### 1 

## Appendix 3

- E: Derivation of Curve Number for SAM

The following guidance was used to generate Curve Numbers based on the hydrologic soil

group, from SSURGO data, and land cover class, from the CDL:

1. Assign the General Land Cover classes to a Curve Number Land Cover class using the Key

for General Land Class and Curve Number Land Cover Classes Table (Table 1).

2. Identify the hydrologic soil group (HSG) for the dominant component in the soil mapping

unit. If the HSG is a combination (i.e., C/D, B/C), select the most runoff

- prone hydrologic

value (runoff vulnerability follows the order D > C > B > A).

3. Determine the Curve Number under crop cover (CN_ag).

a. The curve number is based on the curve number guidance in NRCS T R

- 55

Methodology (USDA NRCS, 2008b), shown in Table 2.

b. The treatment practices are assumed to be contour with crop residue. The

hydrologic conditions for the crop classes are assumed to be fair or good.

c. The curve number cover classes for the developed clas ses are based on the

percent of impervious surface (50

- 75% grass cover for open; 30% impervious for

low; 65% impervious for medium; 85% impervious for high).

d. Table 3 provides a crosswalk between the reclassified General Land Classes from

CDL and the Curve Number Land Cover classes, assigning the curve number

based on both the hydrologic soil group and the land cover class. Each soil map

unit

- land cover combination will have a separate curve number.

4. Determine the curve number for fallow conditions (CN_fall ow)

a. For the crop/agricultural land cover classes, use the curve numbers for the

“Fallow” curve number class in Table 2.

b. For the woods, meadow, brush, and developed/residential classes, use the same

curve number as for the CN_ag

Table 1

- Key for General CDL and Curve Number Land Cover Classes



### CDL General Class Reclass Value Curve Number Land Cover Class

Corn 10 Row Crop

Corn/soybeans 14 Row Crop

Corn/wheat 15 Row Crop

Corn/grains 18 Row Crop

Cotton 20 Row Crop

Cotton/wheat 25 Row Crop

Cotton/vegetables 26 Row Crop

Rice 30 Small Grain

Soybeans 40 Row Crop

Soybeans/cotton 42 Row Crop

Soybeans/wheat 45 Row Crop

Soybeans/grains 48 Row Crop

### 2 CDL General Class Reclass Value Curve Number Land Cover Class

Wheat 50 Small Grain

Wheat/vegetables 56 Row Crop

Wheat/grains 58 Small Grain

Vegetables and ground fruit 60 Row Crop

(ground fruit) 61 Row Crop

Vegetables/grains 68 Row Crop

Orchards and grapes 70 Orchard (woods

- grass)

Other trees 75 Orchard (woods

- grass)

Other grains 80 Small Grain

Other row crops 90 Row Crop

Other crops 100 Row Crop

Pasture/hay/forage 110 Pasture, grassland, or range

Developed

- open 121 Developed

- open

Developed

- low 122 Developed

- low

Developed

- med 123 Developed

- medium

Developed

- high 124 Developed

- high

Forest 140 Woods

Grassland 150 Pasture, grassland, or range

Shrubland 160 Brush

- weed

- grass

Water 180 na

Wetlands

- woods 190 Woods

Wetlands

- herbaceous 195 Meadow

Miscellaneous land 200 Fallow

Table 2

- Curve Number Guidance based on NRCS TR

- 55 Methodology (2008)

.

Cover Type Treatment or Practice Hydrol

. Cond

. HSG A HSG B HSG C HSG D

Fallow Bare Soil

--- 77 86 91 94

Crop residue cover (CR) Poor 76 85 90 93

Good 74 83 88 90

Row crops Straight Row (SR) Poor 72 81 88 91

Good 67 78 85 89

SR + CR Poor 71 80 87 90

Good 64 75 82 85

Contoured (C ) Poor 70 79 84 88

Good 65 75 82 86

C + CR Poor 69 78 83 87

Good 64 74 81 85

Contoured and terraced (C&T) Poor 66 74 80 82

Good 62 71 78 81

C&T + CR Poor 65 73 79 81

Good 61 70 77 80

### 3 Cover Type Treatment or Practice Hydrol

. Cond

. HSG A HSG B HSG C HSG D

Small

grain SR Poor 65 76 84 88

Good 63 75 83 87

SR + CR Poor 64 75 83 86

Good 60 72 80 84

C Poor 63 74 82 85

Good 61 73 81 84

C + CR Poor 62 73 81 84

Good 60 72 80 83

C&T Poor 61 72 79 82

Good 59 70 78 81

C&T + CR Poor 60 71 78 81

Good 58 69 77 80

Close

- seeded or

broadcast legumes

or rotation meadow SR Poor 66 77 85 89

Good 58 72 81 85

C Poor 64 75 83 85

Good 55 69 78 83

C&T Poor 63 73 80 83

Good 51 67 76 80

Pasture, grassland, or range ;

