Content Table

General Description

Afforestation means the development of forest stand on former agricultural areas. This measure can help to reduce soil erosion and surface runoff. While the surface run-off is reduced, infiltration and evapotranspiration increases. A second impact on waterflow is the delay of snow- melt, caused by lower influence of wind and solar radiation. Forests can retain up to twice as much water as grassland areas. Best impact of the measure regarding flood prevention can be reached on soils with low water storage capacity or low permeable geologic formation (Elsaßer 1991). Most suitable for afforestation are hilltops and hillsides within secondary mountain areas were additional surface drainage lines have to be considered. Afforestation projects should use typical tree species but take into account the resilience of the species with regards to anticipated climate changes (temperature regime, water availability, pests).

Afforestation can help to mitigate:

It is building up decentralised retention volume and thus reducing the risks of flash flooding.

Reduction of discharge peaks depend on:

  • Catchment size
  • Steepness
  • Tree population (age, species)
  • Soil condition
  • Weather and climate (high precipitation areas)

According to a modelling study (cited in DWA, 2006) flood peaks were reduced by 6 - 14 % in catchments when 20 % of former agricultural area were afforested. This effect is stronger for frequent flooding events than for rare events.
The measure takes effect in medium to long run. Surface run-off and infiltration characteristics will change relatively soon, evapotranspiration will depend on the surface of the foliage which increases over the time as the young forest stand develops.

A cost benefit analysis has to be done separatly for each individual project. A possible procedure and the coast range for alternative solutions you will find here (link).

Pro and Con


  • improvement of groundwater quality
  • recreational value
  • increase carbon capture
  • cooling and humidifying for local climate
  • use as resource (wood) and for hunt
  • noise reduction


  • only to be used as one component in building up decentralised retention volume due to small effects on flood peaks in extreme events
  • loss of income for farmers
  • risk of nitrate remobilisation
  • possible degrease of groundwater level (important for dry regions)
  • only larger coherent areas of forest can be operated economically sound
  • Sieker, F. (2007): Maßnahmensteckbrief "Aufforstung"; In: Institut für Wasserwirtschaft, Hydrologie und Landwirtschaftlichen Wasserbau (2007): Abschlussbericht zum Forschungsprojekt "Vorbeugender Hochwasserschutz durch Wasserrückhalt in der Fläche unter besonderer Berücksichtigung naturschutzfachlicher Aspekte - am Beispiel des Flusseinzugsgebietes der Mulde"; Research Report
  • DWA-Themenheft (2006): Dezentrale Maßnahmen zur Hochwasserminderung. S. 22f., DWA, Hennef
  • ATV-DVWK (2003): Ursache-Wirkungsbeziehungen zu Hochwasserereignissen, ATV- DVWK Arbeitsbericht, Hennef.
  • DWA Deutsche Vereinigung für Wasserwirtschaft, Abwasser und Abfall (2005) Dezentrale Maßnahmen zur Hochwasserminderung, Kapitel 5 Kostenschätzungen, p.114- 119
  • Elsaßer, Peter (1991): Umweltwirkungen der Aufforstung ackerbaulich genutzter Flächen
  • Physical effects of Afforestation (case studies)
  • reduction- apr08/quanitification- flood- erosion- reduction-apr08.pdf
  • Reforestation or afforestation of land currently under agricultural use (more regarding CO2 reduction)
  • Examples for financing Instruments in Germany:

        -list of instruments relevant in the federal state Bavaria
        -promotional programs in Germany

back to the list of measures/ SAWA profile

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Articles in this Series

Flood hazard maps

Cost range of measures concerning to flood protection

Flood Adaptive Architecture

Cost benefit analysis

Depth Loosening

Rainwater Harvesting

Groundwater Defence

Infiltration and retention structures

Green Corridors / Strips

Conversion to Extensive Grass Land

Decision Support Tools - Planning

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