.
The most common type of traveler used for agriculture in the United
States has a giant gun-type 500-gpm sprinkler that is mounted on a
moving vehicle and wets a diameter of more than 400 ft. The vehicle is
equipped with a water piston or turbine-powered winch that reels in the
cable moving the sprinkler. The cable guides the unit along a path as it
tows a flexible high-pressure lay-flat hose that is connected to the
water supply pressure system. The typical hose is 4 inches in diameter
and is 660 ft. long. After use, the hose can be drained, flattened, and
wound onto a reel. Some models roll up the hose as the system irrigates
so as not to damage the crop by dragging the heavy hose through the
field.

The **center-pivot** systems sprinkle water from a continuously moving
lateral pipeline. The lateral is fixed at one end and rotates to
irrigate a large circular area. The fixed end of the lateral, called the
"pivot point", is connected to the water supply. The lateral consists of
a series of spans ranging in length from 90 to 250 ft. They are carried
about 10 ft above the ground by "drive units", which consist of an
"A-frame" supported on motor driven wheels. Devices are installed at
each drive unit to keep the lateral in a line between the pivot and
end-drive unit; the end-drive unit is set to control the speed of
rotation. The most common center-pivot lateral uses 6-in pipe, is a
quarter mile long (1,320 ft), and irrigates the 126 acres plus 2 to 10
acres more depending on the range of the end sprinklers. However,
laterals, as short as 220 ft and as long as a half-mile, are available.

**Self-propelled linear-move laterals** combine the structure and
guidance system of a center-pivot lateral with traveling water feed
system similar to that of a traveling sprinkler. Linear-move laterals
require rectangular fields free from obstructions for efficient
operation. Measured water distribution from these systems has shown the
highest uniformity coefficients of any system for single irrigation
under windy conditions. Systems that pump water from open ditches must
be installed on nearly level fields. Even if the system is supplied by a
flexible hose, the field must be fairly level in order for the guidance
system to work effectively. A major disadvantage of linear-move systems
as compared to center-pivot systems is the problem of bringing the
lateral back to the starting position and across both sides of the water
supply line. Because the linear-move lateral moves from one end of the
field to the other it must be driven or towed back to the starting
position.

A **Low Energy Precise Application** (LEPA) system is either a
self-propelled linear-move laterals or a center pivot system. Water is
applied below the crop canopy, via a flexible tubing, at extremely low
pressure. Water loss is minimal because the water is applied at rates
less than the basic infiltration rate and evaporation losses are nearly
zero.

-   **Sub-irrigation**

**Subsurface irrigation** occurs when water is applied beneath the
ground surface to create an artificial or perched water table over some
natural barrier that restricts deep percolation. Moisture then reaches
the plant roots through capillary movement. It occurs when irrigation
water is introduced to the field through open ditches, tile drains, or
mole drains. The water table is maintained at some predetermined depth
below the ground surface, usually 12 to 24 inches, depending on the
rooting characteristics of the crop grown.

**Open ditches** are probably most widely used. Feeder ditches are
excavated on the contour and spaced close enough to insure control of
the water table. They are connected to a supply ditch that runs down the
predominant field slope and has control structures as needed to maintain
the desired water level in the feeder ditches.

-   **Trickle**

**Trickle irrigation** is the slow application of water on or beneath
the soil surface by drip, subsurface, bubblier, and micro spray systems.
Water is applied as discrete or continuous drops, tiny streams, or
miniature spray through emitters or applicators placed along a water
delivery line. Water is dissipated from a pipe distribution network
under low pressure in a predetermined pattern. The outlet device that
emits water to the soil is called an "emitter." The shape of the emitter
reduces the operating pressure in the supply line, and a small volume of
water is discharged at the emission point. Water flows from the emission
points through the soil by capillary and gravity forces.

**Bubbler irrigation** uses a point source emitter that applies water to
the soil surface in a small stream or fountain from an opening with a
point discharge rate greater than that for drip or subsurface irrigation
but less than 1 gallon per minute (gpm). The emitter discharge rate
normally exceeds the infiltration rate of the soil, and a small basin is
required to control the distribution of water.

