Fresno State Logo                  Welcome to Wateright

  A Summary Discussion of Nonpoint Source Pollution in Irrigated Agriculture
In This Advisory
Purpose - The purpose of this advisory is to...
  • Define nonpoint source pollution (NPS) as contrasted to point source pollution
  • Explain the model of nonpoint source pollution
  • Explain the most common programs for controlling and reducing nonpoint source pollution through "Best Management Practices"
  • Explain how irrigation and rainfall act as detachment and transport mechanisms
  • List the steps for developing an individual water quality control plan
  • List the most common management practices that can be used to control and reduce NPS as related to irrigation system management
Growers have many things to worry about, weather, pest pressures, commodity prices, regulatory requirements, and water supplies among others.  Added to this will be concerns over nonpoint source pollution of water resources (NPS).  Nationwide, the EPA is stepping up pressure on Confined Animal Feedlot Operations (CAFO) to control manure releases.  In California, livestock producers have developed a statewide Water Quality Program under which each producer must develop a Ranch Water Quality Program.   Combined Resource Management Plans (CRMP) are becoming popular to control water quality on a watershed-wide basis. This paper will be a short primer on some of the most important aspects of NPS and how government is approaching control of the problem.  It will be worth your time to read through this.

There are two categories of water pollution, point and nonpoint.   Point source pollution occurs when the source of the pollution is readily identifiable.  Examples of point source pollution are a discharge pipe from a factory or the outlet from a city's sewage treatment plant.

Much State and Federal effort has been directed at controlling and reducing point source pollution as a result of the 1972 Clean Water Act.  Most significant point sources now operate under discharge permits that define the conditions of the discharge.   These conditions are designed to control the amount of contamination from the individual sources. Because of the great strides in controlling point source pollution, the emphasis in recent years has switched to nonpoint source pollution.  Nonpoint source pollution as defined by the Federal Environmental Protection Agency is:
"...pollution... caused by diffuse sources that are not regulated as point sources..."
A definition developed by the State of Washington is:
"[nonpoint source is] pollution that enters the waters of the state from any dispersed water-based or land-use activities, including, but not limited to, atmospheric deposition, surface water runoff from agricultural lands, urban areas, and forest lands, subsurface or underground sources, and discharges from boats or other marine vessels."
Nonpoint source pollution is diffuse and cumulative in nature.  Typically it is a combination of many small, even insignificant, sources- any one of which may in fact be legal.  However, it is the cumulative effect of many such sources is measurable and leads to significant pollution of ground or surface waters.

Nonpoint source pollution is usually the result of land-use activities.  This would include dairies, irrigated and dryland agriculture, logging, rangeland management, and food processing (disposal of wastes).  However, there are other significant sources of nonpoint pollution.  These include:
  1. Urban and suburban use of pesticides, herbicides, and nutrients.
  2. Runoff from highways and other paved areas.
  3. Maintenance of highway and railroad rights-of-way.
  4. Mosquito abatement activities.
  5. Naturally occurring contamination.
The problems facing agriculture in addressing NPS concerns will not go away soon.   In 1998, of 64 peer-reviewed papers published by the American Society of Agricultural Engineers, 34 of them concerned NPS.  This indicates that State and Federal governments are committing significant resources with the long-term goal of controlling and reducing NPS.

What is at the basis of the concern over nonpoint source pollution?   The simple fact is that water resources are a shared resource .  That is, any one ground or surface water resource will probably have several types of users. These include agriculture, recreation, industrial, domestic, stock watering, and the natural environment.  These uses may be further broken down into such categories as contact (swimming) versus non-contact (fishing and boating only) recreation uses.  These different uses require different quality levels.  For example, the quality level for drinking water is much higher than that for irrigation water. 

Degradation of water quality has an economic impact.  If the degraded quality of a waterbody 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 both recreation and a food supply is lost.   Frequently the values of lost beneficial uses are difficult to estimate accurately.   However, they must be considered when formulating policy or determining required actions.

The assessment of economic impacts of ground water pollution, including both lost benefits and the (appropriate) cost of remediation and control, are complicated by several factors.   These include:

Irreversibility - It may be difficult, if not impossible, and certainly takes substantial time and effort, to clean up a contaminated aquifer.  As the costs of aquifer cleanup are more accurately identified (especially with the experience at Superfund sites), it becomes clearer that it is much less expensive to prevent pollution rather than have to clean up pollution.

