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Why Model Forest Hydrology

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  1. Why Model Forest Hydrology • Test hypothesis (research oriented). To understand the cause-effect relations in a system and provide improved qualitative and quantitative interpretations of a system. • Data extrapolation and synthesis (analysis tools). To extend knowledge from known to unknown and study the dynamics in space and time • Prediction of the future (catchment management). To predict system behavior in improving control or management of the system. • Cost-effective for complex systems in some cases (large scale field experiments are too expensive or impossible) • Planning

  2. Why Model Forest Hydrology South African concerns • Forestry is the only declared SFRA • How much water do other “crops” use? • Do the different forest species use different amounts of water? • What are the impacts of management practices such as: • Site preparation • Thinning • What affect on water resources does the move into marginal areas have? • How much afforestation in a catchment should be permitted • Where, within a catchment, should afforestation be permitted (Riparian Zones?).

  3. Forest Hydrology Models • Simple and Complex • Regression based • e.g. Simple curves based on catchment experiments • Nanni, Scott-Smith Curves (SA), Kuczera Curve (Australia)

  4. Forest Hydrology Models Physical-Conceptual Model - Tree Characteristics • Interception • Canopy capacity • Litter interception • Transpiration • Evergreen • Soil moisture • Infiltration • Litter layer • Soil Structure • Rooting System • Depth of roots • Colonisation

  5. Forest Hydrology Models Physical-Conceptual Model - Plantation Characteristics (SA) • Tree Type • Pine, Eucalypt, Wattle • Site Preparation Method • Pit, rip etc • Management Practices • Thinning, weeding etc • Rotation • Age of Tree

  6. Forest Hydrology Models - ACRU • Various adaptations made to ACRU to account for forest related hydrological processes

  7. Forest Hydrology Models - ACRU Transpiration • Commonly, Crop Coefficients have been used to relate vegetative cover to transpiration rates. • However, sufficient research has been completed on LAIs in South Africa and abroad to use LAI as the "driver" of tree water consumption rather than the crop coefficient concept. • Driven largely by understanding of the changes in LAI with tree growth, thinning, etc. • Maximum transpiration (Etm) is related to LAI by equation developed by Ritchie (1972) and Kristensen (1974). E.g. Etm = 0.71 LAI0.5 - 0.2 (for LAI <3.0)

  8. Forest Hydrology Models - ACRU Interception and Evaporation • Daily canopy interception loss (Il in mm) on a day on which rainfall occurs can be calculated from the Von Hoyningen-Heune (1983) equation. Inputs are gross daily rainfall (Pg in mm) and LAI • Or specific canopy interception values can be provided.

  9. Forest Hydrology Models - ACRU Interception and Evaporation • The wetted forest canopy has an interception storage capacity, Is, is estimated to be 0.5Ii in ACRU • It is assumed that water stored in the canopy is evaporated at rates well in excess of the reference estimated potential evaporation (Er). In ACRU enhanced evaporation from the canopy is estimated as: Ew = Er (0.267LAI + 0.33) for LAI>2.7 Implies that wet canopy evaporation will proceed at 1.67 Er for LAI = 5

  10. Forest Hydrology Models - ACRU Rooting Depth • The proportion of active roots in the topsoil and the degree of colonisation of roots in a soil horizon are variables commonly used to describe rooting patterns • They indicate from where, within the soil profile, the tree is able to extract its soil moisture.

  11. Forest Hydrology Models - ACRU Time Dependence of Model Parameters • A dynamic landuse file is used to account for changes in model parameters as the trees grow. • In the dynamic file the values of LAI, Il, rooting fractions and colonisation change over time according to genus, region, MAP, site preparation and thinning practices

  12. Forest Hydrology Models - ACRU LAI values used in the ACRU FDSS for the three main genera grown commercially, averaged for four major forestry areas in South Africa, two levels of site preparation and two rainfall regimes (Summerton, 1995)

  13. Forest Hydrology Models - ACRU An example of the effects of thinning on the LAI of pines in the Midlands of KwaZulu-Natal on a site with intensive site preparation (Summerton, 1995)

  14. Forest Hydrology Models - ACRU

  15. Forest Hydrology Models - ACRU Forest Decision Support System • Interactive model, developed specifically for application in South Africa, Lesotho and Swaziland, enables objective forest hydrology response related decisions to be made with relative ease and in a short time. • Database of values in which Leaf Area Index (LAI), interception loss (Il) and rooting components are developed for different: • forestry regions in South Africa, • rotation periods, • rainfall regimes, • genera, and • management practices, including site preparation and thinning. • Eucalyptus grandis, Pinus patula and Acacia mearnsii are assumed to be representative of the three main genera, viz. eucalypts, pines and wattle, since not enough is, as yet, known about the comparative water use aspects of different species within a genus.

  16. Forest Hydrology Models - ACRU Typical Use • Assessment of forestry impacts for permit applications. • Use of water by alien invasive vegetation - especially in the Riparian Zone. • Comparison of water use by different crops - including irrigation.


  18. Forest Hydrology Models - ACRU Guidelines for 2004 Prac and other information at www.beeh.unp.ac.za/hydro/