In point source **drip irrigation**, water is applied slowly to the soil
surface as discrete or continuous drops or tiny streams through small
openings. Discharge rates are less than 3 gallons per hour (gph) for
widely spaced individual applicators and less than 1 gph/ft for closely
spaced outlets along a tube (or porous tubing).

**Micro spray** **irrigation**, water is applied to the soil surface as
a small spray or mist. The air is instrumental in distributing the
water, whereas in drip, bubbler, and subsurface irrigation, the soil is
primarily responsible for distributing the water. Discharge rates in
spray irrigation are lower than 30 gph.

**Continuous tape** is generally used to apply water below the surface
(subsurface irrigation). In **subsurface irrigation**, water is applied
slowly below the soil surface through emitters with discharge rates in
the same range as those for drip irrigation. This method of application
is not to be confused with sub-irrigation, in which the root zone is
irrigated through or by water table control. A continuous tape is a
flexible pipe with inline emitters installed regular intervals. The tape
is place at a depth in the soil to allow some tillage over the tape for
multiple year application. It also can be install shallow for a one-year
application.

-   **SURFACE IRRIGATION SYSTEM FACTORS**

For surface irrigation methods, system efficiency is dependent on four
factors while sprinklers depend on six factors. The four factors for
surface systems include:

> **Wc - Water Distribution Control**
>
> **Ce - Conveyance Efficiency**
>
> **L - Surface Topography**
>
> **R - Tailwater Reuse**

For drip irrigation, the system factors **Wc**, **Cc**, and **R** go to
1.0 and **L** is used to determine proper basin shaping for a bubbler.

**[Wc- Water Distribution Control]{.underline}**

Water Distribution Control is a measure of the operator's capability to
control water in the onfarm delivery system. **Table 2** shows the
factors for water distribution control. In order to manipulate the flow
of water from one field to another there must be adequate structural
control. Optimum control includes controlling water to all fields and to
all sets in a field. The value ranges from 0.9 for a system with no
control to 1.0 for a system with control to each field and all sets.

  --------------------------------------------------------- --------------
  **Table 2 Wc-Water distribution Control**                 

  **(description)**                                         **(factor)**

  Cuts in earth ditch bank on field                         0.93

  No water control devices                                  0.90

  Notched ditches                                           0.94

  Pivot, Surge, or Continuous Tape                          1.00

  Siphon tubes or Gated pipe                                0.97

  Spiles (tubes in ditch wall)                              0.95

  Sprinkler                                                 0.98

  Turnout to field                                          0.92
  --------------------------------------------------------- --------------

**[Ce - Conveyance efficiency]{.underline}**

Conveyance efficiency is a function of seepage rates by soil groups.
**Table 3** shows the conveyance efficiency factor (EFF) for different
materials. The factor EFF along with the ditch length is used to compute
conveyance efficiency (**Ce**) of the field delivery system. EFF is
dependent upon the type of material that water flows through.

  ----------------------------------------------------- -----------------
  **Table 3 EFF-Conveyance Efficiency**                 

  **(description)**                                     **(factor)**

  Clay (\>40%)                                          0.995

  Drip                                                  1.000

  Gravel (15-35%)                                       0.880

  Lined Ditch                                           0.998

  Loam to Clay Loam                                     0.990

  Pipe                                                  0.998

  Sand to Sandy Loam                                    0.950

  Very Gravely (\>35%)                                  0.800
  ----------------------------------------------------- -----------------

To manually compute the conveyance efficiency, first select a value for
EFF, then raise EFF to the power of length in l000s.

**Ce = EFF^(length/l000)^**

Many systems will have more than one EFF for the system. In that
situation, the factor EFF is determined by computing a weighted average
based each length and the type of EFF for the reach. For example, a
2000-foot long ditch that has 800 feet of sandy, 500 foot of clay and
700 feet of concrete lining would use a EFF factors that compute to an
average **Ce** = 0.957 as computed below.