Uniqueness - There may not be a substitute supply in areas where aquifers are a primary water supply.  Then the cost of pollution prevention and cleanup are necessary requirements.  An aquifer may be designated as a "sole-source" aquifer by the Federal Environmental Protection Agency.  A sole-source aquifer is one that supplies 50% or more of an area's drinking water and to which contamination would create a significant health hazard.

Indivisibility - Aquifers serve many uses and many users.  Different parts of aquifers cannot be "fenced off' like real property.  If one user pollutes the aquifer, it is generally polluted for all users. Note that some contamination can be accommodated for some types of uses.   Nitrate concentrations in water that preclude its use as drinking water do not adversely affect its use as an irrigation supply.

Uncertainty - There is some uncertainty as to what is a genuine health risk due to certain contaminants, especially for long-term exposures.  Numerical limits are identified by State and Federal Law and are based on a wide variety of health~related studies.  An important question is when do pollution prevention measures become required based on tests that approach the limit.   There is also uncertainty about what efforts are required to prevent pollution.  Many factors governing pollution vary widely from area to area including crops, topography, climate, and aquifer depth and size.   Efforts that may be successful one location may not be effective somewhere else.

Acceptable contamination - As stated, there are many uses and many users of ground and surface waters.   What is contamination for one class of users may not be contamination for another.   A continual concern for policymakers is putting too much of an economic burden on one class of users to prevent contamination for another class.  This is especially important in areas of high agricultural activity where the economic viability of the region depending on the economic health of the agricultural sector.

The Federal Environmental Protection Agency has ultimate responsibility for maintaining water quality in the U.S.   However, for the most part they have delegated this responsibility to the different states.  In California, it is the State Water Resources Control Board that has responsibility for water quality, among other things.   They administer their responsibilities through the different Regional Water Quality Control Boards.
Each Regional Board develops a Basin Management Plan.  This plan basically:
  1. Identifies the water bodies in the basin
  2. Identifies the uses for these water bodies
  3. Identifies the water quality needed to maintain these uses
  4. Undertakes testing to ensure that the quality is being maintained
The Regional Boards are responsible for developing the Plans and then putting them into action.  This may involve taking action against identified polluters, or, in the situation of nonpoint source pollution, develop additional programs for control and reduction of NPS

It is difficult, by its nature, to assign specific responsibility for nonpoint source pollution when it occurs.  Thus, the current state and federal strategies for reducing and controlling nonpoint source pollution rest heavily on education and voluntary adaptation of those actions which reduce the potential for pollution.  Typically, impacted areas are studied for the most likely causes of current or potential NPS and education/demonstration/assistance programs developed to address local conditions.   The most common term associated with these types of programs is "best management practices" or BMPs.

The term "best management practice" comes from language in early laws that essentially said that "best management practices shall be used as of ..".   BMPs are actions involving the hardware, process, or management, or all three, that will act to reduce or control nonpoint source pollution.  The education/demonstration/assistance programs mentioned above will seek to identify the practices that will reduce or control nonpoint source pollution.  Then, through education, demonstration, and assistance, individual farmers are encouraged to adopt these identified practices.

There is much controversy over the use of the term "best management practice".  The fear is that a list will be written and approved, thus forcing farmers to adopt certain practices, regardless of the circumstances.  Many professionals now recognize this problem.  In fact, a manual identifying practices that could help reduce/control NPS that I helped write for the State of Washington did not use the term "best management practice".   Our approach was to define six Overall Management Objectives (Improving Irrigation Performance was one of them) and then identify various practices that could be used to achieve the Objectives.  The specific practices that would be implemented by anyone farmer would depend on the situation.  Thus, that manual recognized, as we all know, that there are no "best" management practices.  There are some practices that work in some situations, others that work in other situations.

