Runoff

Cintia Bertacchi Uvo

VVR111 – Hydrology and Aquatic Ecology

Learning Goals

• Environmental effects of runoff

• Measuring methods

• Hydrograph analysis

• Unit hydrograph – effective rainfall (Peffective)

– direct runoff (Q direct)

– base flow

– phi-index (Φindex)

– calculation and use of unit hydrograph

VVR111 – Hydrology and Aquatic Ecology

Runoff

• Visible surface water

– Rivers, brooks, lakes, ponds, reservoirs…

• Moves towards the lowest point (normally the sea). Gravity.

• May be delayed due to temporary storage

– Infiltration, lakes, evaporation…

VVR111 – Hydrology and Aquatic Ecology

Runoff Delay

• Runoff residence time on a lake

Lake volume / total inflow

Tresidence = Vlake/Σ Qinflow

VVR111 – Hydrology and Aquatic Ecology

Wetland vs. Drained land

VVR111 – Hydrology and Aquatic Ecology

Base flow

• Flow from the ground water

a. Drought or cold winters

b. Rainy period or melting conditions

Dry conditions

Wet conditions

VVR111 – Hydrology and Aquatic Ecology

Sax-Bra Rivers Catchment 1812-20

http://www.landskrona.se/Documents/Landskrona/Documents/Gamla dokument K/kunsksaxbra.pdf

VVR111 – Hydrology and Aquatic Ecology

Sax-Bra Rivers Catchment 1980

VVR111 – Hydrology and Aquatic Ecology

What happens with the runoff when it rains?

VVR111 – Hydrology and Aquatic Ecology

Hydrograph theory

Losses

Time (h)

Time (h)

Ptot (mm/h)

Qtot (m3/s)

Base flow

Qdirect

Φindex

Peffective Peffective = Qdirect

Ptot = Peffective + Φindex

Qtot = Qdirect + QBase flow

VVR111 – Hydrology and Aquatic Ecology

Unit Hydrograph

• The runoff hydrograph for Peffective = 1mm

Time (h)

Time (h)

Peff (mm/h)

Qtot (m3/s)

Qdirect

Peffective = Qdirect

1

VVR111 – Hydrology and Aquatic Ecology

Unit Hydrograph – Assumptions

• Rainfall is evenly distributed over the entire catchment.

• Rainfall intensity is constant for each individual time interval.

• The runoff is linearly proportional to the rainfall.

• The runoff is independent of seasonal variation.

• The duration of runoff depends only on the duration of rainfall and not on the rainfall intensity.

VVR111 – Hydrology and Aquatic Ecology

Unit Hydrograph – Calculation

• Start from an observed hydrograph

• Extract Qbase flow from Qobs at each time step

• Divide Qobs-Qbase by the total Peffective at each step.

q = (Qobs-Qbase)/ Peffective

VVR111 – Hydrology and Aquatic Ecology

Unit Hydrograph – Calculation

• Runoff (l/s) has been observed from a 500 ha large catchment after a total rainfall of 10 mm during 30 min. What is the unit hydrograph for the area?

Time (min) Q (l/s)

0 5.0

30 40.0

60 150.0

90 690.0

120 310.0

150 150.0

180 80.0

210 5.0

q = Qtot-Qbase/ Peffective

Qbase flow = ? Peffective = ?

VVR111 – Hydrology and Aquatic Ecology

Unit Hydrograph – Calculation

Time (min) Q (l/s) Qtot -Qbase q

0 5 0 0

30 40 35 70

60 150 145 290

90 690 685 1370

120 310 305 610

150 150 145 290

180 80 75 150

210 5 0 0

Qbase = 5 m3/s; Qdirect ~2500 m3 and Peffect = 0.5 mm

VVR111 – Hydrology and Aquatic Ecology

Unit Hydrograph – Calculation

• Set up a table with 4 columns. Put time step in first column and total runoff in the second column

• Subtract base flow and put direct runoff in the third column

• Calculate direct runoff volume in m3 by adding up all rows in column 3 and multiplying with Δt. Divide direct runoff volume with catchment area to get effective rainfall in mm.

• Divide direct runoff in column 3 with effective rainfall to get the unit hydrograph. Put results in column 4.

VVR111 – Hydrology and Aquatic Ecology

Unit Hydrograph - Use

• Once you have the unit hydrograph, it is extremely simple to find the hydrograph on any other rainfall.

• Ex: A catchment has a one-hour unit hydrograph according to

• What is the direct runoff from an effective rainfall of 5 mm/h

Time (h) 0 1 2 3 4

Q (m3/(s · mm) 0 3 2 1 0

VVR111 – Hydrology and Aquatic Ecology

Unit Hydrograph - Use

• What is the direct runoff from an effective rainfall of two rains during two hours according to first 5 and then 10 mm?

• What is the total runoff from the rains in b) if the base flow is a constant 1 m3/s?

VVR111 – Hydrology and Aquatic Ecology

Unit Hydrograph - Use

Time

(h)

q(m3/(s·mm) 5·q (m3/s) 10·q(m3/s) ΣQ (m3/s) ΣQ+base

flow (m3/s)

0 0 0 0 1

1 3 15 0 15 16

2 2 10 30 40 41

3 1 5 20 25 26

4 0 0 10 10 11

5 0 0 1

VVR111 – Hydrology and Aquatic Ecology

• A catchment (area = 300 km2) gets 30 mm rain during one day. During the next five days the following flow is measured in the river that drains the catchment.

Time (d) 0 1 2 3 4 5 Q (m3/s) 2 5 10 8 4 2 • Assume that the baseflow is constant Qbase = 2

m3/s during the period. • a) What was the total direct runoff during the

period (m3)? • b) What were the losses (mm)?

VVR111 – Hydrology and Aquatic Ecology

• Find below the discharge values for a 30 min Unit Hydrograph. a) What is the size of the catchment area? b) Calculate and plot the hydrograph for a 60 minute rain with an effective intensity of 10 mm/h.

Time (min) Discharge (m3/s) 0 0 15 4.5 30 10 45 12.5 60 11 75 9 90 6.5 105 4 120 2.5 135 1 150 0