GIS-based watershed characterization and morphometric analysis in Bohol Watersheds, Philippines

Figures & data

Figure 1. The geographic location of the study area. A) The major watersheds, drainage networks, and fault lines overlay on an elevation map. B) The mouth of Loboc Watershed. C) The midstream agricultural area of Wahig – Inabanga Watershed.

Figure 2. The process flow of morphometric analysis in Bohol watersheds.

Table 1. List of morphometric parameters used in the study, equations, and the source of information.

Morphometric parameters (Notation)[Unit]	Equation	References/Source
Linear aspects
Stream order (U)	Hierarchical rank	(Strahler, 1952)
Stream number (Nu)	Nu = N1+ N2 + …Nn	(Horton, 1932)
Stream length (LT)	LT = L1+ L2 + …Ln	(Horton, 1932)
Mean stream length (Lm)	Lm = Lu/Nu	(Strahler, 1957)
Stream length ratio (RI)	RI = Lu/L(u-1)	(Horton, 1932)
Bifurcation ratio (Rb)	Rb = Nu/N(u+1)	(Strahler, 1957)
Mean bifurcation ratio (Rbm)	Rbm = average of Rb	(Horton, 1932; Schumm, 1956)
Watershed length (Lb) [km]	GIS
Main stream length (LM) [km]	GIS
Areal aspects
Watershed area (A) [km^2]	GIS
Perimeter (P) [km]	GIS
Form factor (Rf)	Rf = A/(Lb)^2	(Horton, 1932)
Elongation ratio (Re)	Re = 2/Lb*(A/π)^1/2	(Schumm, 1956)
Circulatory ratio (Rc)	Rc = 4πA/P^2	(Schumm, 1956)
Drainage density (Dd)	Dd = Lu/A	(Horton, 2004)
Drainage texture (Dt)	Dt = Nu/P	(Horton, 1932)
Stream frequency (Fs) [km^−2]	Fs = Nu/A	(Horton, 2004)
Compactness coefficient (Cc)	Cc = 0.2841 × (P/A)^0.5	(Horton, 1932)
Relief aspects
Maximum (Emax)	GIS
Minimum (Emin)	GIS
Mean (Emean)	GIS
Relief (R)	R = (Emax – Emin)	(Strahler, 1952)
Relief ratio (Rr) [m/km]	Rr = H/Lb	(Schumm, 1956)
Relative relief (Rhp)	Rhp = (Emax × 100/ P)	(Schumm, 1956)
Ruggedness number (Rn)	Rn = Dd* R/1000	(Asfaw & Workineh, 2021)
Hypsometric curve	GIS
Hypsometric integral (HI)	HI = (Emean – Emin)/(Emax – Emin)	(Strahler, 1952)

Table 2. Stream network characteristics and the linear aspects of LW and WIW.

Parameters	LW								WIW
U	1	2	3	4	5	6	-	1	2	3	4	5	6	-
Nu	951	304	56	11	3	1	1362	852	194	48	10	3	1	1108
Lu	0.7	1.1	2.9	7.0	17.6	39.2	-	0.6	1.4	3.4	6.6	7.7	85.0	-
LT	666	347	160	77	53	39	1342	557	264	163	66	23	85	1158
Rl,u	-	0.52	0.46	0.48	0.69	0.74		-	0.47	0.62	0.40	0.35	3.69
Rl,m	0.58							1.11
Rb	3.13	5.43	5.09	3.67	3.00	-	-	4.39	4.04	4.80	3.33	3.00	-	-
Rbm	4.06							3.91
Lw	38.6							47
LM	88							105

U – Stream order; N_u – Stream number; L_u – stream length [km]; L_T – Total stream length [km]; R_l,u – Stream length ratio; R_l,m – mean stream length ratio; R_b – Bifurcation ratio; R_bm – Mean bifurcation ratio; L_w – Watershed length [km]; L_M – Main stream length [km]; LW – Loboc Watershed; WIW – Wahig–Inabanga watershed

Figure 3. Drainage network organization and the stream orders in the watersheds under study.

Table 3. Results of the areal aspect analysis in LW and WIW.

Watersheds	A	P	Rf	Re	Rc	Dd	Dt	Fs	Cc
LW	676	164	0.454	0.552	0.315	1.99	7.77	1.89	1.78
WIW	610	203	0.276	0.556	0.186	1.90	5.57	1.85	2.32

A – area [km²]; P – perimeter [km]; R_f – form factor; R_e – elongation ratio; R_c – circulatory ratio; D_d – drainage density; D_t – drainage texture; F_s – stream frequency [km⁻²); C_c – compactness coefficient

Table 4. Relief aspect analysis and the computed values in LW and WIW.

Watershed	Emax	Emin	Emean	R	Rr	Rhp	Rn	HI
LW	732	0	271	732	14	0.004	1.457	0.37
WIW	869	0	195	869	17	0.004	1.653	0.22

E_max – Elevation, maximum [m]; E_min – Elevation, minimum [m]; E_mean – Elevation, mean [m]; R – Relief [m]; R_r – Relief ratio [m/km]; R_hp – Relative relief; R_n – Ruggedness number; HI – Hypsometric integral

Figure 4. Elevation profile of the LW and WIW main rivers from the headwaters to their outlets. The main trunk’s length and peak elevations are 89 km and 732 m, and 105 km and 869 m for LW and WIW, respectively.

Figure 5. Hypsometric curve and hypsometric integral (HI) of the watersheds under study.

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