Typically, programs to address NPS have three tiers.
  1. Tier 1 programs rely on voluntary adaptation of management practices that will reduce, prevent, and control NPS.  These programs involve research to clearly define the problem and its causes, clearly define the management practices that can compbat the problem(s), design and implement education and outreach programs, design and implement demonstration projects (to demonstrate how the management practices can be implemented, and obtain grants and low-interest loans.
  2. Tier 2 is implemented if Tier 1 (the voluntary adaptation of management practices) is seen not to be effective.  In Tier 2 there may be a reappraisal of the problem and proposed solutions.  However, the main difference will be that responsible parties may have to sign binding agreements with regulatory agencies.   This is called a "regulatory-encouraged" phase.  The responsible parties still have some flexibility in what actions they take but now there is a binding agreement in place to ensure that these actions actually are implemented.
  3. Tier 3 is implemented if Tier 2 is not effective.  Tier  3 is termed "regulatory-enforced".  At this stage of the game, "cease-and-desist" orders may be issued, in which case whatever activities are seen to be causing the pollution must cease.  Another  possible action in this stage is the issuance of Waste Discharge Permits, in which case the activity is allowed, but under strict control as to discharges of potential contaminents.
The Tier 1 programs have many of the following characteristics:
  • The problem is correctly identified, not only to program designers, but also, and more importantly, to the customer. It is important to note that at this stage the customer, through industry leadership, is part of the problem identification process. It is likely that this is the critical stage for development of a successful program. If the problem is not correctly identified then resources will be wasted. If all participants do not "buy-in" to the problem identification then the program will be plagued with political problems.
  • Viable solutions are identified (a common term for these solutions is "Best Management Practice"). Again, customers are part of this process to ensure that solutions are practical and economical.
  • Education and outreach efforts are put in place to make all concerned aware of the problem and the potential solutions. Customers must see that there is a problem, that the problem is theirs, and that there is something that can be done about it. Supply-side "players" are encouraged to stock the appropriate parts and/or learn the design, installation, and management techniques required for the solutions.
  • Demonstration projects are funded so that customers can see the solutions in action.
  • Financing assistance may be put in place depending on the situation.
  • Monitoring is put in place to judge whether the program is effective or not, and to indicate where and when changes should be made.
  • There are clear and quantifiable goals identified that signify success
Agriculture is not the only source of NPS.  However, it is to our benefit to address the issue in a manner that will minimize any impacts from agriculture activities.  To deal effectively with NPS, and to intelligently choose the actions that anyone individual might take, requires an understanding of the NPS process and where irrigated agriculture fits.  Pollution is the result of a series of processes.   These can be generally categorized as availability, detachment, and transport.

  1. Availability -Availability means that there is a potentially polluting substance in some amount and in some place.  The potential pollutant could be sediment (from a highly erosive soil), nutrient (excess fertilizer in or on the soil, or from mineralization of crop residues), pesticide, bacteria, or some other harmful matter.
  2. Detachment -Detachment means that the potential pollutant or its environment is modified so that the substance can be moved from where it is supposed to be to where it should not be.  For example, a pesticide is sprayed on a field.  The residue adsorbs to soil particles.  Due to excess irrigation or rainfall, or just a highly erosive soil in a high wind, the soil particles separate from the rest of the field.   That is detachment.  A substance dissolving into water or changing form may also be considered a form of detachment since in many cases the substance will move readily with percolation.  This type of detachment mayor may not result in significant pollution depending on the substance.  For example, the ammonium form of dissolved nitrogen (NH4) does not move readily with water while the nitrate (NO3) form is highly leachable.  Hereafter, when the term "detachment" is used, it implies either:
a. a physical separation of soil particles (with or without adsorbed chemicals or nutrients),
b. the dissolving of a substance that allows it to move readily with surface runoff or deep percolation.  The surface runoff or deep percolation could be the result of natural rainfall or irrigations.  Or,
c. the transformation of a chemical that allows it to move readily with surface runoff or deep percolation.
  1. Transport -Transport means that the pollutant is moved to where it may be harmful.   For example, the soil particle carrying pesticide residues is carried off the field with surface runoff from irrigations or rainfall, or high winds.  The runoff carrying the sediment then enters a river or lake.  Another example is nitrate (NO3 ) fertilizer being leached into ground water through over-irrigation (intentional or not).
To summarize, pollution occurs through availability, detachment, and transport. There is a potentially polluting substance in some amount in some place (available). The substance can be separated from where it is supposed to be (detachment). And finally, the substance moves to where it becomes harmful to man or the environment (transport).

