Curve Number Module description

When rainfall occurs in excess of absorption by soil, it causes runoff which increases with time and length of slope. Runoff is influenced by multiple factors like intensity and duration of rainfall, existing land use, initial abstraction, rate of infiltration, slope gradient and length, percolation rate, presence of hard substratum, antecedent moisture and so on. 

Runoff is a critical factor in deciding the type of conservation needed, number and location of water harvesting and recharge structures, formulation of appropriate cropping pattern and crop selection and the water balance and water availability at the watershed scale. 

Some important runoff estimation models that are in use are SCS Curve Number method, which is empirical method of estimating excess precipitation, Constant infiltration based method in which saturated soil conductivity is used as infiltration rate; Horton equation, which is based on mathematical equation; SAC-SMA (Sacramento Soil Moisture Accounting) which attempts to mimic physical constraints of water movement in a natural system and Rational method (Ramser’s method). 

Under this DSS, along with the SCS Curve Number, Infiltration Method and Rational methods, Runoff model developed based on LRI database (Infiltration) and precipitation available from KSNMDC is included to estimate the amount of runoff that can be expected to occur at different levels in a watershed area.

 Following is the example of estimation of daily runoff using SCS Curve number method. http://www.isprs.org/proceedings/XXXVII/congress/2_pdf/1_WG-II-1/11.pdf

Step By Step Process

• The web page will have the drop down for selecting District, Taluk, Village, Watershed name, Survey Number, Runoff Model.
• User will select the District, Taluk, Village, Micro Watershed, Survey number and Runoff model.
• Depending upon the Runoff model selected by the user

SCS Curve Number method

• If user select the SCS
• System will fetch the information related to the Soil texture, Slope, Landform ( Black, Red / lateritic) from the Parcel characteristics table for the respective Parcel.
• Based on the below table decide the Soil Hydrological group ( Table Master_Hydrological_Soil_Texture)

HSG	Soil Textures
A	Sand, loamy sand, or sandy loam
B	Silt loam or loam
C	Sandy clay loam
D	Clay loam, silty clay loam, sandy clay, silty clay, or clay

 

• A query will be executed to find current Land use for the selected survey number.
• If current land use is having Crop names, then system will fetch the information for the Crop_Class for the respective crop.
• Execute query on below table to find the Curve number (CN) depending upon the soil hydrological group and current landuse.

Land_Cover	Hydrology Soil Group
	A	B	C	D
Good crop	62	71	78	81
Fair crop	0	0	0	0
Poor crop	0	0	0	60
Water body	0	0	0	0
Forest	0	0	0	0
Fallow	77	86	91	94
Settlement	0	0	0	0
Uncultivable	0	0	0	0

 

• For getting the AMC condition
  • Execute query on Rainfall_data for the respective village to get 5 days antecedent rainfall as well as current day rainfall.
    • If rainfall < 35 mm then AMC = AMC-I
    • If rainfall between 35 to 52.5 mm then AMC = AMC-II
    • If rainfall > 52.5 then AMC = AMC-III
  • Execute Query on below table using derive curve number and AMC condition to get multiplication Factor (MF).

Curve Number AMC-II	AMC-IDry	AMC-III Wet
10	0.4	2.22
20	0.45	1.85
30	0.5	1.67
40	0.55	1.5
50	0.62	1.4
60	0.67	1.3
70	0.73	1.21
80	0.79	1.14
90	0.87	1.07
100	1	1

 

• Get adjusted CN number

AdjCN = CN * MF

Where AdjCN = Adjusted Curve Number

             CN = Curve Number

             MF= Multiplication factor

• Adjust Curve Number taking consideration of slope factor

Where,

             SACN - Slope adjusted CN

             CN – Curve Number

AdjCN – Adjusted Curve Number

a - Soil slope (m/m)

• Estimate Potential Maximum Soil Moisture Retension of Runoff (P_e) and Initial Abstraction (I_a) factor using slope Adjusted Curve Number.

Where,

S – Potential Maximum Soil Moisture Retension (in inches)

SACN - Slope adjusted CN

• Estimate Initial using following equation depending on the type of Soil and AMC.
  • For black soil region (AMC I) and Red soil/Laterite soil region (AMC I, AMC II, & AMC III):
  • For black soil region (AMC II & AMC III):

Where,

I_a – Initial Abstraction (in inches)

S - Potential Maximum Soil Moisture Retension (in inches)

• Finally, Runoff is estimated using Daily Rainfall, the Potential maximum Soil Moisture Retension after runoff begins and Initial Abstraction. Multiply Daily rainfall by 25.4 to convert it into inches

Where,

             P_e – Runoff (in inches)

             P – Rainfall (in inches)

             I_a – Initial Abstraction (in inches)

             S - Potential maximum Soil Moisture Retension (in inches)

• Output will be converted from inches to mm by dividing the obtained value by 25.4.
• Fetch the Geometry from the Cadastral table against the Cadastral_ID.
• Read the Coordinates of the geometry and create GeoJson polygon. Send GeoJson to Leaflet to display the polygon with Cyan color and overlay it on the map.
• Display the result in a table showing the information such as Survey No, Farmer Name, Area in Hectare, Interval, Runoff(mm).
• Report Button will be provided along with the result row in table. On click of report Button, Report of the result along with the map will be generated in pdf format.