The impacts of forestry on catchment runoff

Rob Vertessy

• Differences between grassed and forested catchments

• Changes to mean annual streamflow

• Stand age effects on streamflows

• Changes to peak and low flows

• Afforestation of agricultural land: water yield impacts

A roadmap to this presentation

• Forests have higher and more persistent leaf area

• Forests are aerodynamically rougher

• Forests are deeper rooted

• Forests have lower albedo and thus absorb more energy

• ET from grassland is usually less than 700 mm

• ET from forest can reach 1400 mm

• So runoff from forests is less

Forest ET exceeds grassland ET

0 20 40 60 80 100

Reduction in forest cover (%)

0

100

200

300

400

500

600

700A

nnua

l str

eam

flow

incr

ease

(m

m)

Source: Bosch and Hewlett (1982)

Reducing forest cover increases annual runoff

0 400 800 1200 1600 2000 24000

200

400

600

800

Mean annual rainfall (mm)

Ann

ual s

trea

mflo

win

crea

se (

mm

)

The largest impacts of forest clearance are experienced in the highest rainfall areas

Source: Bosch and Hewlett (1982) and Holmes and Sinclair (1986)

(HSR)

Reduction in forest cover (%)

500

400

300

200

100

00 25 50 75 100

Ann

ual s

trea

mflo

win

crea

se (

mm

)Conifer

Hardwood

Scrub

Source: Bosch and Hewlett (1982)

Different covers respond in different ways

Catchment Mean annual rainfall(mm)

Streamflow increase per 10% offorest cleared (mm)

Kokata 1599 70

Bollygum 1532 56

Coachwood 1472 43

Corkwood 1694 43

Jackwood 1391 37

Barrata 1657 0

Source: Cornish and Vertessy (submitted)

Estimated maximum annual runoff increases per 10% of forest cleared: Karuah, NSW

RF2

BS3

BS2

BS1BJ

PI

ST5400

300

200

100

00 20 40 60 80 100

Reduction in forest cover (%)

Max

imum

incr

ease

inan

nual

str

eam

flow

(m

m)

Source: Nandakumar and Mein (1993)

Maximum annual runoff increases after forest clearance: results from Victoria

0

200

400

600

800

1000

1200

1400

0 40 80 120 160 200

Stand age (years)

Mea

n an

nual

stre

amflo

w (

mm

)

Source: Kuczera (1985)

Mean annual runoff can vary as forests age: the mountain ash forest experience

Picaninny catchment - after clearfall in 1972Picaninny catchment - after clearfall in 1972

Source: Vertessy et al. (1998)

-300

-200

-100

0

100

200

300

1 3 5 7 9 11 13 15 17 19 21 23Years after clearance (1973-1995)

Ann

ual s

trea

mflo

wdi

ffer

ence

(m

m)

Annual runoff changes after clearance and regeneration of a mountain ash forest

Old growthOld growth RegrowthRegrowth

Mountain ash forests before and after fire

0

1

2

3

4

5

6

0 50 100 150 200 250

leaf

are

a in

dex

stand age (years)

TOTAL

ASH

UNDER

Leaf area index changes over time in a mountain ash forest

Leaf area index changes over time in a mountain ash forest

Source: Vertessy and Watson (1999)Source: Vertessy and Watson (1999)

A water balance for mountain ash forests with 1800 mm rainfall

A water balance for mountain ash forests with 1800 mm rainfall

15 Years 240 Years Young Old

OVERSTOREY LAIUNDERSTOREY LAI

TOTAL LAI

3.80.4

4.2

1.22.4

3.6

2.62.0

0.6

O/S TRANSPIRATIONU/S TRANSPIRATIONINTERCEPTIONSOIL EVAPO RATION

TOTAL ET

76050

400130

1340

260300260100

920

50025014030

420

RUNOFF 460 880 420

15 Years 240 Years Young Old

OVERSTOREY LAIUNDERSTOREY LAI

TOTAL LAI

3.80.4

4.2

1.22.4

3.6

2.62.0

0.6

O/S TRANSPIRATIONU/S TRANSPIRATIONINTERCEPTIONSOIL EVAPO RATION

TOTAL ET

76050

400130

1340

260300260100

920

50025014030

420

RUNOFF 460 880 420

Source: Vertessy and Watson (1999)Source: Vertessy and Watson (1999)

