EB1810

     
INTRODUCTION  
     

This ready reference is a brief summary of Irrigation Management Practices to Protect Ground Water and Surface Water Quality - State of Washington. We use a short title, the Manual, for simplicity. The Manual is intended to be read by growers and their advisors, and is necessarily long to include the breadth of subjects required. This ready reference was developed to present the major elements of the Manual in a form that will serve many purposes for the irrigation professional and the grower. The Manual is available at no cost through all Washington State University Cooperative Extension offices and the Cooperative Extension Bulletin Office, Cooper Publications Building, Pullman, WA 99164-5912.

Water pollution results from a series of processes: availability, detachment/transformation, and transport of contaminants. For contamination to occur, contaminants must be available at the source of supply. Mechanisms with strong forces separate (detach) contaminants from the source and move (transport) them to a water resource where they may harm people or the environment. At some concentration of contaminant(s) the water is considered polluted for one or several uses. Reduce the pollution potential by minimizing
     

1 availability of the contaminant,
2 detachment/transformation of the contaminant,
3 transport of the contaminant,
     

The primary purpose of the Manual is to present an educational overview of the six Overall Management Objectives (OMO) for irrigated agriculture in Washington State. Achieving the Objectives will help minimize availability, detachment/ transformation, and transport of contaminants. In addition, the OMO can reduce water diversions and water pumped from wells for irrigation, minimize soil erosion, and improve farm profit. A list of Implementation Practices (IP or Practice) that have been shown to achieve the OMO is provided with each OMO. The Practices may involve changes in hardware, management, or both. The OMO and IP are listed in this bulletin for convenience.

The secondary purpose of the Manual is to provide summary discussions concerning nonpoint source pollution (NPS). These include the regulatory environment, current State and Federal strategies for combating NPS, recommended planning procedures for the individual, background science of important contaminants, basic soil-water-plant relationships, irrigation performance, and available resources for planning and implementing an individual NPS program.
     
 WHAT THE MANUAL IS NOT  
     

The Manual is not a complete list of Practices. Objectives and Practices listed in the Manual are generally recognized to be effective in reducing the potential for point and nonpoint source pollution if applied properly. However, there may be other Practices not presented in the Manual that are also effective. Science and practical experience will develop new Practices in the future. The Manual was designed to be a "living" document that can be modified locally and periodically revised to update the Practices.

The Manual is not a regulatory document. There is no State or Federal law using the Manual as a guide in deciding which Practices must be used by any individual. However, some Practices listed in the Manual are part of State law. An example is the recommendations concerning chemigation.

The Manual is not an engineering guide. Some practices will require detailed engineering plans developed for the specific situation. The Manual identifies available engineering resources.

Some parts of Washington State law call for the use of ". . . all known, available, and reasonable methods of prevention, control and treatment . . ." (referred to by the acronym "AKART") when dealing with current or anticipated polluting activities. The Practices presented in the Manual, if implemented correctly and appropriately, are designed to satisfy the intent of AKART. It is important to note, however, that use of one or more of the Practices may not be sufficient to protect water quality to the degree necessary and additional actions could be required.
     
 NONPOINT SOURCE POLLUTION  
     

NPS as defined by the U.S. Environmental Protection Agency is ". . . pollution . . . caused by diffuse sources that are not regulated as point sources ...." Further, the Washington Legislature has defined nonpoint source pollution as "pollution that enters the waters of the State from any dispersed water-based or land-use activities, including . . . surface water runoff from agricultural lands, urban areas, and forest lands, subsurface or underground sources ...." One source of nonpoint source contamination may be insignificant, but the cumulative effect of many sources is measurable and can lead to pollution of ground or surface waters. The diffuse nature of NPS pollution makes it difficult to locate causes and assign individual responsibility.

NPS is an economic and social issue. If the degraded quality of a water body prevents a beneficial use, the economic value of that use is lost. For example, if stream or lake quality is impaired to such a degree that fisheries are not supported, the economic value of fishing as recreation and a food supply is lost. Home and land values may be reduced if located in an area with known water quality problems. The values of lost beneficial uses are difficult to estimate accurately; however, they must be considered when setting policy or determining required actions.

