Irrigation Water Management CNMP Core Curriculum Section 5 — Nutrient Management

Irrigation Water Management

CNMP Core Curriculum

Section 5 — Nutrient Management

CNMP Development Core Training Curriculum

These course materials have been developed as a cooperative effort between five land-grant universities and The Natural Resources Conservation Service.

Ames, Iowa 50011, (515) 294-4111.

Copyright © 1995-2006, Iowa State University of Science and Technology. All rights reserved.

Copyright Information

Objectives

• The student should become familiar with the basic concepts of irrigation water management and what is required for documentation.

Required Materials

• National Statement of Work for Irrigation Water Management www.nrcs.usda.gov/technical/standards/nhcp.html

• National Conservation Practice Standard for Irrigation Water Management www.nrcs.usda.gov/technical/standards/nhcp.html

http://www.nrcs.usda.gov/technical/standards/nhcp.html

http://www.nrcs.usda.gov/technical/standards/nhcp.html

Supplemental Materials

• Local Irrigation District Water Management Plans and Irrigation Guides.

• National Engineering Handbook, Part 652, Irrigation Guide www.info.usda.gov/CED/

• National Engineering Handbook, Part 623, Irrigation, Section 15, Chapters 1 through 12 www.info.usda.gov/CED/

http://www.info.usda.gov/CED/





• Irrigation Water Management is the process of determining and controlling the volume, frequency, and application rate of irrigation water in a planned, efficient manner.


• Why is Irrigation Water Management Important?

• Manage soil moisture to promote desired crop response.

• Optimize the use of available water supplies.

• Minimize irrigation-induced erosion.• Decrease non-point source pollution of

surface and groundwater resources.• Manage salts in the crop root zone.• Manage air, soil or plant micro-climate.



Statement of Work for Irrigation Water Management

Irrigation Water Management• Minimum Documentation:

– Crops to be Grown, Cropping Sequence and Soils Information.

– Volume of Water Needed per Irrigation and for the season.

– Application rate of irrigation water.

– Records Showing Date and Amount of Water Applied.• Include type of irrigation scheduling

technique used by the client.

– Evaluation of the Irrigation System

– Environmental Considerations

Irrigation Water Management• Determining Volume of Water Needed


– Crop Consumptive Use (CU)

The amount of water used by the crop in transpiration and building of plant tissue, and that evaporated from adjacent soil or intercepted by plant foliage. It is expressed as depth in inches or as volume in acre inches per acre. It can represent the daily, design, monthly, or seasonal quantity of water needed for plant growth. Often referred to as Crop Evapotranspiration (ETc ).


– Net Irrigation Water Requirements (Fn )

• Crop Evapotranspiration (ETc ).

• Auxiliary water needs such as leaching, temperature modification and crop quality (Aw ).

• Effective precipitation (Pe ).

• Groundwater contribution (GW).

• Change in soil water content for the period of consideration (ΔSW).






http://www.wcc.nrcs.usda.gov/nrcsirrig/irrig-mgt-models.html




Crop

Evapotranspiration in Inches

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Corn Silage0.00 0.00 0.00 0.00 1.22 4.17 7.13 5.91 1.14 0.00 0.00 0.00

Grass Pasture0.00 0.00 1.46 3.31 4.53 5.24 6.22 5.28 4.02 2.48 0.43 0.00

• Determining Volume of Water Needed– Net Irrigation Water Requirements (Fn )

Crop

Net Irrigation Water Requirements in Inches

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Corn Silage0.00 0.00 0.00 0.00 0.47 3.62 7.13 5.83 1.06 0.00 0.00 0.00

Grass Pasture0.00 0.00 0.08 1.57 3.15 4.57 6.22 5.12 3.46 0.87 0.00 0.00


– Net Irrigation Water Requirements (Fn ), Auxiliary Water Needs for Salinity Management

• Soils in arid areas have the potential to become saline or sodic.

• Saline or sodic soils will cause poor seed germination and reduced yields.

• Additional water must be added to soils with a potential to have saline or sodic problems to leach excess salts.

• Where the soluble salt content of wastewater is high enough to cause problems, the wastewater must be diluted with good quality water or applications must be limited.







Aw = ETc

(1/Lr – 1)where:

Aw = Auxiliary Water Needs for Salinity Management in inches.

ETc = Seasonal Crop Evapotranspiration in inches.

Lr = Leaching Requirement from Figure 2-33 of the NRCS National Engineering Handbook, Part 623, Chapter 2 - Irrigation Water Requirements.


– Net Irrigation Water Requirements (Fn ), Contribution of Ground Water (GW)



Fn = ETc + Aw – Pe – GW - ΔSW

where:

ETc = Crop evapotranspiration in inches

Aw = Auxiliary water needs for leaching, temperature modification, crop quality in inches.

Pe = Effective precipitation in inches

GW = Estimated contribution from ground water in inches.

ΔSW = Change in soil water content for the period considered.


– Available Water Capacity of Soils (AWC)


http://soils.usda.gov/

• Determining Volume of Water Needed– Available Water Capacity of Soils (AWC)


http://soildatamart.nrcs.usda.gov/



What depth to use in computing AWC?






Given:Silty Clay Loam Soil

Soil Depth = 3 feet

AWC = 0.20 in/in from 0 to 24 inches of soil depth

0.10 in/in from 24 to 36 inches of soil depth

Managed Crop Rooting Depth = 4 feet

What is the total plant available soil water capacity for the crop being grown?


