Application of geomorphometric characteristics to prioritize watersheds for soil and water conservation practices in the Lake Tana Basin, Ethiopia

Figures & data

Figure 1. Location map of the study area: (a) East African countries with elevation background (b) the Lake Tana basin and its four major watersheds.

Table 1. Major watershed characteristics of the Lake Tana Basin.

Watersheds	Gilgel Abay	Gumara	Rib	Megech
Area, km^2	1,654.35	1,352.4	1,413.3	514.5
Basin length, km	75	81	91	41
Elevation range, m	1,884–3,534	1,787–3,709	1,789–4,109	1,860–2,976
Slope, %
Flat, <3%	5.8	4.3	5.1	1.5
Gently sloping, 3–8%	34.3	22.5	26.1	12.6
Sloping, 8–15%	24.9	23.4	21.3	19.5
Moderately steep, 15–30%	22.8	33.2	25.8	33.5
Steep, 30–50%	9.0	13.8	14.1	22.3
Very steep, >50%	3.2	2.8	7.6	10.6
Major soils, %
Luvisols	14.9	29.9	19.8	25.2
Vertisols	12.7	10.1	19.0	2.7
Leptisols	18.5	33.1	25.1	28.1
Fluvisols	1.2	0.4	0.6	0.0
Nitosols	24.9	9.9	8.8	14.6
Alisols	17.8	7.9	7.5	5.7
Regosols	2.7	3.4	12.1	8.8
Cambisols	4.7	4.0	3.5	8.6
Ferralsols	1.9	0.9	1.9	–
Towns	0.7	0.4	1.7	6.3

Table 2. Discharge-sediment rating curve constants developed by Ministry of Water and Energy using power equation (S = aQ^b where S is sediment yield in t ha⁻¹yr⁻¹ and Q is discharge in m³ s⁻¹).

Rivers	Area	Rating curve constants
		a	b
Gilgel Abay	1654.4	4.00	1.65
Gumara	1352.4	17.50	1.48
Ribb	1413.3	30.00	1.59
Megech	541.5	15.10	1.35

Table 3. Geomorphometric characteristics of the watershed and methods for assessing the variables.

Category	Morphometric parameter	Methods used	Source
Basin geometry	Drainage area (A)	Projected area enclosed by watershed boundary	Schumn (1956)
	Shape perimeter (P)	Length of horizontal projection of watershed divide	Schumn (1956)
	Basin Length (Lb), km	Distance from outlet to farthest point on watershed boundary	Schumn (1956)
Linear aspect	Stream order	Hierarchical Rank	Strahler (1964)
	Number of stream orders	$N_u=N_1+N_2+·\; ··+N_n$	Horton (1945)
	Length of stream orders, km	$L_u=L_1+L_2+·\; ··+L_n$	Strahler (1964)
	Mean stream length	$L_{sm}=\frac{L_u}{N_u}$	Schumn (1956)
	Average bifurcation ratio	$R_b=\frac{N_u}{N_{u+1}}$	Schumn (1956)
	Length of overland flow, km	$L_o=\frac{1}{2}{D}_d$	Horton (1945)
Areal aspects	Drainage density, km km^−2	$D_d=\frac{\sum L_u}{A}$	Horton (1932, 1945)
	Stream frequency, number km^−1	$F_s=\frac{\sum N_u}{A}$	Horton (1932, 1945)
	Drainage texture	$D_t=\frac{N_u}{P}$	Horton (1945)
	Compactness coefficient	$C_c=\frac{0.282*P}{\sqrt{A}}$	Horton (1945)
	Infiltration number	$I_n=D_d*F_s$	Miller (1953); Strahler (1964)
	Elongation ratio	$R_e=\frac{2\sqrt{A/\pi }}{L_b}$	Schumn (1956)
	Form factor	$R_f=\frac{A}{L_b^2}$	Strahler (1964)
	Circularity ratio	$R_c=\frac{A}{A_c}$	Miller (1953); Strahler (1964)
Relief aspect	Basin relief	$H=Z-z$	Analyzed using GIS approach
	Relief ratio	$R_h=\frac{H_m}{L_b}$	Schumn (1956)
	Ruggedness number	$N_o=H_m*D_d$	Strahler (1964)

where N_u is total number of stream segments of order u, Nu₊₁ is total number of stream segments of the next higher order, A is area of the basin in km², P perimeter of the basin in km, L_b is length of the basin in km, Ac is area of the circle having the same circumference with watershed perimeter in km², Z is maximum elevation in the basin in m, z is elevation at the basin outlet in m, H_m is maximum basin relief.