NPS is usually the result of land-use activities including dairies, feedlots, irrigated and dry land agriculture, logging, rangeland management, food processing (disposal of wastes), and urban and industrial development. Other significant NPS include urban use of chemicals and fertilizers3 highway and railroad maintenance, and naturally occurring contaminants. Since NPS is usually the result of land-use activities, the focus for reduction and prevention of NPS is modification of these activities. State and Federal strategies emphasize voluntary adaptation of practices that will reduce and prevent NPS. The Manual is an educational component of these strategies that identifies those practices.
     

3 Terrninology: fertilizer and nutrient are used interchangeably herein and in the Manual.
     
     
 INDIVIDUAL WATER QUALITY PROGRAM PLANNING  
     
Planning an individual water pollution reduction/control program need not be a complicated process. It is important to remember the overall objectives of minimizing availability, detachment/ transformation, and transport. A feasible planning process can include the following steps.
     

1 Assess the current situation - This includes assessing the susceptibility of the environment to pollution, identifying current contamination amounts and types, identifying existing regional programs, and ongoing demonstration projects. The objective is to identify the contaminants and then how they are made available, being detached, and being transported. Base the assessment on a reasonable probability of water contamination, such as the 10-, 25-, 50-, or 100-year recurrence intervals.
2 Assess the future - Are specific contaminants under control or will they be an increasing problem?
3 Identify specific objectives - Which Overall Management Objective(s) is(are) not being achieved?
4 Identify the Practices that will help - More than one Practice is frequently necessary to achieve an Objective. Implementing one Practice can require the implementation of others.
5 Choose the most applicable Practice(s) The choice may be made based on effectiveness, economics, or both. Be aware of what Practices are being used locally. Consult local resources such as conservation districts, the Natural Resources Conservation Service (NRCS), WSU Cooperative Extension, and consultants.
6 Develop a realistic timeline for implementation, considering economics and coordinating with current management systems.
7 Monitor the situation - Choices of Practices to implement and how to implement them are very often subjective. Changes in water quality or indications of program effectiveness take considerable time, especially if your land is a small fraction of a large watershed. It is necessary to correctly evaluate results to guide future decisions.
     
Even if the original assessment (Step 1 above) finds no currently serious problems, a plan should be developed to prevent potential problems from becoming acute. Plans commonly include proper use and maintenance of irrigation systems and fertilizer/chemical application equipment, an irrigation scheduling method, use of applicable IPM techniques, and protection of domestic water supplies from contaminated surface water.
     
 OVERALL MANAGEMENT OBJECTIVES AND  IMPLEMENTATION  PRACTICES
     

Objective 1.00 - Minimize water losses in the on-farm distribution system
  The on-farm distribution system moves water from the primary supply (well, canal outlet, river pump) to the field(s). It may be very simple or complex. Depending on configuration, the onfarm distribution system may cause erosion of unlined ditches and deep percolation from seepage. The purpose of this OMO is to reduce detachment in unlined ditches and reduce transport by reducing deep percolation in the distribution system. Listed Practices are:
   
 
IP 1.00.01 Install concrete slipform ditches to replace earthen ditches
     
 
IP 1.00.02 - Convert earthen ditches to pipelines or gated pipe
     
 
IP 1.00.03 - Install flexible membrane linings in earthen ditches or reservoirs
     
 
IP 1.00.04 - Install swelling clays or other engineered material in earthen ditches or reservoirs
     
 
IP 1.00.05 - Maintain ditches and pipelines to prevent leaks
     

Objective 2.00 - Improve irrigation system performance and management
in order to minimize deep percolation and surface runoff
     
 

Excessive surface runoff or deep percolation from poor irrigation performance have the potential to transport pollutants. The goal is efficient, effective irrigations. Efficient irrigations make the best use of available water resources while minimizing negative effects on water quality due to excessive surface/subsurface losses. Effective irrigations do what is intended and help produce a profitable crop. Efficient, effective irrigations are the result of knowing when, how much, and how to irrigate and having the capability to execute.

When to irrigate is a production decision. Timing of irrigations will enhance the total cultural system. Commonly, irrigations are timed to avoid stress due to lack of soil water. Sometimes desired crop development will dictate some stress. Irrigations may also be timed to fertilizer applications. The irrigator must know how much to irrigate for all situations. Normally this is the amount of water required to refill the effective root zone of the plant, plus required leaching for salt control (during crop rotation), plus unavoidable losses to deep percolation, surface runoff, or immediate evaporation. It is valuable to know when to irrigate and crucial to know how much to irrigate. Finally, however, the irrigator must know how to irrigate. How to irrigate does not refer to the mechanics of setting up a booster pump or laying out sprinkler pipe. Rather, it is the capability to achieve good irrigation performance by applying the desired amount of water while minimizing unavoidable losses to immediate evaporation, deep percolation, and surface runoff. Note that it is impossible to irrigate without some losses unless parts of the field are under-watered due to inherent nonuniformity of applying water.