Thus, the main factors in reducing potential pollution are:
  1. Minimized availability of the potential pollutant in the environment - this is why you see management practices focused on reducing the amount and manner of chemical sprays and fertilizer applications (i.e. integrated pest management programs and plant/soil sampling to guide fertilizer applications)..
  2. Minimized detachment of the substance - this is why you see practices aimed at stabilizing soil (i.e. consevation tillage), or the encouraging the use of chemical/fertilizer compounds that do not transform as readily (i.e. use of less volatile chemicals, or those with shorter half-lives).
  3. Minimized transport of the substance - surface runoff or deep percolation from irrigation and rainfall are prime transport mechanisms for irrigated agriculture,   This is why you see the management practices aimed at improving irrigation performance (i.e. irrigation scheduling, irrigation system evaluations).
Water is extremely important in the detachment and transport processes.  High flows in furrows, or excessive application rates under a sprinkle irrigation system can cause soil erosion which can create a sedimentation problem.  It also creates the potential to transport chemicals attached to the soil particles.  Excess deep percolation can leach nutrients and other chemicals to ground water.  Thus, when applying water it is well to consider the fate of that water.
Figure 1 - Schematic of the hydrologic cycle
The fate of applied water can be better understood if the total hydrologic cycle is understood first.  The hydrologic cycle, illustrated in Figure 1, describes the movement of water through its different forms and locations.   Important processes in the hydrologic cycle are:
  1. Evaporation -the transformation of liquid water into water vapor from free water surfaces.
  2. Precipitation (rain or snow).
  3. Runoff -water moving overland or in a river or stream.
  4. Infiltration -the movement of water into the soil.
  5. Percolation -the movement of water through the soil.
  6. Freezing - liquid water turning into ice.
  7. Thawing - melting of ice
  8. Transpiration - the movement of water vapor out through plant/animal tissue surfaces into the atmosphere.
A term that is used constantly by agriculturists is "evapotranspiration".   This refers to the total extraction of water from the soil when it is cropped.   It consists of direct evaporation from the soil surface and transpiration from the plant's surfaces.

The hydrologic cycle essentially indicates that there is a set amount of water in the world.  It may be in various forms (ice, liquid, vapor) and in different places (oceans and lakes, rivers and streams, the atmosphere, or in ground water aquifers).   Importantly, it may be in different qualities.

For example, a possible path in the cycle may start with evaporation of water from the ocean into vapor that forms clouds. These clouds then move over land and cause rainfall.   Some of the rain percolates into the soil and moves to a ground water aquifer.   A farmer pumps from the aquifer to apply to a crop.  Then, the crop takes up the soil water and evaporates it back to the air.  In the air it forms clouds which again rain and the cycle repeats.

Another path would be for some of the rainfall to runoff to a creek.  The creek turns into a river and the river flows to the ocean.  The water then evaporates from the ocean to turn into clouds again.

To further explain the above examples, when water is applied to a field through irrigation or rainfall, none of it is "lost". Different portions of that water will move through different paths in the hydrologic cycle.  Some of the paths are more desirable than others.  For example, it is desirable that most water applied be stored in the root zone so that it is available for plant uptake.   Figure 2 is a schematic diagram of the root zone during an irrigation showing the different fates of the applied water.
Figure 2 - SchFigure 2 - Schematic of the different "fates" or "destinations" of water during and after an irrigation

Specific fates of water applied to a field due to an irrigation or rainfall are illustrated in Figure 2.  They include:
  1. Immediate evaporation - some water will immediately evaporate during an irrigation or rainfall.  Evaporation losses during a sprinkle application may range from 6 to 15% or more depending on temperature, humidity, and wind conditions.
  2. Surface runoff -if applied water does not infiltrate into the soil it will run off the surface. This runoff may go to a creek or stream, or may be picked up by a runoff reuse system on the farm, or on another farm downstream.
  3. Deep percolation -water that infiltrates the soil may be picked up by the crop to become evapotranspiration, or may percolate below the root zone. The deep percolation could end up in a usable aquifer for later pumping and reapplication to the farm.
  4. Root zone storage -which eventually will be picked up by the crop and evapotranspired back to the atmosphere.
A major manageA major management objective for growers is to minimize the amounts of surface runoff and deep percolation.  Surface runoff can be an important detachment mechanism depending on the erosivity of the soil.   Deep percolation and surface runoff are the primary transport mechanisms causing contamination.  They move sediment, chemicals, and fertilizers from the field to surface and ground waters.

