Setting Runoff Coefficients from GIS Data

SCS runoff curve numbers or SRM runoff coefficients can be calculated directly from GIS data. To set these values the following data is required:

The GIS data can be read straight from the file. Loading the two files as background layers will allow data to be viewed before carrying out the calculation process.

Only subcatchments that have their Curve number or SRM runoff coefficient field set to the #G flag will be updated.

The two GIS files should contain polygon data including an additional field defining the land use or soil type.

The lookup table is a CSV format data file. An example is given below. Land Use is listed down the left hand column, and Soil Type across the top row. The lookup table should map all the possible combinations of land use and soil type onto valid curve numbers/runoff coefficients.

Land Use,

A,

B,

C,

D

N,

40,

41,

42,

43

O,

50,

51,

52,

53

P,

60,

61,

62,

63

Q,

70,

71,

72,

73

Curve Number Lookup Table

To load curve numbers/runoff coefficients from GIS data:

  1. Open a network in the GeoPlan Window.
  2. Select subcatchments if updating the Curve Number or SRM runoff coefficient field for a selection of subcatchments only.
  3. Choose Subcatchment | CN/Runoff coefficient lookup from the Model menu. This displays the CN/Runoff Coefficient Lookup dialog.
  4. Set parameters in the CN/Runoff Coefficient Lookup dialog:
    • Select the subcatchment field to be updated: SCS Curve Number or SRM Runoff Coefficient.
    • Choose the source of the Land Use and Soil Type data
    • Choose the source of the curve number/SRM runoff coefficient Lookup Table and enter number of Soil Types (number of lookup columns) in table
    • Check the Update Selected Subcatchments Only option to update only the currently selected subcatchments. Uncheck this option to set values for all subcatchments for which data is available
    • Select a flag to be applied to updated data.
    • Select options for displaying a log of the update process.
  5. Click OK to start the curve number/runoff coefficient calculation process

Curve numbers/runoff coefficients will only be calculated for subcatchments that have their Curve number or SRM runoff coefficient field set to the #G flag.

Curve numbers/runoff coefficients will only be calculated for subcatchments where there is an overlap of land use and soil data AND a corresponding value in the lookup table. If there is more than one overlap then the curve number/runoff coefficient will be in proportion to the relative areas of overlap.

CN values

CN values for different conditions are available from various sources. Table 1 summarises many of these for the initial CN (for AMC II; see below) dependent on land use, cover quality and soil type. Definitions of these latter two parameters are provided in the accompanying tables (Tables 2 and 3).

Note that the curve number values apply for InfoWorks and SWMM networks.

(U.S.A. Bureau of Agriculture)

Land Cover Class

Land Treatment/practice/treatment

Stormflow Potential

Hydrological Soil Group

A

B

C

D

Fallow

1 = Straight row

2 = Straight row

3 = Straight row + conservation tillage

 

High

Low

77

75

74

86

84

83

91

89

87

94

92

90

Row Crops

1 = Straight row

2 = Straight row

3 = Straight row + conservation tillage

4 = Straight row + conservation tillage

5 = Planted on contour

6 = Planted on contour

7 = Planted on contour + conservation tillage

8 = Planted on contour + conservation tillage

9 = Planted on contour + winter rainfall region

10 = Conservation structures

11 = Conservation structures + conservation tillage

12 = Conservation structures + conservation tillage

Low

High

Low

High

Low

High

Low

High

Low

High

Low

72

67

71

64

70

65

69

64

66

62

65

61

81

78

79

75

79

75

78

74

74

71

73

70

88

85

86

82

84

82

83

80

80

78

79

76

91

89

89

85

88

86

87

84

82

81

81

79

Garden Crops

1 = Straight row

2 = Straight row

Low

High

45

68

66

75

77

81

83

84

 

Land Cover Class

Land Treatment/practice/treatment

Stormflow Potential

Hydrological Soil Group

A

B

C

D

Small Grain

1 = Straight row

2 = Straight row

3 = Straight row + conservation tillage

4 = Straight row + conservation tillage

5= Planted on contour

6= Planted on contour

7 = Planted on contour + conservation tillage

8 = Planted on contour + conservation tillage

9 = Planted on contour + winter rainfall region

10 = Conservation structures

11 = Conservation structures

12 = Conservation structures + conservation tillage

13 = Conservation structures + conservation tillage

 

High

Low

High

Low

High

Low

High

Low

Low

High

Low

High

Low

65

63

64

60

63

61

62

60

63

61

59

60

58

76

75

74

72

74

73

73

72

70

72

70

71

69

84

83

82

80

82

81

81

79

78

79

78

78

76

88

87

86

84

85

84

84

82

81

82

81

81

79

Close seeded legume or rotational meadow

1 = Straight row

2 = Straight row

3 = Planted on contour

4 = Planted on contour

5 = Conservation structures

6 = Conservation structures

High

Low

High

Low

High

Low

66

58

64

55

63

51

77

72

75

69

73

67

85

81

83

78

80

76

89

85

85

83

83

80

 

Land Cover Class

Land Treatment/practice/treatment

Stormflow Potential

Hydrological Soil Group

A

B

C

D

Sugar Cane

1 = Straight row: trash burnt

2 = Straight row: trash mulch

3 = Straight row: limited cover

4 = Straight row: partial cover

5= Straight row: complete cover

6= Conservation structures: limited cover

7 = Conservation structures: partial cover

8 = Conservation structures: complete cover

 