A brief discussion of irrigation scheduling and explanations of the fundamentals of irrigation performance, distribution uniformity, and application efficiency are presented at the end of this ready reference.

There are four sections within Objective 2. Three list Implementation Practices for different irrigation system types and one lists Practices applicable to any kind of irrigation system. Practices that can be used with any kind of irrigation system include:
     
 
IP 2.01.01 - Measure all water applications accurately
     
 
IP 2.01.02 - Monitor pumping plant efficiency
     
 
IP 2.01.03 - Evaluate the irrigation system using NRCS, American Society of Agricultural Engineers, American Society of Civil Engineers, or WSU Cooperative Extension procedures
     
 
IP 2.01.04 - Evaluate water quality and soil chemistry to determine required annual leaching ratios to maintain salt balances in the root zone and identify any chemical amendment needs to prevent/alleviate infiltration problems
     
 
IP 2.01.05 - Use irrigation scheduling as an aid in deciding when and how much to irrigate
     
 
IP 2.01.06 - Practice total planning of individual irrigations
     
 
IP 2.01.07 - Use two irrigation systems in special situations (sprinklers for pre-irritations then furrows; portable gated pipe to reduce furrow lengths for pre-irritations; sprinklers to germinate crops irrigated by micro-irrigation; over-tree sprinkler for cooling/frost control with under-tree irrigation)
     
 
IP 2.01.08 - Consider changing the kind of irrigation system
     
 
IP 2.01.09 - Use aerial photography to help identify patterns that indicate problems with irrigation/drainage; consider soil variation and topography
     

Implementation Practices for surface (furrow/rill, border strip) irrigation systems include:
     
 
IP 2.02.01 - Increase furrow flows to maximum non-erosive streamsize during advance
     
 
IP 2.02.02 - Use torpedoes to form a firm, obstruction free channel for furrow flow
     
 
IP 2.02.03 - Use surge-flow techniques
     
 
IP 2.02.04 - Decrease the length of furrow runs
     
 
IP 2.02.05 - Install a suitable field gradient using laser-controlled land and grading where topsoil depth allows
     
 
IP 2.02.06 - Irrigate a field in two cycles, one cycle with water in the compacted furrows, one in the uncompacted furrows to match inflow and set times with the different infiltration rates
     
 
IP 2.02.07 - Drive a tractor with no tools in the uncompacted rows to equalize the infiltration rates in adjacent furrows
     
 
IP 2.02.08 - Use laser-controlled land grading to remove high and fill low spots in a field
     
 
IP 2.02.09 - Rip hardpans and compacted soil layers to improve infiltration rates
     
 
IP 2.02.10 - Use cutback furrow flows to reduce surface runoff
     
 
IP 2.02.11 - Install runoff-reuse systems
     
 
IP 2.02.12 - After advance, reduce furrow flows to minimum necessary to ensure down-row uniformity if excess runoff is a problem
     
 
IP 2.02.13 - Control the total application of water
     
 
IP 2.02.14 - Apply water only in every other furrow
     

Implementation Practices for sprinkle irrigation systems include:
     
 
IP 2.03.01 - Have an irrigation engineer/ specialist check hand-line and side-roll sprinkle field layouts to ensure correct combinations of spacing, operating pressure, sprinkler head, and nozzle size/ type
     
 
IP 2.03.02 - Have an irrigation engineer/ specialist check field layouts for flow uniformity - use flow control nozzles, pressure regulators as necessary
     
 
IP 2.03.03 - Maintain sprinkle systems in good operating condition
     
 
IP 2.03.04 - Use the "lateral offset" technique with hand-line, side-roll, or "big gun" field sprinklers to improve overlap uniformity
     
 
IP 2.03.05 - Operate in low-wind situations if possible
     
 
IP 2.03.06 - Modify hand-line and sideroll sprinkle system layouts to smaller spacings and lower pressures if wind is a problem
     
 
IP 2.03.07 - Ensure that center pivot sprinkler/ nozzle packages match the infiltration rate of the soil
     
 
IP 2.03.08 - Minimize surface runoff from sprinkle-irrigated fields
     
 
IP 2.03.09 - Use reservoir tillage (dammer/diker) techniques to reduce field runoff
     
