NATURAL RESOURCES CONSERVATION SERVICE
CONSERVATION PRACTICE STANDARD
Filter Strip
(ACRES)
code 393
Definition
A strip or area of herbaceous vegetation situated between crop land, grazing land, forest land or disturbed land and environmentally sensitive areas.
PURPOSE
· To reduce sediment, particulate organic, and sediment adsorbed contaminant loading in runoff
· To reduce dissolved contaminant loading in runoff
· To serve as Zone 3 of a Riparian Forest Buffer, Practice Standard 391
· To reduce sediment, particulate organic, and sediment adsorbed contaminant loading in surface irrigation tailwater
· To restore, create or enhance herbaceous habitat for wildlife and beneficial insects.
· To maintain or enhance watershed functions and values
· To increase carbon in biomass and soils
CONDITIONS WHERE PRACTICE APPLIES
This practice applies (1) in areas situated downslope of crop land, grazing land, forest land or disturbed land (2) where sediment, particulate organic matter and/or dissolved contaminants may leave these areas and are entering environmentally sensitive areas; (3) in areas where permanent vegetative establishment is needed to enhance wildlife and beneficial insects, or maintain or enhance watershed function; (4) in areas requiring filter strips as part of a waste management system; (5) in areas above conservation practices.
This practice applies when planned as part of a resource management system.
CRITERIA
General criteria applicable to all purposes
The establishment of this practice will support and comply with all federal, state, tribal, and local laws, rules and regulations as specified in the reference section.
Filter strips shall be designated as vegetated areas to treat runoff and are not part of the adjacent cropland rotation.
Overland flow entering the filter strip shall be primarily sheet flow except in areas of the Northwest Wheat and Range Region where rill flow shall also be allowed.
Concentrated flow shall be dispersed before it enters the filter strip with use of berms, diversions, shallow trenches or other practices.
The filter strip shall be established to permanent herbaceous vegetation, except for the purpose of “Reducing sediment, particulate organic, and sediment adsorbed contaminant loading in surface irrigation tail water”, where small grains are allowed for temporary filter strips.
The vegetation shall consist of a single species or a mixture of grasses, legumes and/or other forbs.
The vegetation will be adapted to the soil, climate, and nutrients, chemicals, and practices used in the current management system. Refer to “Vegetative Species Characteristics and Adaptability for Conservation Buffers”Vegetative Species Characteristics and Adaptability for s guidance found as a supplement to the Washington and Oregon Guide for Conservation Seeding and Planting. .
The plant species used in filter strips shall have dense stem and root development with minimum vegetative canopy cover of 80 percent. The filter strip shall meet the purpose identified.
Grazing shall not be permitted in the filter strip unless a prescribed grazing system is implemented. Grazing will be permitted under a prescribed grazing system only when soil moisture conditions support livestock traffic without excessive compaction or damage to the filter strip.
Control undesired weed species, especially state-listed noxious weeds.
Additional criteria to reduce sediment, particulate organic, and sediment-adsorbed contaminant loading in runoff
Filter strip flow lengthflow width shall be determined based on hydrologic soil characteristics, density and height of the filter strip vegetation, and runoff volume associated with run-off producing events as per Table 1. The minimum flow width for this purpose shall be 20 feet.
Table 1. Minimum Filter Strip Flow LengthFlow Widths to Reduce Sediment, Particulate Organic, and Sediment-adsorbed Contaminant Loading in Runoff.
Width of Flow (ft)10-yr 24-hr event (in)1/. / HSG A 2/ / HSG B / HSG C / HSG D
0-1 / 20 / 20 / 25 / 30
>1-2 / 20 / 25 / 30 / 35
>2-3 / 25 / 30 / 35 / 40
>3-4 / 30 / 35 / 40 / 45
>4-5 / 35 / 40 / 45 / 50
>5-6 / 40 / 45 / 50 / 55
>6-7 / 50 / 55 / 60 / 65
1). Western U.S. Precipitation Frequency Maps for 10 year 24 hour storm events may be viewed at: http://www.wrcc.dri.edu/pcpnfreq/wa10y24h.gif
Runoff values were determined using the SCS Rainfall-Runoff Depth Relation.
2). HSG = Hydrologic Soil Group of up-gradient area using Chapter 2 of the Engineering Field Handbook (EFH) to determine HSG.
* Values in Table 1 are representative of a filter strip with soils having a depth to 5 feet. If the soil depth is less, adjust the filter flow width adding 5 feet of width for every foot decrease in soil depth.