  ----------------- ------------------ ---------------- ------------------
  **Material**      **Length (ft)**    **EFF**          **Length x EFF**

  Sandy Soil        800                0.92             736

  Clay Soil         500                0.97             485

  Lined Ditch       700                0.99             693

  Total             **2000**                            **1914**
  ----------------- ------------------ ---------------- ------------------

The Weighted Average EFF =1914/2000 = 0.957 or **Ce** = 0.957

FIRS uses the above technique to compute a weighted average when more
than one reach is used. Field conveyance efficiency should be computed
beginning at the **field delivery points** (not farm delivery). An EFF
value for each soil type or lining along the field distribution system
should be entered. The corresponding value for **Ce** is computed for
each reach and a weighted average is used.

The delivery system efficiency should not be included in this
computation. If the farm unit has a delivery system with seepage loss,
the loss should be evaluated using the second page of the spreadsheet
labeled Delivery Efficiency Section.

**[L - Surface topography]{.underline}**

Surface topography has an effect on efficiency. The value for surface
topography should be based on the relief class as defined in the section
15, Chapter 12, NRCS National Engineering Handbook. Laser leveling and
proper length of run will improve irrigation water application,
particularly uniformity. Length of run, which should be part of the IWM
plan, may be computed using a computer, calculator or estimated from the
irrigation guide. Laser leveling greatly enhances uniformity of smaller
(2 or 3 inch) applications. A uniform laser leveled slope would be rated
1.0. Some of the poorest mountain meadow irrigation with an improper
length of run will be about 0.75. Select the appropriate class from
**Table 12-1** on the next page.

![](media/image1.png){width="9.091666666666667in"
height="6.497222222222222in"}

**[R - Tailwater Reuse]{.underline}**

Reuse is dependent on percent of runoff that can be captured on the same
field (for example, a pump back system). **Table 4** **R-Tailwater
Reuse** shows the factors for the tailwater reuse. Care should be taken
when estimating tailwater reuse. The reuse factor is your estimate of
amount of runoff that can be reused. The first number in the table is
the percent of the water applied that can be reused. The second is an
estimate of the system efficiency. Select the best fit.

  ------------------------ -------------- ------------------------- --------------
  **Table 4 R-Tailwater                                             
  Reuse**                                                           

  **(reuse %, application  **(factor)**   **(reuse %, application   **(factor)**
  eff %)**                                eff %)**                  

  0% reuse, 10% Appl. Eff. 1.00           50% reuse, 10% Appl. Eff. 1.48

  0% reuse, 25% Appl. Eff. 1.00           50% reuse, 25% Appl. Eff. 1.37

  0% reuse, 50% Appl. Eff. 1.00           50% reuse, 50% Appl. Eff. 1.25

  0% reuse, 75% Appl. Eff. 1.00           50% reuse, 75% Appl. Eff. 1.12

  30% reuse, 10% Appl.     1.26           80% reuse, 10% Appl. Eff. 1.75
  Eff.                                                              

  30% reuse, 25% Appl.     1.20           80% reuse, 25% Appl. Eff. 1.60
  Eff.                                                              

  30% reuse, 50% Appl.     1.15           80% reuse, 50% Appl. Eff. 1.40
  Eff.                                                              

  30% reuse, 75% Appl.     1.07           80% reuse, 75% Appl. Eff. 1.17
  Eff.                                                              
  ------------------------ -------------- ------------------------- --------------

-   **SPRINKLER IRRIGATION SYSEM FACTORS**

The six factors used for sprinklers include:

> **Wc - Water distribution control factor**
>
> **Ce - Conveyance efficiency factor**
>
> **R - Tailwater reuse factor**
>
> **Sd - Sprinkler design factor**
>
> **W - Wind factor**
>
> **C - Climate factor**

**We, Ce, R** (common to both surface and sprinkler).

**[Wc - Water distribution control]{.underline}**

The computation of water control is described above for surface systems.
**Table 2** shows the factors for water distribution control. Normally
water control is improved to all fields and sets for a sprinkler. This
factor does not include the distribution system if the operator is
pumping from a farm distribution ditch or the pipe to the pump. The
water control factor does not automatically become a 1.0 unless the
entire delivery system is well controlled. The best water distribution
control system would a well connected to a center pivot with a pipe. In
that case, the factor would be one.