 
IP 2.03.10 - Install runoff-reuse systems (see IP 2.02.11)
     

Implementation Practices for microirrigation systems include:
     
 
IP 2.04.01 - Consult experienced agronomists/ engineers to ensure that the appropriate volume of soil is being wet by the system design to satisfy plant needs
     
 
IP 2.04.02 - Have an irrigation engineer/ specialist check the design for emission uniformity (pressure uniformity, correct pressure for the device) - use pressure regulators and pressure compensating emitters as necessary
     
 
IP 2.04.03 - Have the irrigation water analyzed to enable design of an adequate system of water treatment and filtration
     
 
IP 2.04.04 - Test all combinations of irrigation water/fertilizer/other additives in the system simultaneously to ensure compatibility and prevent clogging of the system before injection
     
 
IP 2.04.05 - Practice correct maintenance to ensure designed system performance.
     

Objective 3.00 - Manage fertilizer program to minimize excess fertilizer available for transport
     
 

This Objective seeks to minimize the availability of nutrients, primarily nitrogen and phosphorus, for detachment/transformation and ultimate transport. Important practices include analyzing soil, plant tissue, and water for residual nutrients to reduce over-application. Other practices address proper application. Stevens et al. (1993) present a detailed discussion of common plant nutrients, their pollution potential, and sound management practices to reduce the pollution potential.

Fertigation is the practice of applying nutrients by injecting them directly into the stream of irrigation water. It is an effective and convenient method for applying nutrients and is safe for the environment when used properly. Pacific Northwest Extension Publication 360 (Trimmer et al., 1992) contains detailed information concerning the proper implementation of fertigation. It is important to note that WACs 16-200-742 and 228-232 contain the regulations governing fertigation and chemigation.
     

  The Practices listed for Objective 3 are in three sections:
 

a overall good practices,
b prevention of excess applications,
c proper application techniques.
     

Overall good practices include:
     
 
IP 3.01.01 - Assess the risk of contamination of ground and surface water due to fertilizer/ chemical leaching or runoff. Berms around supply tanks are required
     
 
IP 3.01.02 - Consider conservation tillage methods to reduce erosion
     
 
IP 3.01.03 - Consider cropping patterns that include deep-rooted crops to scavenge residual fertilizer
     
 
IP 3.01.04 - Maintain records of all tissue tests, fertilizer tests, cropping rotations, yields, and applications
     

Practices that can help prevent excess applications include:
     
 
IP 3.02.01 - Analyze fields for residual fertilizer
     
 
IP 3.02.02 - Analyze irrigation water for nitrogen content
     
 
IP 3.02.03 - Analyze plant tissue to identify fertilizer requirements
     
 
IP 3.02.04 - Test manure or other waste materials for nutrient content
     
 
IP 3.02.05 - Apply seasonal fertilizer requirements with multiple applications that match plant needs
     
 
IP 3.02.06 - Use slow-release nitrogen fertilizers
     
 
IP 3.02.07 - Develop realistic yield goals
     

Practices to ensure accurate applications include:
     
 
IP 3.03.01 - Calibrate application equipment, including manure spreaders, to apply the correct, planned amount
     
 
IP 3.03.02 - Use the appropriate application technique (chemigation, broadcast, banding, foliar) for the particular situation
     
 
IP 3.03.03 - Schedule fertilizer applications to avoid periods of irrigation for leaching for salt control, plant cooling, or frost control
     
 
IP 3.03.04 - Avoid wind drift during applications. Apply proper droplet sizes and consider time of day
     
 
IP 3.03.05 - Incorporate surface applied fertilizers immediately to reduce any volatilization
     
 
IP 3.03.06 - Use nitrification inhibitors in combination with applications of ammoniacal forms
     
 
IP 3.03.07 - Ensure uniformity of application with manure
     
 
IP 3.03.08 - Do not apply manure to frozen ground, especially on sloping fields
     
 
IP 3.03.09 - Analyze irrigation water for compatibility with fertilizer to be applied by fertigation
     
 
IP 3.03.10 - Use fertigation properly and according to regulations
     

Objective 4.00 - Manage crop protection program to minimize
chemical residues available for transport
     
 

This Objective seeks to minimize the availability of chemical residues for detachment/ transformation and ultimate transport. Ramsay et al. (1991) and Mulla et al. (1989), thoroughly discuss ground water pollution by pesticides and other water soluble chemicals.