Minimizing deep percolation and surface runoff (or its control) is the result of proper management of the irrigation.  This means achieving good distribution uniformity (an even application of water throughout the field) and controlling the total application of water.
You may be called upon to develop your own water quality control plan if you fall under the auspices of a Tier 1 program.  Planning an individual water pollution reduction/control program doesn't have to be a complicated process. It is important to remember the overall objectives of minimizing:
  1. Availability
  2. Detachment/transformation
  3. Transport
A feasible planning process could include the following steps
  1. Assess the current situation - This would include assessing the susceptibility of the environment to pollution, identifying current contamination levels and types, identifying regional programs that might be in place and on-going demonstration projects. The objective is to identify what, and how, contaminants are being made available, being detached, and being transported
  2. Assess the future - Are specific contaminants under control or are they an increasing problem?
  3. Identify specific objectives - Which Overall Management Objective(s) is(are) not being achieved?
  4. Identify the Practices that will help - There may be more than one Practice that will help achieve an Objective. Also note that implementing one Practice might force the implementation of others.
  5. Choose the most applicable Practice(s) - The choice may be made on effectiveness or economics, or both. Be aware of what Practices have been used locally already. Check with local resources such as Conservation Districts, the WSU Cooperative Extension, and local consultants.
  6. Develop a realistic timeline for implementation
  7. Monitor the situation - Choices for Practices to implement, and how they are actually implemented, are very often subjective. Changes in water quality or indications of program effectiveness can take time to appear. It is important that some type of program evaluation procedures be used for guidance on future decisions.
Even if the original assessment (step 1. above) finds no serious problems, some type of program should be in place to prevent problems from arising. Common components of such programs would include proper maintenance of irrigation systems and fertilizer/chemical application equipment, some form of irrigation scheduling, use of applicable IPM techniques, and proper use of chemigation/fertigation equipment.

A good exaA good example of an overall Manual of non-point source pollution in irrigated agriculture is Publication EM4885 "Irrigation Management Practices to Protect Ground Water and Surface Water Quality - State of Washington". This Manual was developed by the Washington State University Cooperative Extension. The primary purpose of the Manual is to present six Overall Management Objectives (OMO or Objective) for irrigated agriculture in Washington State. The Objectives, if achieved, should help to minimize availability, detachment/transformation, and transport of potential pollutants. In addition, they should help to minimize water diversions for irrigation, minimize soil erosion, and produce a profitable crop. A list of Implementation Practices (IP or Practice) that have been shown to aid in achieving the OMO is provided with each OMO. The Practices may involve a change in hardware, change in management, or both. The OMOs and IPs are listed in this bulletin. The specific IPs to use will vary with a given situation. However, the OMOs are constant and should be achieved by all.

Objective 2 addresses irrigation performance. A summary of that Objective and the Practices identified for it are seen below.

Objective 2 - Improve irrigation system performance and management in order to minimize deep percolation and surface runoff.

There are four sections within this Objective, three listing Implementation Practices for different irrigation system types and one listing Practices that are applicable to any irrigation system type. Those practices that can be used with any irrigation system type 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 SCS or WSU Cooperative Extension procedures
  • IP 2.01.04 - Know required leaching ratios to maintain salt balances
  • 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-irrigations then furrows; portable gated pipe to reduce furrow lengths for pre-irrigations; sprinklers to germinate crops irrigated by micro-irrigation; over-tree sprinkler for cooling with undertree irrigation)
  • IP 2.01.08 - Consider changing the irrigation system type
  • IP 2.01.09 - Use aerial photography to identify patterns that indicate problems with irrigation/drainage

Implementation practices for surface (furrow/rill, border strip) irrigation systems include:
  • IP 2.02.01 - Increase furrow flows to maximum non-erosive streamsize if water advance is slow
  • 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 grading where soil 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
  • 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 take out high and 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 - 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 side-roll sprinkle systems 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 micro-irrigation systems include:
  • IP 2.04.01 - Consult experienced agronomists/engineers to ensure that the appropriate volume of soil is being wetted by the system design
  • 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 - Have a chemical analysis of irrigation water/fertilizer/other additives to ensure compatibility and prevent clogging of the system
  • IP 2.04.05 - Practice good maintenance procedures to ensure that the system performs as designed