Shakesby & Doerr(2006)による〔『Wildfire as a hydrological and geomorphological agent』(269p)から〕

『水文学的ならびに地形学的因子としての野火』


Abstract
 Wildfire can lead to considerable hydrological and geomorphological change, both directly by weathering bedrock surfaces and changing soil structure and properties, and indirectly through the effects of changes to the soil and vegetation on hydrological and geomorphological processes. This review summarizes current knowledge and identifies research gaps focusing particularly on the contribution of research from the Mediterranean Basin, Australia and South Africa over the last two decades or so to the state of knowledge mostly built on research carried out in the USA.
 Wildfire-induced weathering rates have been reported to be high relative to other weathering processes in fire-prone terrain, possibly as much as one or two magnitudes higher than frost action, with important implications for cosmogenic-isotope dating of the length of rock exposure. Wildfire impacts on soil properties have been a major focus of interest over the last two decades. Fire usually reduces soil aggregate stability and can induce, enhance or destroy soil water repellency depending on the temperature reached and its duration. These changes have implications for infiltration, overland flow and rainsplash detachment. A large proportion of publications concerned with fire impacts have focused on post-fire soil erosion by water, particularly at small scales. These have shown elevated, sometimes extremely large post-fire losses before geomorphological stability is re-established. Soil losses per unit area are generally negatively related to measurement scale reflecting increased opportunities for sediment storage at larger scales. Over the last 20 years, there has been much improvement in the understanding of the forms, causes and timing of debris flow and landslide activity on burnt terrain. Advances in previously largely unreported processes (e.g. bio-transfer of sediment and wind erosion) have also been made.
 Post-fire hydrological effects have generally also been studied at small rather than large scales, with soil water repellency effects on infiltration and overland flow being a particular focus. At catchment scales, post-fire accentuated peakflow has received more attention than changes in total flow, reflecting easier measurement and the greater hazard posed by the former. Post-fire changes to stream channels occur over both short and long terms with complex feedback mechanisms, though research to date has been limited.
 Research gaps identified include the need to: (1) develop a fire severity index relevant to soil changes rather than to degree of biomass destruction; (2) isolate the hydrological and geomorphological impacts of fire-induced soil water repellency changes from other important post-fire changes (e.g. litter and vegetation destruction); (3) improve knowledge of the hydrological and geomorphological impacts of wildfire in a wider range of fire-prone terrain types; (4) solve important problems in the determination and analysis of hillslope and catchment sediment yields including poor knowledge about soil losses other than at small spatial and short temporal scales, the lack of a clear measure of the degradational significance of post-fire soil losses, and confusion arising from errors in and lack of scale context for many quoted post-fire soil erosion rates; and (5) increase the research effort into past and potential future hydrological and geomorphological changes resulting from wildfire.

Keywords: wildfire; forest fire; rock weathering; soil erosion; mass movement processes; sediment yield; soil hydrophobicity; infiltration; peakflow; total flow; palaeofire; global warming』