  

43

45

67

49

39

65

25

6

65

66

78

69

61

75

59

35

77

77

85

79

74

82

75

70

82

83

89

84

80

86

83

79

Range and Pasture

1 = Range in poor condition

2 = Range in fair condition

3 = Range in good condition

4 = Pasture planted on contour

5= Pasture planted on contour

6= Pasture planted on contour

High

Moderate

Low

High

Moderate

Low

68

49

39

47

25

6

79

69

61

67

59

35

86

79

74

81

75

70

89

84

80

88

83

79

Irrigated Pasture

  

Low

35

48

65

70

Meadow

  

Low

30

58

71

81

Wood and Scrub

1 = Woods

2 = Woods

3 = Woods

4 = Brush - winter rainfall region

High

Moderate

Low

Low

45

36

25

28

66

60

55

44

77

73

70

60

83

79

77

66

Orchards

1 = Winter rainfall region, understory of crop cover

  

39

53

66

71

 

Land Cover Class

Land Treatment/practice/treatment

Stormflow Potential

Hydrological Soil Group

A

B

C

D

Forests & Plantation

1 = Humus depth 25mm; compactness:

2 = Humus depth 25mm;

3 = Humus depth 25mm;

4 = Humus depth 50mm; compactness:

5 = Humus depth 50mm;

6 = Humus depth 50mm;

7 = Humus depth 100mm; compactness:

8 = Humus depth 100mm;

9 = Humus depth 100mm;

10 = Humus depth 150mm; compactness:

11 =Humus depth 150mm;

12 = Humus depth 150mm;

Compact

Moderate

Loose / friable

Compact

Moderate

Loose / friable

Compact

Moderate

Loose / friable

Compact

Moderate

Loose / friable

52

48

37

48

42

32

41

34

23

37

30

18

72

68

60

68

65

57

64

59

50

60

56

47

82

78

71

78

75

67

74

69

61

71

66

57

87

85

77

85

81

74

80

75

67

77

72

65

Urban/ Suburban Land Uses

1 = Open spaces, parks, cemetries

2 = Open spaces, parks, cemetries

3 = Commercial/business areas

4 =Industrial districts

5 = Residential lot: size 500m2

6 = Residential: lot size 1000m2

7 = Residential: lot size 1350m2

8 = Residential: lot size 2000m2

9 = Residential: lot size 4000m2

10 = Paved parking lots, roofs etc

11 = Streets/roads: tarred, with storm sewers, curbs

12 = Streets/roads: gravel

13 = Streets/roads: dirt

14 = Streets/roads: dirt-hard surface

75% grass cover

75% grass cover

85% impervious

72% impervious

65% impervious

38% impervious

30% impervious

25% impervious

20% impervious

 

 

 

 

 

39

49

89

81

77

61

57

54

51

98

98

76

72

74

61

69

92

88

85

75

72

70

68

98

98

85

82

84

74

79

94

91

90

83

81

80

78

98

98

89

87

90

80

84

95

93

92

87

86

85

84

98

98

91

89

92

Cover Quality

Definition

Poor

(Heavily grazed/regularly burned areas)

Less than 50% ground surface protected by plant cover or brush and tree canopy.

Fair

(Moderate cover)

50-75% ground surface protected by plant cover.

Good

(Dense cover)

More than 75% ground surface protected.

Soil Type

Definition

A

(Low runoff potential)

High infiltration rates even when thoroughly wetted.

Chiefly deep, well/excessively drained sands or gravels.

High rate of water transmission.

B

Moderate infiltration rates when thoroughly wetted.

Chiefly moderately deep/deep, moderately well/well drained soils with moderately fine/moderately coarse textures.

Moderate rate of water transmission.

C

Slow infiltration rates when thoroughly wetted.

Chiefly soils with a layer that impedes downward movement of water, or soils with a moderately fine/fine texture.

Slow rate of water transmission.

D

(High runoff potential)

Very slow infiltration rates when thoroughly wetted

Chiefly:

  • clay soils with a high swelling potential
  • soils with a permanent high water table
  • soils with a clay pan or clay layer at or near the surface
  • shallow soils over nearly impervious material

Very slow rate of water transmission.

CN values obtained using the above tables are adjusted dependent on antecedent moisture conditions (AMC) as defined in Table 4.

AMC

Total 5-day antecedent rainfall (mm)

Dormant Season

Growing Season

I

<12.5

<35.5

II

12.5 - 28.0

35.5 - 53.5

III

>28.0

>53.5

Finally, the relationship between CN for different AMC classes is shown in Table 5. These relationships are used within the software to adjust the curve number value.

CN for AMC II

Corresponding CN for

AMC I

AMC III

100

100

100

95

87

99

90

78

98

85

70

97

80

63

94

75

57

91

70

51

87

65

45

83

60

40

79

55

35

75

50

31

70

45

27

65

40

23

60

35

19

55

30

15

50

25

12

45

20

9

39

15

7

33

10

4

26

5

2

17

0

0

0

SCS Model

Simple Runoff Model (SRM)

CN/Runoff Coefficient Lookup Dialog