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

Reduction in forest cover (%) Annual streamflow increase (mm) Source: Bosch and Hewlett (1982) Reducing forest cover increases annual runoff

Mean annual rainfall (mm) Annual streamflow increase (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 (%) Annual streamflow increase (mm) Conifer Hardwood Scrub Source: Bosch and Hewlett (1982) Different covers respond in different ways

Source: Cornish and Vertessy (submitted) Estimated maximum annual runoff increases per 10% of forest cleared: Karuah, NSW

RF2 BS3 BS2 BS1 BJ PI ST Reduction in forest cover (%) Maximum increase in annual streamflow (mm) Source: Nandakumar and Mein (1993) Maximum annual runoff increases after forest clearance: results from Victoria

Stand age (years) Mean annual streamflow (mm) Source: Kuczera (1985) Mean annual runoff can vary as forests age: the mountain ash forest experience

Picaninny catchment - after clearfall in 1972

Source: Vertessy et al. (1998) Years after clearance ( ) Annual streamflow difference (mm) Annual runoff changes after clearance and regeneration of a mountain ash forest

Old growth Regrowth Mountain ash forests before and after fire

leaf area index stand age (years) TOTAL ASH UNDER Leaf area index changes over time in a mountain ash forest Source: Vertessy and Watson (1999)

A water balance for mountain ash forests with 1800 mm rainfall Source: Vertessy and Watson (1999)

Source: Cornish and Vertessy (submitted) Annual runoff changes after clearance and regeneration of six Karuah catchments

Observed annual streamflow decrease (mm) Predicted annual streamflow decrease (mm) Source: Cornish and Vertessy (submitted) Predicting annual runoff changes in the Karuah catchments (ASR = *SD *BAI *CC, r 2 = 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) JanFebMarAprMayJunJulAugSepOctNovDec Month Relative change in flow (%) Reduced share Increased share Annual runoff increase ~ 290 mm

Month ONDJFAMMJJASO 0 Mean monthly streamflow (mm) Source: Bosch and von Gadow (1990) Effects of afforestation on runoff seasonality at Cathedral Peak, South Africa Pines Grass

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 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 ….

Mean annual rainfall (mm) Forest Grassland Mean annual evapotranspiration (mm) 1400 Source: Holmes and Sinclair (1986) Evapotranspiration from forest versus grassland

0 50 km N The Middle Murrumbidgee Basin (26,000 km 2 )

Annual runoff (mm) The Middle Murrumbidgee Basin Mean annual runoff predicted by HSR for current cover

Observed mean annual runoff (mm) y = 0.913x, r 2 = 0.82 Predicted mean annual runoff (mm) Observed mean annual runoff versus the elevation corrected HSR prediction for the 28 subcatchments

Change in annual runoff (mm) Predicted change in mean annual runoff assuming grassland to pine conversion everywhere

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