Filter strip location requirements:
The filter strip shall be located upgradient of areas to be protected. To the extent practical the field filter interface shall be on the contour.
The average annual sheet and rill erosion rate above the filter strip shall be no greater than the soil loss tolerance for the most predominant soil in the upgradient portion of the field. On other soils in the field, the average annual sheet and rill erosion rate above the filter strip shall not exceed 5 tons per acre.
Additional criteria to reduce dissolved contaminants in runoff
Filter strip flow width width required to reduce dissolved contaminants in runoff shall be based on management objectives, contaminants of concern, and the volume of runoff from the filter strip’s drainage area. Table 2 is based on these criteria as well as the filter strip’s area and infiltration capacity. The minimum flow width for this purpose shall be 30 feet.
Filter strips designed to serve this purpose shall be harvested with biomass removed from the strips.
Table 2. Minimum Filter Strip Flow LengthFlow Widths to Reduce Dissolved Contaminants in Runoff.
Width of Flow (feet)10-yr 24-hr event (in) 1/ / HSG A 2/ / HSG B / HSG C / HSG D
0-1 / 30 / 30 / 35 / 40
>1-2 / 30 / 35 / 40 / 45
>2-3 / 35 / 40 / 45 / 50
>3-4 / 40 / 45 / 50 / 55
>4-5 / 45 / 50 / 55 / 60
>5-6 / 50 / 55 / 60 / 65
>6-7 / 60 / 65 / 70 / 75
1). Western U.S. Precipitation Frequency Maps for 10 year 24 hour storm events may be viewed at: http://www.wrcc.dri.edu/pcpnfreq/wa10y24h.gif
Runoff values were determined using the SCS Rainfall-Runoff Depth Relation.
2). HSG = Hydrologic Soil Group of up-gradient area using Chapter 2 of EFH.
*Values in Table 2 are representative of a filter strip with soils having a depth to 5 feet. If the soil depth is less, adjust the filter flow width adding 5 feet of width for every foot decrease in soil depth.
Additional criteria to serve as Zone 3 of a Riparian Forest Buffer, Practice Standard 391
A Zone 3 shall be added to the riparian buffer when it is necessary to entrap sediment, nutrients or other soluble contaminants and to re-establish sheet flow.
The criteria used for designing Zone 3 of the Riparian Forest Buffer, Practice Standard 391 shall follow this Filter Strip (393) standard for the intended purpose.
Additional criteria to reduce sediment, particulate organic, and sediment adsorbed contaminant loading in surface irrigation tailwater
Irrigation water management shall be applied on fields above filter strips in accordance with the requirements of Section IV, FOTG, Practice Standard, Irrigation Water Management (449).
Filter strip vegetation may be a small grain or other suitable annual seeded vegetation at a rate to achieve stem density along the row according to Table 3.
Table 3: Stem spacing for temporary small grains in filter strips (consider 1,000,000 plant stems per acre).
Drill Row Spacing (in) / Stems per foot7.5 / 14
9.0 / 17
10 / 19
12 / 22
Filter strips shall be established early enough prior to the irrigation season so that the vegetation can withstand sediment deposition from the first irrigation.
The minimum flow width for this purpose shall be 15 feet.
Additional criteria to restore, create, or enhance herbaceous habitat for wildlife and beneficial insects
Additional filter strip flow wwidth idth devoted to this purpose must be in addition to the width required for the other purpose(s).
Any addition to the flow width width for wildlife or beneficial insects shall be added to the downhill slope of the filter strip. Vegetation to enhance wildlife may be added to that portion of the filter strip devoted to other purposes to the extent they do not detract from its primary functions.
Plant species selected for this purpose shall be for permanent vegetation adapted to the targeted wildlife or beneficial insect population(s) as per “Vegetative Species Characteristics and Adaptability for Conservation Buffers”
Optimal cover stand heights for waterfowl or upland birds are 18-24 inches.
A minimum width of 30 feet is necessary to limit the chance of predators.
Dispersed woody vegetation may be used to the extent it does not interfere with herbaceous vegetative growth, or purpose or operation and maintenance of the filter strip.
The filter strip shall not be mowed during the nesting season of the target wildlife.
Livestock and vehicular traffic in the filter strip shall be excluded during the nesting season of the target species.
