CONTOUR FARMING

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CONTOUR FARMING

• How does contour farming reduce the average
  annual soil erosion rate from a watershed?
• Why is contour farming not effective in
  controlling erosion on very steep land?
• Text, page 107

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CONTOUR STRIP CROPPING REDUCES EROSION BY

• Each year only part of the field is open to the
  highest erosion rates
• Erosion from open strips is trapped in the
  vegetation on the close-growing strips.

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FUNCTIONS OF TERRACESFrom ASAE Standard ASAE
S268.4

• Reduce Soil Erosion
• Retain Soil Moisture For Crop Use
• Remove Surface Runoff w/o erosion
• Create level land for farmability
• Control Sediment Loss from Fields
• Reduce Peak Runoff Rates
• Improve Water Quality

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TERRACES FOR EROSION CONTROL

• Terraces affect the topographic factor on the
  USLE (LS).
• To be effective for field erosion control
  terraces need to be spaced at nearly constant
  vertical intervals.
• For Sediment Control terraces need to be level or
  at a very flat slope, particularly at the lower
  end.

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Terrace System Classification

• Parallel or Non Parallel
• Gradient or Level
• Cross-Section
• Outlet Type

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PARALLEL GRADIENT TERRACE SYSTEM
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PARALLEL LEVEL TERRACE SYSTEM
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Nonparallel Gradient Terrace System
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Parallel Gradient Terrace System
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Broadbase TerraceCross-section
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Broadbase Terrace

• Designed to fit smoothly in the field so that
  machinery can work the entire area.
• Used for erosion control and to retain soil
  moisture
• May be constructed with level or graded channels.
• Limited to field slopes lt8

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Steep-Backslope Terrace Cross Section
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Steep-Backslope Terrace

• Developed for use on field slopes too steep for
  Broadbase terraces.
• Push-up construction reduces overall field slope.
• Backslope is a barrier to machine travel.
  Normally planted to permanent grass.
• Less effective for field erosion control

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Narrow-Base TerraceCross-Section
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Narrow-Base Terrace

• Steep Ridge on both sides planted to grass
• Less expensive to construct than broadbase or
  steep backslope cross-sections.

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Ridgeless-Channel Terrace Cross-Section
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Ridgeless-Channel Terrace

• Designed for nearly level land
• Cut soil spread between terraces
• Little interference with field machine
  operations.
• Low cost construction

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Conservation Bench Terrace Cross-section
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Conservation Bench Terrace

• Moisture Conservation in semi-arid locations with
  field slopes lt5
• Layout includes a watershed area above and the
  flat channel to catch runoff from the watershed.
• Watershed area 2 to 4 times channel area,
  depending on expected runoff.

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Bench TerraceCross-section
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Bench Terrace

• Used to create level land for farming,
  particularly with surface irrigation.
• Concept used in ancient mountainside farming
  systems in Asia and South America.
• Used in irrigated areas in the American west.

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Bench Terraces in Japan
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Bench Terraces in the US
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Terrace System Spacing
Slope VI / HI x 100
VI
HI
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VI XsYEq. 7.1, p. 116 in Text

• VI is the Vertical Interval between terraces in
  Feet
• s is the Field Slope between terraces in percent
• HI Horizontal Interval or spacing
• s VI / HI 100 (definition of field slope)
• HI VI / s 100

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VI XsYEq. 7.1, p. 116 in Text

• X is a factor that depends on the climate
  (rainfall) in the area
• Related to R in the USLE
• See Fig. 7.10, page 117 in the Text
• 0.4 lt X lt 0.8
• Higher value for less rainfall
• Use 0.7 for Iowa

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VI XsYEq. 7.1, p. 116 in Text

• Y is a factor that depends on the soil
  erodibility and cover protection
• Related to K and C in the USLE
• Lower Value for easily eroded soil with little
  cover protection
• 1 lt Y lt 4
• Use Y 2 for average field conditions

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TERRACE SPACING

• Estimate the required terrace spacing for average
  field conditions on a 5 slope in Northern
  Arkansas.
• From Fig. 7.10 X 0.5
• For average field conditions Y 2
• VI (0.5)(5)2 2.52 4.5 ft
• HI (4.5/5)(100) 90ft.

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GRADES IN TERRACE CHANNELS
Would there be more erosion at the top or bottom
of a slope? Why?
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GRADES IN TERRACE CHANNELS
A
B
C
Which of these slopes would have the most
erosion, and why?
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GRADES IN TERRACE CHANNELS

• The allowable terrace channel grade is greatest
  at the upper end of the terrace.
• When water removal is a concern, terraces have a
  minimum grade requirement to prevent ponding
• See table 7.2, page 119 in text.

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MAXIMUM TERRACE LENGTH

• The maximum terrace length is based on a maximum
  drainage area.
• Terrace channels should drain no more than 3
  acres of watershed
• Length x Spacing Drainage Area

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Parallel Gradient Terrace System
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Terrace Channel Water Storage

• Runoff Volume Length x Spacing x Depth of
  Runoff
• Storage Volume Length x X-section Area
• Required X-section for Storage Spacing x Depth
  of Runoff
• L x S x D L x A
• S x D A

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Terrace Channel Water Storage

• Terrace Length 500 ft
• Terrace Spacing 100 ft
• Runoff Depth 0.2 ft
• Runoff Volume 500 x 100 x 0.2 10000 ft3
• Storage Volume 500 x Area
• 10000 500 x Area
• Area 10000/500 20 100 x 0.2