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The impacts of forestry on catchment runoff

The impacts of forestry on catchment runoff. Rob Vertessy. A roadmap to this presentation. Differences between grassed and forested catchments Changes to mean annual streamflow Stand age effects on streamflows Changes to peak and low flows

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The impacts of forestry on catchment runoff

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  1. The impacts of forestry on catchment runoff Rob Vertessy

  2. A roadmap to this presentation • 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

  3. Forest ET exceeds grassland ET • 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

  4. 700 600 500 400 Annual streamflow increase (mm) 300 200 100 0 0 20 40 60 80 100 Reduction in forest cover (%) Reducing forest cover increases annual runoff Source: Bosch and Hewlett (1982)

  5. 800 600 Annual streamflow increase (mm) 400 200 0 0 400 800 1200 1600 2000 2400 Mean annual rainfall (mm) The largest impacts of forest clearance are experienced in the highest rainfall areas (HSR) Source: Bosch and Hewlett (1982) and Holmes and Sinclair (1986)

  6. 500 400 Conifer 300 Annual streamflow increase (mm) 200 Hardwood 100 Scrub 0 0 25 50 75 100 Reduction in forest cover (%) Different covers respond in different ways Source: Bosch and Hewlett (1982)

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

  8. 400 ST5 PI 300 Maximum increase in annual streamflow (mm) 200 BS3 BJ RF2 100 BS1 BS2 0 0 20 40 60 80 100 Reduction in forest cover (%) Maximum annual runoff increases after forest clearance: results from Victoria Source: Nandakumar and Mein (1993)

  9. 1400 1200 1000 Mean annual streamflow (mm) 800 600 400 200 0 0 40 80 120 160 200 Stand age (years) Mean annual runoff can vary as forests age: the mountain ash forest experience Source: Kuczera (1985)

  10. Picaninny catchment - after clearfall in 1972

  11. 300 200 100 Annual streamflow difference (mm) 0 -100 -200 -300 1 3 5 7 9 11 13 15 17 19 21 23 Years after clearance (1973-1995) Annual runoff changes after clearance and regeneration of a mountain ash forest Source: Vertessy et al. (1998)

  12. Mountain ash forests before and after fire Old growth Regrowth

  13. 6 5 4 TOTAL 3 leaf area index UNDER 2 ASH 1 0 0 50 100 150 200 250 stand age (years) Leaf area index changes over time in a mountain ash forest Source: Vertessy and Watson (1999)

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

  15. 80 60 40 20 Annual streamflow change per 10% Forest area treated (mm) 0 -20 -40 Pre-treatment -60 Post-treatment -80 77 79 81 83 85 87 89 91 93 95 97 Water Year Annual runoff changes after clearance and regeneration of six Karuah catchments Source: Cornish and Vertessy (submitted)

  16. 600 500 400 Predicted annual streamflow decrease (mm) 300 200 100 0 100 200 300 400 500 600 0 Observed annual streamflow decrease (mm) Predicting annual runoff changes in the Karuah catchments (ASR = 1368 - 480.8*SD - 37.418*BAI - 16.76*CC, r2 = 82%) Source: Cornish and Vertessy (submitted)

  17. 20 15 Annual runoff increase ~ 290 mm 10 5 Increased share Relative change in flow (%) 0 -5 -10 Reduced share -15 -20 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Month 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) Source: Haydon (1993)

  18. 150 100 Mean monthly streamflow (mm) 50 0 O N D J F M A M J J A S O Month Effects of afforestation on runoff seasonality at Cathedral Peak, South Africa Grass Pines Source: Bosch and von Gadow (1990)

  19. GH1 (control) GH2 (afforested) 20 20 (A) Pre-planting (B) Post-canopy closure n = 3 n = 4 16 16 n = 4 n = 7 12 12 Mean flood peak (l s-1 ha-1) n = 8 8 8 n = 15 n = 40 n = 49 4 4 0 0 > 15 > 15 2 - 5 2 - 5 5 - 10 5 - 10 10 - 15 10 - 15 Storm size class at control (l s-1 ha-1) Afforestation reduces mean flood peaks: results from Glendhu, South Island, New Zealand Source: Fahey and Jackson (1997)

  20. Well established generalisations • 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

  21. Generalisations supported by limited evidence • 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

  22. Still speculating …. • 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

  23. Forest Grassland Evapotranspiration from forest versus grassland 1400 1200 1000 Mean annual evapotranspiration (mm) 800 600 400 500 1000 1500 2000 2500 Mean annual rainfall (mm) Source: Holmes and Sinclair (1986)

  24. Harden Yass Gundagai Wagga Wagga Tumut Canberra Cooma km 50 0 The Middle Murrumbidgee Basin (26,000 km2) N

  25. 0 200 800 100 400 1950 The Middle Murrumbidgee Basin Mean annual runoff predicted by HSR for current cover Annual runoff (mm)

  26. 1000 y = 0.913x, r2 = 0.82 800 600 Predicted mean annual runoff (mm) 400 200 0 0 200 400 600 800 1000 Observed mean annual runoff (mm) Observed mean annual runoff versus the elevation corrected HSR prediction for the 28 subcatchments

  27. 500 400 300 Annual runoff change (mm) 200 100 0 0 0.25 0.5 0.75 1 Proportion of catchment 0 240 480 120 360 600 Predicted change in mean annual runoff assuming grassland to pine conversion everywhere Change in annual runoff (mm)

  28. …… Thanks for listening …...

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