**[Ce - Conveyance efficiency]{.underline}**

The computation of conveyance efficiency is described above for surface
systems. Sprinkler systems, involve using a pipeline to deliver water
from the pump to the mainline. See **Table 3**.

**[R - Tailwater reuse]{.underline}**

Tailwater reuse is explained above for surface systems. See **Table 4**.
Application efficiencies should be such that little or no runoff occurs
for a sprinkler system.

**[Sd - Sprinkler Design factor]{.underline}**

A sprinkler will work at optimum efficiency if the system is well
designed, maintained, and operated properly. **Table 5** explains the
condition description and the factor value. Proper system design
requires a nozzle selection that applies water at a rate less than the
intake rate of the soil. Previous studies have shown that systems apply
water most efficiently when the pressure variation along the sprinkler
line is less than 20 percent. Obviously, a pump operating at less than
optimum may not directly affect application efficiency, but it most
certainly increases the cost of pumping.

  ------------------------------------------------------------- ------------
  **Table 5 Sd - SPRINKLER DESIGN FACTOR**                      

  **Condition Description**                                     **Factor**

  Designed according to NRCS criteria - nozzle pressure         1.00
  variation \< 20%, nozzle discharge rate \< soil intake rate   

  Nozzle pressure varies 20-25 % - nozzle discharge rate less   0.96
  than intake rate, pump operates within 90-95% of optimum      

  Nozzle pressure varies 25-30% - nozzle discharge rate less    0.93
  than intake rate, pump operates within 80-90% of optimum      

  Nozzle pressure varies 30---35% - nozzle discharge rate       0.89
  greater than intake rate, pump operates within 70-80% of      
  optimum                                                       

  Nozzle pressures vary more than 35% - nozzle discharge rate   0.86
  greater than intake rate, pump operates outside of 70% of     
  optimum                                                       
  ------------------------------------------------------------- ------------

**[W - Wind factor]{.underline}**

Evaporation and drift loss is evaluated by spray type. **Table 6** shows
the wind factors. As wind speeds increases or spray particle size
decreases, efficiency is reduced and water loss is increased. Wind speed
will have less effect on application efficiency with sprinklers on
drops. This factor will be 1.0 for LEPA systems (systems with close
spacing, every other row, and hose drag.

  ----------------------------------------------------- -----------------
  **Table 6 W-Wind factor**                             

  **(description)**                                     **(factor)**

  \>10 mph, med spray (20-30 psi)                       0.91

  \>10 mph, hp impact (\>50 psi)                        0.87

  \>10 mph, LP w/drops (\<20 psi)                       0.95

  1-4 mph, hp impact (\>50 psi)                         0.94

  1-4 mph, LP w/drops (\<20 psi)                        0.98

  1-4 mph, med spray (20-30 psi)                        0.96

  4-10 mph, hp impact (\>50 psi)                        0.90

  4-10 mph, LP w/drops (\<20 psi)                       0.96

  4-10 mph, med spray (20-30 psi)                       0.94

  All drip systems                                      1.00

  Under Tree Micro Spray                                0.99
  ----------------------------------------------------- -----------------

**[C - Climatic factor]{.underline}**

The climatic factor relates average daily evapotranspiration to
efficiency. **Table 7** shows the values for climate. A very dry climate
has more evaporation loss. Peak consumptive use values from the
irrigation guide are used to determine the value of C.

  ----------------------------------------------------- -----------------
  **Table 7 C-Climate factor**                          

  **Peak CU Mo**                                        **(factor)**

  Max 0.15 in/day                                       1.01

  Max 0.30 in/day                                       1.00

  Max 0.375 in/day                                      0.99

  Max 0.45 in/day                                       0.98
  ----------------------------------------------------- -----------------

-   **MANAGEMENT EFFICIENCY ELEMENT**

Management efficiency involves making decisions. Some decisions concern
agronomic practices while others are based on scientific measurements.
Management efficiency is determined by six factors. The factors are
applicable to a surface, subsurface or sprinkler irrigation system. The
six management factors are:

**Md - Use of flow measuring devices**

**S - Soil moisture monitoring and scheduling\
I - Irrigation skill and action\
M - Maintenance program for system\
O - Water delivery constraint\
Sc - Soil condition**

**[Md - Use of flow measuring devices]{.underline}**

Water must be **measured to each field** for optimum irrigation water
management. **Table 8** shows the factor values for flow measuring
devices. The most commonly used measuring devices are weirs or parshall
flumes, but there are other means to estimate flow. For instance, if a
staff gage is used to determine the amount of water being delivered, the
technician might select a value greater than .9 and less than .95.