Figure 2. Stream orders, drainage lines and shapes of watersehds: (a) Gilgel Abay (b) Gumara (c), Rib, and (d) Megech.

Figure 3. Aspect map of watersheds: (a) Gilgel Abay (b) Gumara (c), Rib, and (d) Megech.

Table 4. Parameters of basin geometry and linear aspect for Gilgel Abay, Gumara, Rib and Megech watersheds.

Category	Morphometric parameter		Watersheds
			Gilgel Abay	Gumara	Rib	Megech
Basin geometry	Drainage area, km^2		1654.4	1352.4	1413.3	514.5
	Shape perimeter		311.3	291.6	308.3	155.5
	Basin length, km		75	81	91	41
Linear aspect	Maximum stream order		5	5	5	4
	Number of streams	1st	219	178	192	55
		2nd	43	33	47	10
		3rd	10	9	8	3
		4th	3	2	2	1
		5th	1	1	1	–
		Total	276	223	250	69
	Length of streams, km	1st	436	368	384	133
		2nd	241	158	243	73
		3rd	193	134	106	41
		4th	79	42	22	10
		5th	7	26	43	–
		Total	955	728	796	257
	Mean stream length, km		3.46	3.26	3.19	3.72
	Bifurcation ratio	1:2	5.09	5.39	4.09	5.50
		2:3	4.3	3.67	5.88	3.33
		3:4	3.33	4.50	4.00	3.00
		4:5	3.00	2.00	2.00	–
		Mean	3.93	3.89	3.99	3.94
	Length of overland flow, km		0.87	0.93	0.89	1.00

Figure 4. Relationship of the length of overland flow versus 1st order stream number (blue) and stream length (orange).

Figure 5. Drainage density map of watersheds: (a) Gilgel Abay (b) Gumara (c), Rib, and (d) Megech.

Table 5. Aspect and relief morphometric parameter values of studied watersheds.

Category	Morphometric parameter	Watersheds
		Gilgel Abay	Gumara	Rib	Megech
Areal aspect	Drainage density, km km^−2	0.58	0.54	0.56	0.50
	Stream frequency, number km^−2	0.17	0.16	0.18	0.13
	Drainage texture	0.89	0.76	0.81	0.44
	Compactness coefficient	2.16	2.24	2.31	1.93
	Infiltration number	0.096	0.089	0.99	0.067
	Elongation ratio	0.61	0.51	0.47	0.62
	Form factor	0.29	0.21	0.17	0.31
	Circularity ratio	0.21	0.20	0.19	0.27
Relief aspect	Elevation range, m	1,884–3,534	1,787–3,709	1,789–4,109	1,860–2,976
	Basin relief	1650	1922	2320	1116
	Relief ratio	0.022	0.024	0.025	0.027
	Ruggedness number	0.95	1.03	1.31	0.56

Figure 6. Relative relief map of watersheds: (a) Gilgel Abay (b) Gumara (c), Rib, and (d) Megech.

Figure 7. Ruggedness index map of watersheds: (a) Gilgel Abay (b) Gumara (c), Rib, and (d) Megech.

Table 6. Correlation matrix for geomorphometric parameters of the four major watersheds of Lake Tana basin.