Pesticides refer to chemicals that are applied to control insects, weeds, or plant diseases. Applied pesticides may evaporate, be carried off the field attached to soil particles or in solution, be broken down into other substances, or be taken up by plants or insects as intended. The primary factors that affect the chemical's fate are the pesticide properties (adsorptivity, degradation rate, solubility, and volatility), soil properties, site conditions, and the application practices. A major Practice cited in IP 4.01.02 is Integrated Pest Management (IPM). IPM is a collection of management actions that seeks to reduce the overall use of synthetic chemicals (pesticides, herbicides, fumigants). Many Practices that are listed separately are considered parts of a comprehensive IPM program, such as, effective scouting for proper timing of chemicals and use of natural bio-controls. Chemigation is the practice of applying chemicals by injecting them directly into the stream of irrigation water. It is an effective and convenient method for applying chemicals and is safe for the environment when used properly. Information concerning the proper implementation of chemigation is detailed in Pacific Northwest Extension Publication 360 (Trimmer et al., l992). WACs 16-228-232 and 16-200742 contain regulations governing chemigation and fertigation.
     

  The Practices listed for Objective 4 are in two sections;
 

a overall good practices and
b proper application techniques.
     

Chemical applicators must be state licensed and injection equipment must be registered and inspected.
Overall good practices include:
     
 
IP 4.01.01 - Assess the risk of contamination of ground and surface waters due to chemical leaching and runoff
     
 
IP 4.01.02 - Practice Integrated Pest Management techniques where applicable
     
 
IP 4.01.03 - Schedule applications for maximum effectiveness
     
 
IP 4.01.04 - Maintain records of all chemicals bought and applied as well as scouts and individual applications
     
 
IP 4.01.05 - Read and follow all label instructions
     
 
IP 4.01.06 - Transport and store chemicals properly
     
 
IP 4.01.07 - Mix and load pesticides properly
     
 
IP 4.01.08 - Store and dispose of used containers properly
     
 
IP 4.01.09 - Maintain equipment properly to reduce spills or leaks
     
 
IP 4.01.10 - Clean equipment properly after use
     
 
IP 4.01.11 - Consider conservation tillage methods to reduce erosion
     

Practices to ensure accurate and effective applications include:
     
 
IP 4.02.01 - Calibrate application equipment
     
 
IP 4.02.02 - Use the appropriate application technique (chemigation, broadcast, air, ground)
     
 
IP 4.02.03 - Schedule chemical applications to avoid periods of irrigation for leaching for salt control, plant cooling or frost control
     
 
IP 4.02.04 - Analyze irrigation water for compatibility with chemicals to be applied by chemigation
     
 
IP 4.02.05 - Use chemigation properly and according to regulations
     

Objective 5.00 - Reduce contamination of surface water by sedimentation
  Surface runoff from irrigation or rainfall creates the potential for contamination by sedimentation. Sediment is a contaminant and can also carry adsorbed chemicals to surface water. This Objective seeks to reduce erosion and sediment loads in any surface runoff that is returned to surface waters.
     
 

 IMPORTANT!!!

Practices which reduce erosion and sediment loads generally increase infiltration rates and the total infiltration of water. Reducing erosion can increase leaching of nutrients and pesticides, thereby increasing the potential for ground water contamination. Water applications must always be controlled carefully. Plan for predicted rains. Achieving Objectives 3 and 4 will reduce the availability of nutrients and pesticides that might contaminate surface and ground water.
     

The Practices listed for Objective 5 are in two sections,
 

a reduction and prevention of erosion
b methods for managing sediment-laden runoff. Practices that can reduce or prevent erosion include:
     
 
IP 5.01.01 - Use cover crops on unprotected, easily erodible soils
     
 
IP 5.01.02 - Manage crop residues to reduce surface water contamination
     
 
IP 5.01.03 - Install straw mulching in furrows
     
 
IP 5.01.04 - Use reduced tillage, such as paraplow systems
     
 
IP 5.01.05 - Use pressed (slicked) furrows with furrow/rill irrigation systems
     
 
IP 5.01.06 - Grade the land to optimize furrow/rill slopes to reduce soil erosion
     
 
IP 5.01.07 - Install tailwater drop structures
     
 
IP 5.01.08 - Install buried tailwater drops and collection pipes
     

Practices that address treatment of sediment laden water include;
     