**Measurement at the farm delivery point** can be translated to each
field if the water is not split and used on separate fields \-- use the
value for water measured to "each field" for this condition. When gated
pipe is used, even on multiple fields, the value for **measurement to
each field** may be used. Although, the actual amount applied may not be
known, all the water is applied. Select **none**, if the farm operator
makes no attempt to assure that flow down each furrow is that
recommended in the Irrigation Water Management Plan, \-- the operator
does not have control to each field or on each field.

  --------------------------------------------------- -------------------
  **Table 8 Md-Use of flow Measuring devices**        

  **(description)**                                   **(factor)**

  At each field                                       1.00

  At farm delivery                                    0.95

  None                                                0.90
  --------------------------------------------------- -------------------

**[S -- Soil moisture monitoring and scheduling]{.underline}**

The soil moisture deficit in the root zone must be measured (monitored)
and irrigations scheduled to obtain good water management. **Table 9**
shows the factors for soil moisture monitoring and scheduling. For an
irrigator using the check book method of scheduling, (**moisture
monitoring and scheduling**) a factor of 1.0 should be used. Using some
form of moisture monitoring, such as tension meters or resistance meters
to schedule irrigation should also be a 1.0 factor. Looking at the
condition of plants (**Irrigation when plant shows stress**) should be a
0.95. Irrigating when there is water at the head gate (on a set
rotation) or from a well without regard to soil moisture is a 0.90 or
**None**.

  -------------------------------------------------------- --------------
  **Table 9 S-Soil moisture monitoring and scheduling**    

  **(description)**                                        **(factor)**

  Irr. when plant shows stress                             0.95

  Moisture monitoring and scheduling                       1.00

  None                                                     0.90
  -------------------------------------------------------- --------------

**[I - Irrigation skill and action]{.underline}**

Good management requires either an experienced operator or an operator
trained in how to apply water. Determining the proper time of set and
length of run to replace depleted soil moisture is important in
achieving water management. An irrigation water management (IWM) plan is
essential to apply water correctly. **Table 10** shows the choices for
this factor. Structural work may help implement the plan, but without
the IWM plan, proper time of set and length of run is pure speculation.

In some cases, installation of a structural feature will result in an
improvement in this factor. For example, installing gated pipe will
result in some improvement as the irrigator using gated pipe will
require less skill than one using siphon tubes.

An automated system can be properly managed to substitute for a trained
irrigator. More mechanical skill will be required to manage a drip,
sprinkler, or bubbler system. Experience that is technically sound and
based on a good IWM plan may substitute for training. Training to
improve the irrigator skill level may be obtained by attending a NRCS
sponsored irrigation water management workshop or from some university
or junior college water management course. A full time experienced
irrigator following an irrigation water management plan will have a
factor of 1.0.

  ----------------------------------------------------- -----------------
  **Table 10 I-Irrigation skill and action**            

  **(description)**                                     **(factor)**

  Following IWM plan                                    1.00

  Nearly following IWM plan                             0.98

  No IWM plan                                           0.82

  Not following IWM plan                                0.90
  ----------------------------------------------------- -----------------

**[M - Maintenance program for the system]{.underline}.**

Proper system maintenance is imperative for a system to be managed at
its potential. **Table 11** shows the factor values for system
maintenance. Generally, this factor will not change as a result of
installing any structural features.

Nozzles on sprinklers have to be replaced, concrete ditches and
structures for water control have to be replaced when they wear out or
repaired when damaged. All leaks in the system should be kept to a
minimum.