Determine total plant available soil water capacity:

Use 3 feet to determine total plant AWC since the soil depth is less than the managed crop rooting depth.

AWC = 0.20 in/in x 24 inches = 4.8 inches

0.10 in/in x 12 inches = 1.2 inches

Total Plant AWC = 6.0 inches



MAD is defined as the percentage of the available soil water that can be depleted between irrigations without serious plant moisture stress. MAD is expressed as:– a percentage of the total Plant Available Water

Capacity (AWC),– a soil-water deficit (SWD) in inches, or– an allowable soil-water tension level.

• Determining Volume of Water Needed– Management Allowed Depletion (MAD)


• Determining Volume of Water Needed– Management Allowed Depletion (MAD)


– Net Irrigation Application (Fa ) based on Plant AWC:

Fa =

MAD * Plant AWC 100%where:

Fa = Net Irrigation Application in inches

MAD = Moisture Allowed Depletion in percent(50% for Corn Silage)

Plant AWC = Plant Available Water Capacity of soils (6 inches)

Fa = 3 inches


– Net Irrigation Application (Fa ) based on Plant AWC for Corn Silage:

Fa = 50% * 6 inches 100%

– Net Irrigation Application (Fa ) based on Net Irrigation Requirement (Fn ):

Fa = 7.13 inches for Corn Silage Fa = 6.22 inches for Pasture

Use this value


– Gross Irrigation Application Requirements

d =Fa

Ea

where:d = Gross application, inchesFa = Net application, inches

Ea = Application efficiency of

the irrigation system





Use a net irrigation application (Fa ) of 3.00 inches. A traveling big gun irrigator with an application efficiency of 60% will be used to irrigate Corn Silage and Pasture.

d =3.0 inches x

100%60%

d = 5 inches


– Irrigation Frequency (f):

where: MAD = Management Allowed Depletion (%).

Plant AWC = Plant Available Water Capacity (inches).

Daily ETc = Daily Evapotranspiration or Consumptive Use for the crop being grown (inches/day).

MAD x Plant AWC

Daily ETc

f =


– Irrigation Frequency (f):

where: MAD = 50% for Corn Silage

Plant AWC = 6.0 inches Daily ETc = Daily Evapotranspiration or

Consumptive Use for Corn Silage in inchesDaily ETc

=7.13 inches for July

31 daysDaily ETc = 0.23 in/day50% x 6.0 inches

100% x 0.23 in/dayf =

f = 13 Days


– Irrigation System Capacity Requirements

Q = 453 * A * d f * H

where:Q = flow rate (gpm) A = area to be irrigated is 40 acres d = gross application depth is 5.0

inches f = irrigation frequency is 13 daysH = 16 hours of operation per day is

desired by the client.



Q = 453 * 40 acres * 5.0 inches

13 days * 16 hours/dayQ = 436 gpm or 0.97 cfs

Note:

The existing or planned irrigation system must be able to supply 436 gpm and irrigate the 40 acres of Corn Silage every 13 days to meet the crop evapotranspiration needs during the peak water use period in July.


– Irrigation System Application RateAssume a traveling “Big Gun” will be used to irrigate with. Select the system flowrate and wetted diameter from Table 6-6:


– Irrigation System Capacity RequirementsThe application rate for a traveling “Big Gun” is computed using the following formula:

It = C * Q R2 * Deg

where: It = average application rate in

inches/hour C = unit conversion constant, 13,624 Q = Gun discharge in gallons per minute R = wetted radius in feetDeg = portion of circle receiving water in

degrees, usually does not exceed 270o.


– Irrigation System Capacity RequirementsThe application rate for a traveling “Big Gun” is computed using the following formula:

It = 13,624 * 225 1632 * 270

It = 0.43 in/hr



< I = 0.43 in/hr

Irrigation Water Management• Irrigation Scheduling

Irrigation Water Management• Irrigation SchedulingImportant factors to keep in mind when developing a irrigation scheduling tool for a client:

• The scheduling tool must consider information about the crop, soil, climate, irrigation system, water deliveries and management objectives.

• An irrigation scheduling tool needs only be accurate enough to determine how much water to apply and when.

• A good rule of thumb to follow when developing an irrigation scheduling tool is to keep it simple and easy for the client to understand.

Irrigation Water Management• Irrigation Scheduling

Irrigation Water Management• Irrigation System Evaluation


Irrigation system evaluation is the analysis of any irrigation system and management based on measurements taken in the field under normal conditions and management.

There are three levels of irrigation system evaluations that can be performed:

Simplified

Abbreviated

Detailed

Irrigation Water Management• Irrigation System Evaluation There are 3 levels of Irrigation System Evaluations:

Simplified – This type of evaluation provides enough information to the landowner/operator to make management and operation decisions. This evaluation usually takes a few hours to complete.

Abbreviated – This type of evaluation provides enough information for the landowner/operator to make management and operation decisions, plus identify any problems with the system. This evaluation takes a half to full day to complete.

Detailed – This type of evaluation provides the landowner/ operator with a report and a comprehensive irrigation system operation and maintenance plan. This evaluation can take one to five days to complete.


Irrigation Water Management• Irrigation Water Management Jobsheet





Class Exercise

Questions?

Documents

Irrigation Water Management CNMP Core Curriculum Section 5 — Nutrient Management