Additional criteria to maintain or enhance watershed functions and values
Filter strips shall be strategically located to enhance connectivity of corridors and non-cultivated patches of vegetation within the watershed.
Space/color patterns/diversity in animal/plant species shall be maintained to enhance aesthetics of the watershed
Plant species selected for this purpose shall be for establishment of native permanent vegetation, where feasible.
Filters designed for this purpose shall be prepared in accordance with the requirements of local watershed management plans as per RCW 90.82, “The Watershed Planning Act”.
Additional Criteria to Increase Carbon in Biomass and Soils
Maximize width and length of the filter strip. The minimum width required for the filter is 20 feet.
Select plants that have higher rates of plant biomass and are adapted to the site. Maintain vegetation health and vigor.
Harvest and remove vegetation only to control pests and promote vigor.
considerations
Filter strips should be strategically located to reduce runoff, and increase infiltration and ground water recharge throughout the watershed.
Filter strips for the single purposes of wildlife/beneficial insect habitat or to enhance watershed function should be strategically located to intercept contaminants thereby enhancing the water quality of the watershed.
A mixture of sod-forming and bunch-forming species may be utilized where wildlife cover, food and soil fertility is important.
Establish and maintain a diverse plant community to maximize the number of wildlife species able to benefit from the establishment of a buffer strip.
Consider using this practice to enhance the conservation of declining species of wildlife, including those that are threatened or endangered.
Plan buffer strips to benefit many species of wildlife, not target species, unless the resource concerns demand it.
Consider using this practice to protect National Register listed or eligible (significant) archaeological and traditional cultural properties from potential damaging contaminants.
Filter strip size should be adjusted to a greater flow lengthflow width to accommodate harvest and maintenance equipment.
Plans and Specifications
Based on this standard, plans and specifications shall be prepared for each specific field site where a filter strip will be installed. A plan includes information about the location, construction sequence, vegetation establishment, and management and maintenance requirements.
Specifications will include:
a) Purpose for which the filter is being installed.
b) Location of filter strip.
c) Width, length, and slope of the filter strip to accomplish the planned purpose (width refers to flow path width across the filter strip).
d) Soils and Hydrologic Soil Group
e) Establishment procedures including species, seeding or sprigging rates, seedbed preparation, planting dates, nutrient management and mulching, irrigation, and maintenance.
f) Management applied to the upgradient area as well as the filter strip, including supporting conservation to meet design erosion control rate requirements.
g) Operation and maintenance requirements.
Special guidance and specifications for designing for “additional criteria to maintain or enhance watershed functions and values”.
These plans shall include:
q definition of the area of concern,
q the purpose of the plan, and
q individuals involved in developing the plan.
Operation and Maintenance
For the purposes of:
q Filtering contaminants to reduce sediment, particulate organic, and sediment adsorbed contaminant loading in runoff;
q Reducing dissolved contaminant loading in runoff and, and;
q Serving as Zone 3 of a Riparian Forest Buffer, Practice Standard 391;
permanent filter strip vegetative plantings should be harvested according to species requirements to encourage dense growth, maintain an upright growth habit, and harvest and remove contaminants that are contained in the plant tissue.
For all purposes:
Refer to Washington Weed Law RCW 17:10.080(4) for the Washington State Noxious Weed List. Noxious weeds will not be allowed to establish in the filter strip and will be controlled if present.
Herbicide activity in adjacent areas shall be compatible with the vegetation in the filter strip
Prescribed burning may be used to manage and maintain the filter strip where an approved burn plan has been developed.
When necessary, use aeration equipment to reduce compaction problems. If the area is a cultural resource site, do not use this equipment unless approved by a Cultural Resource Specialist.
Inspect the filter strip after major storm events and repair any gullies that have formed, redistribute unevenly deposited sediment accumulation greater than 6 inches that will disrupt sheet flow. Reseed disturbed areas.
To maintain or restore the filter strip’s function, periodically regrade the filter strip area when sediment deposition at the filter strip-field interface exceeds 6 inches and jeopardizes its function. Reestablish the filter strip vegetation, if needed.
Filters designed for this purpose require frequent harvest of the sequestered material. Plant species selected for entrapping nutrients should take up high amounts of nitrogen and phosphorus, produce ample biomass, and tolerate frequent mowing.
Take measures to prevent concentrated flow through the filter strip. Conservation measures require periodic maintenance and refurbishing of the fill material.
Apply supplemental nutrients (as needed) to maintain the desired species composition and stand density of the filter strip.