Source: Cornish and Vertessy (submitted)

80

60

40

20

0

-20

-40

-60

-80

Water Year

77 79 81 83 85 87 89 91 93 95 97

Pre-treatment

Post-treatment

Ann

ual s

trea

mflo

w c

hang

e pe

r 10

%F

ores

t ar

ea t

reat

ed (

mm

)

Annual runoff changes after clearance and regeneration of six Karuah catchments

600

500

400

300

200

100

0

6005004003002001000

Observed annual streamflow decrease (mm)

Pre

dict

ed a

nnua

l str

eam

flow

decr

ease

(m

m)

Source: Cornish and Vertessy (submitted)

Predicting annual runoff changes in the Karuah catchments

(ASR = 1368 - 480.8*SD - 37.418*BAI - 16.76*CC, r2 = 82%)

Source: Haydon (1993)

Flow seasonality changes caused by thinning in the mountain ash forest: Crotty Creek

(‘39 regrowth thinned to 50% of original basal area over six years)

-20

-15

-10

-5

0

5

10

15

20

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Month

Rel

ativ

e ch

ange

in f

low

(%

)

Reduced share

Increased share

Annual runoff increase ~ 290 mm

150

100

50

Month

O N D J F AM M J J A S O0

Mea

n m

onth

lyst

ream

flow

(m

m)

Source: Bosch and von Gadow (1990)

Effects of afforestation on runoff seasonality at Cathedral Peak, South Africa

Pines

Grass

0

4

8

12

16

20

0

4

8

12

16

20

> 1

5

10 -

15

5 -

10

2 -

5

Storm size class at control (l s-1 ha-1)

GH1 (control)GH2 (afforested)

(A) Pre-planting (B) Post-canopy closure

n = 40 n = 49

n = 15

n = 7

n = 4

n = 8

n = 4

n = 3

Mea

n flo

odpe

ak (

l s-1 h

a-1)

> 1

5

10 -

15

5 -

10

2 -

5

Source: Fahey and Jackson (1997)

Afforestation reduces mean flood peaks: results from Glendhu, South Island, New Zealand

• Pine runoff < Eucalypt runoff < Grassland runoff; differences are amplified with increasing rainfall

• Afforestation reduces low AND high flows as well as mean runoff

• Streamflow changes linearly with % forest area cleared or planted

• Following clearing, peak changes usually occur within 2-3 years; recovery usually takes 4-10 years but may take as long as 25 years

Well established generalisations

• Forest age affects ET rates in moist eucalypt forest; old growth stands yield more runoff than stands aged 20-30 years

• Afforestation of grasslands reduces low flows proportionally more than median and high flows

• Forest thinning has similar impacts as patch cutting in terms of magnitude of response, provided a similar basal area is treated

• Effects of patch cutting are felt longer than for thinning

Generalisations supported by limited evidence

• Forest age affects ET rates in dry eucalypt forest; there is little hydrometric data to substantiate this

• Transpiration per unit leaf area declines with forest age; two good case studies of this exist but require confirmation in other forest areas

Still speculating ….

500 1000 1500 2000 2500

Mean annual rainfall (mm)

Forest

Grassland

400

600

800

1000

1200M

ean

annu

al

evap

otra

nspi

ratio

n (m

m)

1400

Source: Holmes and Sinclair (1986)

Evapotranspiration from forest versus grassland

0 50

kmN

Canberra

Yass

Harden

Tumut

GundagaiWagga Wagga

Cooma

The Middle Murrumbidgee Basin(26,000 km2)

Annual runoff (mm)

0

100

200

400

800

1950

The Middle Murrumbidgee BasinMean annual runoff predicted by HSR for

current cover

0

200

400

600

800

1000

0 200 400 600 800 1000

Observed mean annual runoff (mm)

y = 0.913x, r2 = 0.82P

redi

cted

mea

n an

nual

run

off

(mm

)

Observed mean annual runoff versus the elevation corrected HSR prediction for the 28

subcatchments

0

120

240

360

480

600

Change in annual runoff (mm)

Predicted change in mean annual runoff assuming grassland to pine conversion

everywhere

0

100

200

300

400

500

0 0.25 0.5 0.75 1

Proportion of catchment

An

nu

al r

un

off

ch

ang

e (m

m)

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