  ------------------------------------------------------ ----------------
  **Table 11 M-System Maintenance**                      

  **(description)**                                      **(factor)**

  Maintained occasionally                                0.93

  Minor leakage (some leakage)                           0.97

  Seldom maintained                                      0.90

  Well maintained (no leaks)                             1.00
  ------------------------------------------------------ ----------------

**[D - Water delivery constraint]{.underline}**

Irrigating the crop when it needs water requires that water be available
when needed at the rate required for optimum application. **Table 12**
shows the factors for water delivery. A demand system, with full control
of when and how much water is applied, is ideal for good water
management. The three factors that affect water delivery are frequency
of delivery, flow rate and duration of flow.

**Rotation:** In a rotation system, the operator is allowed to use water
periodically, whether you need it or not. With rotational irrigation,
there is a tendency to over irrigate during part of the season, usually
early season (after planting) or late season (after senescence).
Frequently flow rate is fixed or constant throughout the season.

**Arranged:** In an arranged system, you request water from an
irrigation company or district and tell them when you are through with
the water. Arranged delivery usually involves some storage. There can be
restrictions, fixed flow rate, or limited rates during the season, which
reduces efficiency. Typically, Bureau of Reclamation project areas
arrange water delivery \-- normally 24-hour advance notice must be given
to either start or end delivery.

**User has full control of delivery:** With direct diversion, water is
diverted directly from a canal or channel. Water is not usually
available all season, every year. Normally there is no ditch rider or
regulatory authority to determine who can have water or when the user
can have water, the operator can turn water on or off when they decide
to. The physical size of the outlet or the supply system size limits
diversion. The factor value can vary for 0.8 to 1.0. If a system has an
inadequate water control structure, requires creek channel modification
to divert water, or does not have the required controls to deliver the
amount of water needed to meet the IWM plan, the factor should be less
than 1.0. Systems with little or no control should be at the 0.8 level.
Some project type systems have a limited amount that can be delivered,
but can turn the water off at any time.

  ----------------------------------------------------- -----------------
  **Table 12 D-Water delivery constant**                

  **(description)**                                     **(factor)**

  Arranged, fixed duration                              0.90

  Arranged, limited rate                                0.96

  Arranged, restricted                                  0.93

  Arranged, shutoff anytime                             0.97

  Direct diversion-min control                          0.80

  Limited rate, full control                            0.98

  Rotation, fixed freq & dur.                           0.83

  Rotation, modified amount                             0.88

  Rotation, modified freq                               0.85

  Unrestricted                                          1.00
  ----------------------------------------------------- -----------------

**[SC-Soil Condition Index (SCI)]{.underline}**

Conservation tillage, crop residue, subsoiling, and a conservation
cropping rotation are management tools to improve soil tilth and
facilitate better irrigation. **Table 13** shows the values for a given
SCI number. Intake rates are reduced as soil compaction increases and
organic matter is reduced. Roots do not grow into compacted soil. The
index uses the crop rotation, the number of tillage operations and soil
loss to develop a condition index. If the number is positive, the
cropping system is sustainable. If it is negative, the system is not
sustainable over time.

The SCI index ranges from --2.0 to +8.0, and can be calculated using
Agronomy Technical Note No. 42. See the following website for a copy:
<http://www.nm.nrcs.usda.gov/techserv/TechNotes/agro.htm> Note that
longer crop rotations that include legumes or grasses in the rotation
may have higher values. Generally, this factor will not change as a
result of installing any irrigation structural features.

  ------------------------------------------------------ ----------------
  **Table 13 Sc-Soil Condition**                         

  **(SCI Rating Number)**                                **(factor)**

  \< -2.0                                                0.890

  -2.0                                                   0.900

  -1.0                                                   0.905

  0.0                                                    0.910

  1.0                                                    0.920

  2.0                                                    0.930

  3.0                                                    0.950

  4.0                                                    0.980

  5.0                                                    0.990

  \>5.0                                                  1.000
  ------------------------------------------------------ ----------------

-   **DELIVERY EFFICIENCY**

FIRS **does not** account for water losses in a delivery system. The
second page of the spreadsheet is set up to make those calculations
based on what type of structure transports the water (delivers the water
to the field). There may not be large amounts of water lost if the
length is short or the structures have no leaks.

