SCS runoff curve numbers or SRM runoff coefficients can be calculated directly from GIS data. To set these values the following data is required:
- Subcatchment boundaries properly defined in the network. Subcatchments should not contain overlapping areas, and there should not be gaps between subcatchments unless these fall outside the catchment area
- A GIS file containing land use data
- A GIS file containing soil type data
- A lookup table (CSV file) of curve number/runoff coefficient appropriate to a particular land use/soil type pair
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.
Data
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
Instructions
To load curve numbers/runoff coefficients from GIS data
- Open a network in the GeoPlan Window.
- Select subcatchments if updating the Curve Number or SRM runoff coefficient field for a selection of subcatchments only.
- Choose Subcatchment
CN/Runoff coefficient lookup from the Model menu. This displays the
CN/Runoff Coefficient Lookup dialog.
- 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.
- Click OK to start the curve number/runoff coefficient calculation process
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.
Table 1 Initial CN values for AMC II with various land use, cover quality and soil type
(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 |
Table 2 Definition of cover quality
|
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. |
Table 3 Definition of soil type
|
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:
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.
Table 4 Summary of CN values depending on AMC values
|
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.
Table 5 Relationship between CN for different AMC classes
|
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 |