 
IP 5.02.01 - Install sedimentation basins
     
 
IP 5.02.02 - Install vegetative buffering (filter) strips
     
 
IP 5.02.03 - Collect and reuse surface runoff (see IP 2.02.11)
     

Objective 6.00 - Prevent direct aquifer contamination by wells
     
  Deep wells are a direct link from the surface to ground water. Aquifer contamination can occur because of movement of nutrients or pesticides from the surface through or along the well. Contamination can also occur when an unsealed well pierces two or more aquifers. If one aquifer is contaminated and the well is not properly constructed, water from the contaminated aquifer can move to the clean aquifer. Improperly abandoned wells are an open source of contamination. Abandoned wells must be properly filled and capped so there is no path from the surface to the aquifer. Well construction and abandonment are generally covered by Washington State law. Of particular interest is Engrossed Substitute House Bill l806 which amended many sections of the existing law and added new sections. An important new element is that all constructors of wells, whether licensed or not, are to construct wells according to the Department of Ecology's well standards. Well drilling regulations are contained in RCW 18.104. Minimum standards for construction and maintenance of wells are contained in WAC 173-160.
     

The Practices listed for Objective 6 are:
     
 
IP 6.00.01 - Complete wells properly where there is the possibility of cascading flows contaminating a lower aquifer
     
 
IP 6.00.02 - Do not store, load, or mix chemicals near a wellhead or other vulnerable place
     
 
IP 6.00.03 - Prevent back siphonage/ flow of chemicals or nutrients down a well after injection
     
 
IP 6.00.04 - Identify and properly seal all abandoned and improperly constructed wells
     
 ADDITIONAL RESOURCES  
     
  Chapter 7 of the Manual contains a list of publications under major subject headings of Fertilizer Management, Ground Water and Related Information, Integrated Pest Management (IPM), Irrigation System Management, Pesticide Properties, and Pesticide Application and Handling. Major agencies that can provide aid are also identified.
     
 

Regulatory Agencies
Washington State Department of Agriculture
406 General Administration Building, AX-41
Olympia, WA 98504

Washington State Department of Ecology
Water Quality Program
P.O. Box 47600
Olympia, WA 98504

U.S. Environmental Protection Agency
Region 10 - Pesticides Section
1200 Sixth Ave.
Seattle, WA 98101

Service Agencies
WSU Cooperative Extension - Cooperative Extension offices, located in each county, provide information on a wide range of topics related to pest management, fertility, crops, and irrigation water management.

USDA Natural Resources Conservation Service (NRCS), formerly known as Soil Conservation Service, can provide technical assistance through the development of farm management plans. NRCS and local Conservation Districts jointly assist growers in designing management plans.
     
     
 IRRIGATION SCHEDULING  
     

Irrigation scheduling is the general name given to several irrigation water management techniques. These techniques help the irrigator decide when to irrigate, how much water to apply with the irrigation, or both. Generally, one of several available methods is used to measure or predict the soil or plant water content. At a predetermined water content an irrigation is indicated. Irrigation scheduling can be used with any combination of crop and irrigation system. Efficiency can be improved by using irrigation scheduling because either individual applications are reduced or, in extreme cases, some irrigations are unneeded.

Ley (1992, 1995), James et al. (1989), and Trimmer (1994) offer complete discussions of irrigation scheduling. Several Drought Advisories from WSU Cooperative Extension also contain information useful for irrigation scheduling systems (Sorensen, 1992; Ley, l989). The Washington Irrigator (1989) also discusses irrigation scheduling. The NRCS (USDA-SCS, undated) describes a simple, useful way to estimate soil water content by feeling and observing soil samples.

In addition, WSU Cooperative Extension has developed the Washington Public Agricultural Weather System (PAWS). PAWS is a valuable source for irrigation management information. It consists of a network of standardized, calibrated weather stations placed in strategic agricultural production areas. The data from these weather stations are stored in computers and available to users of the service. More importantly, PAWS calculates a daily reference crop evapotranspiration (crop water use). Knowing this reference crop evapotranspiration, the water use of other crops is estimated by using their crop coefficients. This is essential information for use of the checkbook form of irrigation scheduling. The PAWS user manual (Wright and Ley, 1990), is available from Cooperative Extension offices.
     
