Question

For the table show, determine the Direct Runoff depth. Express in the nearest whole number value in mm. Note that the given is observed Runoff, not yet Direct Runoff. Observed Observed Time Rainfall Runoff (mm) (m3/s) 9:00 43 23 9:30 53 65 10:00 5 161 10:30 5 270 11:00 312 11:30 233 12:00 122 12:30 63 13:00 51 13:30 35 14:00 20 watershed area 18.22 km2 baseflow 11.34 m3/s

Answer 1

Base flow = 11.34 m³/s

Watershed area = 18.22 km²

Direct run-off = Observed run-off - Base flow

Time	Observed rainfall	Observed Runoff	Base flow	Direct runoff
hours	mm	m3/s	m3/s	m3/s
09:00	43	23	11.34	11.66
09:30	53	65	11.34	53.66
10:00	5	161	11.34	149.66
10:30	5	270	11.34	258.66
11:00		312	11.34	300.66
11:30		233	11.34	221.66
12:00		122	11.34	110.66
12:30		63	11.34	51.66
13:00		51	11.34	39.66
13:30		35	11.34	23.66
14:00		20	11.34	8.66
			Sum=	1230.26	m3/s

Direct run-off depth = Total volume of direct run-off/Area of basin

$=\frac{(\Sigma O)*t*60*60}{A*10^{6}} = 0.36*\frac{{(\Sigma O)*t}}{A} cm$

where t = Time interval between successive ordinates in hours

A = Area of basin in km²

$\Sigma$O = Sum of discharge ordinates (direct run-off) in m³/s

Direct run-off depth = $0.36*\frac{1230.26*0.5}{18.22}= 12.154 cm = \mathbf{\mathit\underline{{12 cm}}}$

Direct run-off depth = 120 mm

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