The wetted perimeter of each type of structure used in the delivery
system must be calculated. Page 2 of the FIRS spreadsheet will calculate
wetted perimeters for five different structures that carry water. Those
values are then used to estimate the seepage loss in the next section of
the form. Users will need to know the length of each reach of the
structure, type of structure, and the type of material the structure is
made of. **Table 14** shows the seepage loss of various conveyance
materials in Ft^3^/Ft^2^/day.

If the delivery system carries water to more than one field, the user
must evaluate the wetted perimeter of the system as if there is only one
field's flow in the structure. For example, if the main ditch from the
turnout carries 8 cfs for four fields, and has a wetted perimeter of 8
feet. When the flow is reduced to 2 cfs for the field to be evaluated,
the wetted perimeter is 5 feet. The user must use the wetted perimeter
for the lower flow.

+----------------------+-----------+--------------------+-------------+
| **Table 14 Seepage   |           |                    |             |
| Losses for various   |           |                    |             |
| materials**          |           |                    |             |
+----------------------+-----------+--------------------+-------------+
| **Conveyance         | **Loss**  | **Conveyance       | **Loss**    |
| Material**           |           | Material**         |             |
|                      | **(       |                    | **(Ft^3^/F  |
| **(name)**           | Ft^3^/Ft^ | **(name)**         | t^2^/day)** |
|                      | 2^/day)** |                    |             |
+----------------------+-----------+--------------------+-------------+
| Ash Loam             | 0.60      | Loam               | 1.25        |
+----------------------+-----------+--------------------+-------------+
| C. Loam              | 0.60      | Pipe               | 0.01        |
+----------------------+-----------+--------------------+-------------+
| C. Loam (hardpan)    | 0.29      | Sand               | 1.70        |
+----------------------+-----------+--------------------+-------------+
| CLD (broken)         | 0.23      | Sandy Clay Loam    | 0.90        |
+----------------------+-----------+--------------------+-------------+
| CLD (new)            | 0.10      | Sandy Loam         | 1.70        |
+----------------------+-----------+--------------------+-------------+
| CLD (old)            | 0.17      | Silt Loam          | 0.60        |
+----------------------+-----------+--------------------+-------------+
| Gr. C Loam           | 0.90      | V. F. Sandy Loam   | 0.74        |
+----------------------+-----------+--------------------+-------------+
| Gr. Sand             | 2.50      |                    |             |
+----------------------+-----------+--------------------+-------------+
| Gr. Sandy Loam       | 1.70      |                    |             |
+----------------------+-----------+--------------------+-------------+
| Gravels              | 4.50      |                    |             |
+----------------------+-----------+--------------------+-------------+

-   **SUMMARY**

FIRS has its greatest validity in **evaluating change**, not in
estimating the absolute value of onfarm irrigation efficiency. It would
be preferable to call the end product an index of water use. The FIRS
method evaluates both the present and future onfarm irrigation water
use, by using a standard set of system and management modifiers.

FIRS provides a relative rating. It is a product of up to twelve (for
sprinkler) management and system factors which consistently relate the
effectiveness of irrigation practices from one location to another. When
a potential efficiency is selected for a specific field and irrigation
system the index evaluates the difference between the gross volume of
farm delivery and the net consumed by the plant. Additional evaluations
may be needed for special water use, to determine how effectively the
system will apply a two-inch water application for germination, frost
protection, cooling, dust control, or leaching. FIRS represents typical
conditions and will only provide an index of change. It will not
substitute for detailed engineering designs or evaluation.

FIRS can be translated into water conservation terms, such as reduced
demand for water, reduced losses and waste, and amount of water
conserved. FIRS is a valuable tool for identifying the increments of
change in onfarm irrigation water use that can result from improvements
in system or management practices.

-   **EXAMPLE**

![](media/image2.wmf){width="5.998611111111111in"
height="8.097916666666666in"}

![](media/image3.wmf){width="5.998611111111111in"
height="4.694444444444445in"}