     
 IRRIGATION PERFORMANCE DISTRIBUTION UNIFORMITY
 AND APPLICATION EFFICIENCY
     

There are two measures of irrigation performance: distribution uniformity and application efficiency. Distribution uniformity is a measure of how evenly water infiltrates the ground across a field during irrigation. Eight inches of water infiltrating the ground in one part of a field and four inches in another part of the field illustrate poor distribution uniformity. Distribution uniformity is expressed as a percentage between 0 and 100. Although 100% distribution uniformity is theoretically possible, it is virtually impossible to attain in practice. Good distribution uniformity is essential for reducing deep percolation if the entire field is to be watered sufficiently.

Application efficiency (irrigation efficiency) was defined by the American Society of Civil Engineers' On-Farm Irrigation Committee in 1978 as the ratio of the volume of irrigation water beneficially used to the volume of irrigation water applied. Beneficial uses include crop evapotranspiration, deep percolation needed for leaching for salt control, crop cooling, and an aid in certain cultural operations. Differences in specific mathematical definitions of application efficiency are due primarily to the physical boundaries of the measurement (a farm, an irrigation district, an irrigation project, or a watershed) and whether the application efficiency is for an individual irrigation or an entire season. Application efficiencies are expressed as a percentage between 0 and 100. An application efficiency of 100% is unattainable due to immediate evaporation losses during irrigation. However, 95% application efficiency can be approached if a crop is under-watered, or properly designed and managed systems are used to minimize evaporation losses, for example, drip systems. If there were no deep percolation, all water applied and not immediately evaporated would be available for crop use.

Note that "irrigation efficiency'' or "application efficiency" should not be confused with "water use efficiency" (WUE). WUE is generally a measure of crop yield per unit of water applied.
     

Relationships Between Distribution Uniformity and Application Efficiency
     

Two important relationships between distribution uniformity and application efficiency are described in Figures 1 - 4. The relationships are illustrated with a profile view of two adjacent sprinklers in a field with the underlying root zone. The horizontal line in the figures denotes the depth of soil water depletion at the beginning of irrigation.

This is the amount of water the grower must add to the soil to refill the root zone soil profile to field capacity. The depth of water infiltrated during the irrigation is indicated by the line marked depth of water application. Deep percolation is indicated whenever the depth of water infiltration is greater than the depth of soil water depletion. Conversely, under-irrigation occurs when the depth of infiltration is less than the depth of soil water depletion.

Figures 1 and 2 demonstrate the first relationship - there must be good distribution uniformity before there can be good application efficiency if the crop is to be sufficiently watered. In Figure 1, the grower irrigates to sufficiently water the entire field. The poor distribution uniformity, indicated by the steeply sloped depth of water application, produces excessively deep percolation. That is, the deep percolation is much more than necessary to maintain an annual salt balance.

FIGURE 1.

Depiction of irrigation resulting in poor distribution uniformity and excessive deep percolation.

 
   

In Figure 2, the grower acts to prevent excessive deep percolation. Now part of the field remains under-irrigated. Under-irrigation usually results in a high application efficiency as most water applied is stored in the root zone and is available for plant use. But, it may not be an effective way of growing as the resulting water stress on the crop in some parts of the field will usually decrease yields. Further, for leaching to maintain a salt balance, some deep percolation is needed annually. Leaching must be uniform over a number of years to prevent areas of excessive salt accumulation.

FIGURE 2.

Depiction of irrigation resulting in poor distribution uniformity while under-watering the field.

 
   

Figures 3 and 4 indicate the second relationship - good distribution uniformity is no guarantee of good application efficiency. Figure 3 depicts a good irrigation. Distribution uniformity is good as indicated by the more even depth of water application. About the right amount of water is applied. There is little deep percolation (enough for salt control) and the entire field is wet sufficiently. It is assumed that surface runoff is minimal or collected for reuse. Figure 4 depicts an irrigation with the same good distribution uniformity as Figure 3.

FIGURE 3.

Depiction of an irrigation sufficiently watering the entire field with good distribution uniformity and application efficiency.

 
   

However, twice as much water as needed is applied, producing poor application efficiency. A practical example of this situation is the grower using a well-designed and maintained microirrigation system. The hardware provides good distribution uniformity and the potential for good application efficiency.. But, if the grower runs the system twice as long as necessary, the potential is not realized and there will be excessive deep percolation and associated leaching of contaminants.

FIGURE 4.

Depiction of irrigation with good distribution uniformity but excessive deep percolation.

