Question

QUESTION 1

The daily stream flow data for a particular catchment having an area of 6500 km² are given in the table below. Separate the baseflow using Straight line method, Fixed base method, and recession Separation method.

Calculate the total direct runoff for each method.

Table Q1

Time	Flows, Q
(days)	(m3/s)
1	1 600
2	1 550
3	5 000
4	10 300
5	8 600
6	6 500
7	5 000
8	3 800
9	2 800
10	2 200
11	1 850
12	1 600
13	1 330
14	1 300
15	1 280

Answer 1

Given data:

Area of catchment = 6500 km2

Time (days)	Flows (Q) in m3 / sec
1	1600
2	1550
3	5000
4	10300
5	8600
6	6500
7	5000
8	3800
9	2800
10	2200
11	1850
12	1600
13	1330
14	1300
15	1280

Base flow Separation:

First of all plot the graph between discharge and the time.I have plotted using excel sheet and attached below.

Base flow separation using different methods:

1.Method 1- Straight line method:

Identify first point (beginning of direct runoff) and then join with a straight line to a point on the recession limb(end of the direct runoff).First point is easy to identify where the curve changes sharply but the point on recession limb is difficult to catch.Just draw a straight line from the first point towards the end.Go through the below attached graph plotted in excel.

2.Method 2 -Fixed Base Method:

here we are required to find N value.

N= 0.83 *A 0.2 = 0.83 * (6500)0.2 = 4.80 days = 5 days(approximately).

This N value is useful to identify the point on recession limb.

See in the following plot attached below,

From the point A, a line is drawn that follows the previous trend in discharge to the point C ( a point on the line drawn from the peak discharge). Now from the point B(after N days from the peak value).Here peak discharge is obtained at the 4th day and the N value is 5 So B is taken on 9th day(4+5) on the recession limb and draw a backward line to the point C.
Lines AC and BC separates the baseflow and the direct runoff.

3.Method 3- Recession Separation Method:

Here in this method the last point(E) on the recession limb is extended backwards upto point (F ) obtained from a line drawn from the inflection point(C).Now join A and F through a smooth curve .Go through the attached plot drawn in excel.

Direct runoff = Total runoff - Base flow.

From all the three methods, the values of direct runoff are obtained and represented in the table format below.

Time	Total runoff	Baseflow	Direct Runoff
method 1	method 2	method3	method1	method2	method3
days	m3 /sec	m3 /sec	m3 /sec	m3 /sec	m3 /sec	m3 /sec	m3 /sec
1	1600	1600	1600	1600	0	0	0
2	1550	1550	1550	1550	0	0	0
3	5000	1520	1480	1500	3480	3520	3500
4	10300	1500	1400	1450	8800	8900	8850
5	8600	1450	1700	1400	7150	6900	7200
6	6500	1450	1950	1400	5050	4550	5100
7	5000	1450	2300	1400	3550	2700	3600
8	3800	1400	2550	1400	2400	1250	2400
9	2800	1380	2800	1380	1420	0	1420
10	2200	1380	2200	1380	820	0	820
11	1850	1380	1850	1380	470	0	470
12	1600	1350	1600	1350	250	0	250
13	1330	1330	1330	1330	0	0	0
14	1300	1300	1300	1300	0	0	0
15	1280	1280	1280	1280	0	0	0

All the values of baseflow are obtained from the respective graphs and may vary slightly person to person as they are taken from the graphs.Direct runoff is obtained from subtracting baseflow from the total runoff.