Estimation of rainfall-induced surface runoff for the Assam region, India, using the GIS-based NRCS-CN method

Figures & data

Figure 1. Map of the study area.

This figure illustrates a map of the study area showing the location of Assam state, located in the north-eastern part of India. It also illustrates the five administrative divisions of Assam along with the name of the 27 districts.

Figure 2. Methodology for the estimation of surface runoff.

This figure represents the overall methodology applied in the study. It shows the different layers and their source, and the steps required to estimate surface runoff.

Figure 3. (a) Sub-basin with stream network, (b) Land use and land cover, (c) Surface slope, and (d) Hydrologic soil group.

This figure illustrates the different layers required to estimate curve number (CN). Long Description: This figure illustrates the different layers required to estimate curve number (CN). In this figure (a) shows 63 delineated sub-basins of the study area with different stream orders ranging from 4 to 7, figure (b) represents eight classes of land use land cover, figure (c) shows the spatial variation of the slope of the study area, and in figure (d) Hydrologic soil group map of the study area is given, and it is derived from the soil texture map.

Table 1. Accuracy assessment of LULC map.

LULC Class	WB	FA	GL	FV	CL	SL	BLT	BL	Total User	UA (%)
WB	97	0	3	0	0	0	0	3	103	94.17
FA	0	100	8	0	0	2	0	0	110	90.91
GL	1	5	103	2	3	4	0	1	119	86.55
FV	2	0	2	94	4	2	0	1	105	89.52
CL	0	2	3	2	102	3	0	3	115	88.70
SL	2	0	3	0	2	99	0	8	114	86.84
BLT	0	0	0	0	4	0	98	2	104	94.23
BL	3	0	0	0	3	4	3	101	114	88.60
Total Producer	105	107	122	98	118	114	101	119	884
PA (%)	92.38	93.46	84.43	95.92	86.44	86.84	97.03	84.87

WB- Water body; FA- Forest area; GL- Grassland; FV- Flooded Vegetation; CL- Cropland; SL- Scrubland; BLT- Built-up area; BL- Barren land; UA- User’s Accuracy; PA- Producer’s Accuracy

Figure 4. Spatial distribution of (a) Curve number I, (b) Curve number II, (c) Curve number III, (d) Slope-corrected curve number I, (e) Slope-corrected curve number II, and (f) Slope-corrected curve number III.

This figure represents the calculated value of curve number (CN) for the different sub-basins of the study area. Long Description: This figure represents the calculated value of curve number (CN) for the different sub-basins of the study area. In this figure (a) shows CNI for AMCI, figure (b) shows the CNII value for AMC II, and figure (c) describes CNIII for AMC III. The three Curve numbers are then corrected according to the slope and given in figure (d) as slope-corrected CNI, figure (e) as slope-corrected CNII, and figure (f) as slope-corrected CNIII.

Table 2. Estimation of curve number for each LULC-HSG complex of the study area.

Land use land cover (LULC)	Hydrologic soil group (HSG)	Area (km^2)	Curve number (CN)	CN*Area	WCN
Waterbody	A	225.39	98	22088.09	AMC-II-76.37
	B	676.16	98	66263.30	AMC-I-57.58
	C	2408.30	98	236013.43	AMC-III-88.14
	D	128.16	98	12559.68
Forest area	A	1406.89	45	63310.05
	B	12240.90	53	648767.67
	C	11576.73	67	775641.19
	D	5157.67	72	371352.24
Grassland	A	68.66	61	4188.36
	B	163.81	77	12613.39
	C	403.62	85	34307.65
	D	21.83	91	1986.53
Flooded vegetation	A	1.40	77	107.63
	B	78.95	80	6316.12
	C	145.29	83	12059.28
	D	8.12	86	698.14
Cropland	A	1077.33	62	66794.66
	B	8208.76	71	582821.78
	C	15899.17	88	1399126.66
	D	1558.74	91	141845.34
Scrubland	A	307.79	63	19390.85
	B	657.92	77	50660.18
	C	1898.91	85	161407.66
	D	167.77	88	14763.76
Built-up area	A	368.93	89	32835.04
	B	3399.64	92	312767.22
	C	5959.50	94	560193.06
	D	600.35	95	57033.25
Barren land	A	379.02	77	29184.27
	B	334.59	86	28774.43
	C	2905.46	91	264397.27
	D	2.64	94	248.16
	Total	78438.41		5990516.34

Figure 5. (a) Mean annual precipitation and (b) Surface runoff depth.

This figure consists of figure (a) Mean annual precipitation map for the study area and 5(b) Surface runoff depth map for the study area. Long Description: This figure consists of two figures, Figure (a) represents the mean annual precipitation for the study area derived from 16 years of rainfall data, and the unit is given in mm. Figure (b) shows the surface runoff depth map for the study area, estimated from the rainfall data and Curve number values (CN).

Figure 6. Correlation between rainfall and simulated runoff depth.

This figure consists of 6 figures showing a graphical representation of the correlation between rainfall and runoff depth for the sub-basins 6, 11, 26, 38, 40, and 55. The coefficient of determination value for all the selected sub-basins is greater than 0.80 which suggests that the parameters are strongly correlated.

Figure 7. Correlation between simulated and measured runoff depth.

This figure consists of 6 figures showing a graphical representation of the correlation between simulated and measured runoff depth for the sub-basins 6, 11, 26, 38, 40, and 55. The coefficient of determination value for all the selected sub-basins is greater than 0.80 which suggests that the parameters are strongly correlated.

Figure 8. Temporal variation of rainfall, simulated and measured runoff depth.

This figure represents the temporal variation of rainfall, simulated and measured runoff depth for the present study. Long Description: In this figure represents temporal variation and relation between rainfall, simulated and measured runoff depth from 2005 to 2020. It shows linear relation exists in rainfall, simulated and measured runoff depth.

Data Availability Statement

The authors confirm that the data supporting the findings of this study are available within the article and some of the raw data were generated at our laboratory and derived data supporting the findings of this study are available upon reasonable request.