 
   

     
 SUMMARY  
     

Improved irrigation system hardware or management may result in greater distribution uniformity and improve the potential for higher application efficiency. It follows that distribution uniformity is the first concern when improving irrigation system performance. However, achieving high application efficiency ultimately depends on the management of the system.

The overall uniformity of any kind of irrigation system consists of different components. For example, the uniformity of sprinkler systems may be affected by different pressures throughout the system, or different sprinkler heads or nozzle sizes (or worn nozzles). In addition, wind greatly affects the spray patterns. High quality water, uniform pressures (or flow rates), and similar emitters in good operating condition are critical to micro-irrigation systems. Each uniformity component can be evaluated. Some of the Practices listed under Objective 2 address improvements in the components of uniformity. Others are ways to improve knowledge of the correct timing and amount of applications or improving control over the total application.

The primary purpose of the Manual is to present six Overall Management Objectives with Implementation Practices for each Objective. Numerous Practices address changes to management and facilities to reduce and control nonpoint source pollution associated with irrigated agriculture. The Manual educates as well as presents information for designing a farm-specific irrigation management plan to improve the grower's operation and protect the land and water resources.
     
   
 REFERENCES CITED  
     
 

Adams, E.B. Reprint 1994. Protect Your Groundwater: Survey Your Homestead Environment. Washington State University Cooperative Extension Bulletin EB1631.

James, L.G., J.M. Erpenbeck, D.L. Bassett and J.E. Middleton. Reprint 1989. Irrigation Requirements for Washington: Estimates and Methodology. Washington State University Cooperative Extension Bulletin EB 1513.

Ley, T.W. Reprint 1995. Simple Irrigation Scheduling Using Pan Evaporation. Washington State University Cooperative Extension Bulletin EB1304.

Ley,T.W. Rev. 1992. WSU Drought Advisory: Scientific Irrigation Scheduling. Washington State University Cooperative Extension Bulletin EM4825.

Ley, T.W. Reprint 1989. WSU Drought Advisory: Visual Crop Moisture Stress Symptoms. Washington State University Cooperative Extension Bulletin EM4821.

Ley, T.W. 1989. The Washington Irrigator, Issue No. 11. Washington Energy Extension Service and Washington State University Cooperative Extension.

Mulla, D.J., R.E. Hermanson, and R.C. Maxwell. 1989. Pesticide Movement in Soils Groundwater Protection. Washington State University Cooperative Extension Bulletin EB1543.

Ramsay, C.A., C.G. Cogger, and C.B. MacConnell. 1991. Protecting Ground water from Pesticide Contamination. Washington State University Cooperative Extension Bulletin EB1644.

Sorensen, E.J. Rev. 1992. WSU Drought Advisory: Vegetable Crops. Washington State University Cooperative Extension Bulletin EM4830.

Stevens, R.G., D.M. Sullivan, and C.G. Cogger. 1993. How Fertilizers and Plant Nutrients Affect Ground Water Quality. Washington State University Cooperative Extension Bulletin EB1722.

Trimmer,W.L., T.W. Lay, G. Clough, and D. Larsen, Chemigation in the Pacific Northwest. 1992. Pacific Northwest Extension Publication PNW 360.

Trimmer W., and H. Hansen. Reprint l994. Irrigation Scheduling. Pacific Northwest Extension Publication 288.

U.S. Department of Agriculture, Soil Conservation Service. Undated. Estimating Soil Moisture by Feel and Appearance.

Wright, M.A. and T.W. Ley. Rev. 1990. PAWS Users Manual. Washington State University Cooperative Extension Bulletin EB1547.
     
     
 ACKNOWLEDGMENTS  
     

Publication partially funded by Clean Water Act Section 319 funds administered by the Washington Department of Ecology and the U.S. Environmental Protection Agency, Region 10.

By

Ronald E. Hermanson, Ph.D., P.E.
Principal Investigator and Extension Agricultural Engineer, Water Quality Biological Systems Engineering Department and Peter Canessa, P.E., Project Coordinator and Consulting Irrigation Engineer

Washington State University Cooperative Extension

 

Issued by Washington State University Cooperative Extension, and the U.S. Department of Agriculture in furtherance of the Acts of May 8 and June 30, 1914. Cooperative Extension programs and policies are consistent with federal and state laws and regulations on nondiscrimination regarding race, color, gender, national origin, religion, age, disability, and sexual orientation. Evidence of noncompliance may be reported through your local Cooperative Extension office. Published October 1995. Subject code 340. X EB1810

     

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