要旨
 野火は、基盤岩表面を風化しかつ土壌の構造と性質を変えることで直接に、そして水文と地形に関する過程において土壌と植物に変化を与える影響をとおして間接に、その両方によってかなりの水文・地形変化をもたらしうる。このレビューは、米国で行われた研究の上にほとんどの知識が築かれている状況に対して、この20年間ほどに地中海地帯・オーストラリア・南アフリカからの研究が貢献している面にとくに焦点をあてて、現在の知識を要約し、研究の相違点を確認している。
 野火による風化速度は、野火の傾向がある地域での他の風化過程に比べて大きく、岩石露出時間の宇宙線源同位体年代決定法との大事な関係も含めて、凍結作用よりもおそらく1ないし2桁大きいことが報告されている。土壌特性も対する野火の影響はこの20年間の主要な関心事であった。火災はふつうは土壌集合体の安定性を弱め、土壌水の防水性をその到達温度と時間に応じて誘引または促進または破壊する。これらの変化は、浸透・陸地面流水・雨撥ね剥離に関係する。火災の影響に関する報告の大部分は、火災後の水による土壌浸食、とくに小さなスケールでのものに集中している。これらは、地形の安定性が再確立する前に、大きい、とくには非常に大規模な火災後の流失が示されている。単位面積当りの土壌流失は、より大きなスケールでは堆積物の埋蔵の機会が増えることを反映して、測定スケールと一般に反比例する。この20年間では、焼け跡地域の土石流と地すべり活動の形態・原因・タイミングの理解は非常に改善された。従来はほとんど報告されていない過程(例えば、堆積物の生物による移動や風食)についても前進がある。
 火災後の水文的影響も、大スケールよりも小スケールで、浸透と陸地面流水に対する土壌水の防水性の影響に特に注目して、一般に研究されている。流域スケールでは、火災後に強まった最大流が、測定が簡単なことと大きな災害を引き起こすことを反映して、全流の変化より注目を浴びている。河川流路に対する火災後の変化は、今日まで研究は限られるが、複雑なフィードバック・メカニズムを伴って短期と長期の両方にわたって生じている。
 確認された研究のギャップから以下のことが必要である:(1)バイオマス破壊の度合よりも土壌変化に関連した火災シビアリティ指標を開発すること;(2)他の重要な火災後の変化(例えば、リターと植生の破壊)から、火災による土壌水防水性変化の水文・地形的影響を分離すること;(3)火災を起しやすい地域タイプのもっと広い範囲で、野火の水文・地形的影響についての知識を向上させること;(4)小空間および短期間とは違った土壌流出、火災後土壌流出の劣化の意味を明確に測定することの不足、およびたくさんの引用された火災後土壌浸食速度の誤差および関連したスケールが欠けることによる混乱を含めて、山地斜面と流域の堆積物生産量の決定と分析における重要な問題を解決すること;そして(5)野火から生じる水文・地形変化について過去および可能性のある将来に研究努力を増やすこと。』

1. Introduction
2. Wildfire: global significance, causes, frequency and severity
 2.1. Global significance and causes
 2.2. Frequency and severity
3. Direct effects of fire on rock, vegetation and soil
 3.1. Rock weathering
 3.2. Removal of vegetation and litter cover and changes to microbial and faunal activity
 3.3. Changes to the structure and hydrological properties of soil
4. Indirect effects of fire
 4.1. Post-fire hydrological effects
  4.1.1. Infiltration
  4.1.2. Overland flow
  4.1.3. Catchment runoff behaviour
 4.2. Post-fire geomorphological effects
  4.2.1. Soil erosion by water
   4.2.1.1. Water erosion processes
   4.2.1.2. Sediment redistribution by overland flow and runoff and resulting landforms
   4.2.1.3. Quantities and rates of water erosion
  4.2.2. Mass movement processes
   4.2.2.1. Dry ravel
   4.2.2.2. Debris flows
   4.2.2.3. Shallow landslides
   4.2.2.4. Other mass movement processes
  4.2.3. Wind erosion
  4.2.4. Bioturbation and bio-transfer
5. Reconstruction of wildfire patterns and effects in history and prehistory
6. Conclusion: retrospect and prospect
Acknowledgement
References

Table 4 A selection of published post-fire measured rates of sediment yields for catchments. Sediment yields relate to the first year after fire unless otherwise indicated
Location Vegetation Rainfalla
(mm)
Fire Severityb Slopec
(°)
Area
(ha)
Post-fire sediment yield
(t ha-1)
Inburnt sediment yield
(t ha-1)
Notes Author(s)
Burnt Mesa, New Mexico, USA Mixed conifer 825 High 5 0.41 47d n.d. - White and Wells (1982)
Moderate 7 0.08 10 -
Light 5 0.14 32 -
Arizona, USA Ponderosa pine 737 High n.d. 8.1 4.8e 0.003 Bedload and suspended sediment Campbell et al. (1977)
Moderate n.d. 4.0 0.005e
Washington, USA Mixed conifer 580 High 16 514 0.12 0.008-0.100 - Helvey (1980)
15 563 0.26
15 473 0.40
Western Cape Province, South Africa Pine plantation 1296 High 7 200 7.83 n.d. Bedload and suspended sediment Scott et al. (1998)
a Average annual rainfall in upright font, rainfall for measured period in italics.
b An arbitrary classification of fire severity based on authors' descriptions.
c Channel or catchment slope.
d A bulk density of 1.0 g cm-3 is assumed for this paper.
e Measured over 6 months.


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