	A	P	Lb	Nu	Lsm	Rb	Lo	Dd	Fs	Dt	Cc	In	Re	Rf	Rc	H	Rh	No
A	1.00
P	0.98	1.00
Lb	0.87	0.95	1.00
Nu	1.00	0.99	0.90	1.00
Lsm	−0.73	−0.85	−0.97	−0.77	1.00
Rb	−0.02	0.04	0.18	0.06	−0.14	1.00
Lo	−0.97	−0.94	−0.83	−0.98	0.67	−0.21	1.00
Dd	0.97	0.92	0.78	0.97	−0.61	0.15	−1.00	1.00
Fs	0.91	0.95	0.95	0.95	−0.86	0.34	−0.94	0.90	1.00
Dt	1.00	0.98	0.88	1.00	−0.74	0.03	−0.98	0.97	0.93	1.00
Cc	0.82	0.91	1.00	0.86	−0.99	0.17	−0.78	0.72	0.93	0.83	1.00
In	0.27	0.40	0.60	0.35	−0.63	0.84	−0.40	0.32	0.64	0.31	0.62	1.00
Re	−0.40	−0.57	−0.80	−0.46	0.91	−0.28	0.36	−0.27	−0.65	−0.42	−0.85	−0.75	1.00
Rf	−0.46	−0.62	−0.84	−0.52	0.94	−0.30	0.43	−0.34	−0.70	−0.48	−0.89	−0.76	1.00	1.00
Rc	−0.89	−0.96	−1.00	−0.92	0.96	−0.10	0.84	−0.80	−0.94	−0.90	−0.99	−0.53	0.77	0.81	1.00
H	0.71	0.83	0.96	0.77	−0.98	0.34	−0.70	0.63	0.90	0.73	0.98	0.76	−0.92	−0.94	−0.94	1.00
Rh	−0.91	−0.82	−0.59	−0.87	0.41	0.29	0.85	−0.89	−0.67	−0.89	−0.52	0.13	0.01	0.07	0.65	−0.36	1.00
No	0.77	0.87	0.98	0.82	−0.96	0.37	−0.78	0.71	0.94	0.79	0.98	0.76	−0.86	−0.90	−0.95	0.99	−0.43	1.00

Strong positive correlations (r ≥ 0.9 and red color numbers), strong negative correlations (r ≤ −0.9 and light blue color numbers), good positive correlations (0.75 ≤ r < 0.9 and green color numbers), good negative correlations (−0.9 > r ≥ −0.75 and orange color numbers), moderate positive correlation (0.6 ≥ r > 0.75 and purple in color), and moderate negative correlation (−0.75 > r ≥ −0.6 and dark blue in color). A is drainage area, P is shape perimeter, L_b is basin Length, N_u is number of stream orders, L_sm is mean stream length, R_b is average bifurcation ratio, L_o is length of overland flow, D_d is drainage density, F_s is stream frequency, D_t is drainage texture, C_c is compactness coefficient, I_n is Infiltration number, R_e is elongation ratio, R_f is Form factor, R_c is circularity ratio, H is Basin relief, R_h is relief ratio, and N_o is ruggedness number

Table 7. Prioritization of the four watersheds using geomorphometric parameters.

Geomorphologic characteristics	Implications to surface runoff and erosion	Gilgel Abay	Gumara	Rib	Megech	Weight
Total stream length	Direct effect	1	3	2	4	0.3
Average bifurcation ratio	Direct effect	3	4	1	2	0.3
Average length of overland flow	Direct effect	4	2	3	1	0.3
Drainage density	Direct effect	1	3	2	4	0.4
Stream frequency	Direct effect	2	3	1	4	0.4
Form factor	Inverse effect	3	2	1	4	0.4
Drainage texture	Direct effect	1	3	2	4	0.4
Compactness coefficient	Inverse effect	2	3	4	1	0.4
Infiltration number	Direct effect	3	2	4	1	0.4
Elongation ratio	Inverse effect	3	2	1	4	0.4
Circularity ratio	Inverse effect	3	2	1	4	0.4
Relief ratio	Inverse effect	4	3	2	1	0.3
Ruggedness number	Direct effect	3	2	1	4	0.3
Compound rank		11.7	12.2	9.1	14
Priority		2	3	1	4

Table 8. Daily minimum, maximum, and mean streamflow and suspended sediment concentration as well as mean annual streamflow and sediment yield of the studied rivers.

Hydrological and sediment variables	Watersheds
	Gilgel Abay	Gumara	Rib	Megech
Daily minimum flow, m^3 s^−1	0.51	0.27	0	0
Daily maximum flow, m^3 s^−1	465	365	163	530
Daily mean flow, m^3 s^−1	55	37.7	15.6	10.3
Annual mean flow, Mm^3	1,735	1,191	494	326
Daily minimum sediment concentration, g l^−1	0.03	0.16	0.05	0.13
Daily maximum sediment concentration, g l^−1	3.48	10.07	20.00	63.05
Daily mean sediment concentration, g l^−1	1.22	2.69	5.27	3.65
Annual mean sediment yield, t ha^−1yr^−1	11.9	18.9	14.3	4.1

Figure 8. Mean annual streamflow and sediment yield of the studied watersheds.

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Data availability statement

The raw data required to reproduce the above findings are available to the corresponding author Dr. Anwar A. Adem ([email protected]) on reasonable request.