continuous forage for g razing Poor 68 79 86 89

Fair 49 69 79 84

Good 39 61 74 80

Meadow

- continuous grass, protected from grazing;

generally mowed for hay --- 30 58 71 78

Brush

- brush

- weed

- grass mixture w/ brush as the major

element Poor 48 67 77 83

Fair 35 56 70 77

Good 30 48 65 73

Woods

- grass combination (orchard or tree farm)

(based on 50% woods, 50% grass) Poor 57 73 82 86

Fair 43 65 76 82

Good 32 58 72 79

Woods Poor 45 66 77 83

Fair 36 60 73 79

Good 30 55 70 77

Farmsteads

- buildings, lanes, driveways, surrounding lots

---- 59 74 82 86

Developed Open Space <50% grass cover Poor 68 79 86 89

50

- 75% grass cover Fair 49 69 79 84

>75% grass cover Good 39 61 74 80

Impervious: paved lots, roofs, driveways, etc 98 98 98 98

Streets & Roads Paved curbs, storm sewers 98 98 98 98

Paved, open ditches, incl ROW 83 89 92 93

Gravel, incl ROW 76 85 89 91

Dirt, incl ROW 72 82 87 89

Urban Commercial, business 85% imperv 89 92 94 95

Industrial 72% imperv 81 88 91 93

Residential 1/8 ac lots or less

(townhouse) 65% imperv 77 85 90 92

1/4 ac lots 38% imperv 61 75 83 87

1/3 ac lots 30% imperv 57 72 81 86

### 4 Cover Type Treatment or Practice Hydrol

. Cond

. HSG A HSG B HSG C HSG D

1/2 ac lots 25% imperv 54 70 80 85

1 ac lots 20% imperv 51 68 79 84

2 ac lots 12% imperv 46 65 77 82



### Table 3 provides a cross

- reference between the generalized CDL crop groups and the curve

numbers.

Table 3

- Curve Number Determinations for CDL Class

- HSG Combinations.

CDL Reclassified

Values Cover Type Treatment

or Practice Hydrol ogic

Condition HSG_

A HSG_

B HSG_

C HSG_

D

200 Fallow Bare Soil

--- Ag 77 86 91 94

Fallow 77 86 91 94

10, 14, 15, 18, 20,

25, 26, 40, 42, 45,

48, 56, 60, 61, 68,

90, 100 Row crops C + CR Good Ag 64 74 81 85

Fallow 74 83 88 90

30, 50, 58, 80 Small grain C + CR Good Ag 60 72 80 83

Fallow 74 83 88 90

Close

- seeded

legumes/rotation

meadow C Good Ag 55 69 78 83

Fallow 74 83 88 90

110, 150 Pasture, grassland,

range Fair Ag 49 69 79 84

Fallow 49 69 79 84

195 Meadow

- continuous grass,

protected from grazing --- Ag 30 58 71 78

Fallow 30 58 71 78

160 Brush

- brush

- weed

grass Fair Ag 35 56 70 77

Fallow 35 56 70 77

70, 75 Woods

- grass combination

(orchards) Poor Ag 57 73 82 86

Fallow 57 73 82 86

140, 190 Woods Fair Ag 36 60 73 79

Fallow 36 60 73 79

121 Developed Open

Space >75% grass

cover Good Ag 39 61 74 80

Fallow 39 61 74 80

122 Residential

(developed

- low) 1/3 ac lots 30%

imperv Ag 57 72 81 86

Fallow 57 72 81 86

123 Residential

(developed

- medium) 1/8 ac lots

or less 65%

imperv Ag 77 85 90 92

Fallow 77 85 90 92

124 Urban (developed

high) Commercial,

business 85%

imperv Ag 89 92 94 95

Fallow 89 92 94 95