How Do Forest Fires Affect Water Resources?

Effects of Forest Fires

Forests cover approximately 31% (4 billion hectares) of the global land surface and provide a wide range of economic and ecological products and services, including natural storage, filtration and drinking water supply. In many parts of the world, forests provide high quality water for domestic, agricultural, industrial and ecological needs.

Most of the healthy forests are located in areas with abundant annual rainfall and provide relatively clean and large volumes of water flow.

Quality water from forested watersheds also provides significant economic benefits by reducing the costs of comprehensive drinking water treatment facilities and related infrastructure. The value of the natural storage and filtration of water supplied by forests on a global scale is estimated at approximately US$4.1 trillion per year. (one)

Many megacities in the world with rapidly growing populations depend heavily on the water provided by forests. For example, nearly two-thirds of municipalities in the United States and about one-third of the world's largest cities, including Tokyo, Melbourne, Los Angeles, and Rio de Janeiro, get the majority of their drinking water from watersheds with dense forests. (2)

The post-drought water supply shortages in many countries, including the USA, seem to have made it easier to understand the critical importance of forests in regulating water quantity and quality. (3)

Hydrological Cycle Affects

Forest fires can affect hydrological processes (influencing inflow, infiltration and evapotranspiration) that affect the timing of changes in stream flows throughout the year and the magnitude of the flow. (4)

The destruction of forest vegetation by fire prevents the reduction of the rate and intensity of precipitation to the soil and also reduces evaporation in forests through evapotranspiration. Thus, rain and snow reach the soil faster, and runoff and stream flows may increase, which can create floods (5).

In addition, solar energy reaching the snow layer more in temperate climates causes snowmelt to start earlier. This, in turn, may have adverse effects on the reservoir storage operation and the aquatic ecosystem balance. For example, in the Northern Rocky Mountains in the USA, the amount of snow that turns into water in unburned woodlands is twice as high as in burnt woodland. Complete melting of the snow cover occurred approximately 9-15 days ago due to the double increase in snow melting rates in regions that have experienced forest fires (6).

The Balance of the Ecosystem Is Completely Disrupted

Some experts argue that after a fire, the hydrological process, though complex to explain, may be affected by high temperatures that damage the soil. In some cases, after forest fires, a water-repellent (hydrophobic) layer forms on or near the soil surface, while in other cases, a natural water-repellent layer forms in the background after the protective vegetation is burned and removed. (7)

This layer can seriously reduce or prevent water from seeping into the ground during rain or snowmelt, allowing the falling precipitation to flow quickly. (8)

The degree of water impermeability of the soil caused by the fire and its lifespan after forest fire are most evident in coarse-grained dry soils and after large fires (9).

Very large fires can produce complex responses that vary depending on soil properties (eg structure, water content, amount of organic matter) (10).

These differences in soil properties may also affect the infiltration feature of the soil, causing more and faster flow of water (11).

The amount and duration of runoff from the land after the fire may also be affected by the post-fire ash layer thickness, surface sealing due to fine sediment and coniferous deposition. (12)

In general, forest fires can affect hydrological processes, causing frequent rainfall that will increase flash floods. In addition, these floods can be difficult to predict (13).

However, long-term records of their flows in a forest fire area are rarely available, making it difficult to assess the effects of the fire. However, in the few cases where data are available, 2- to 5-fold increases in peak flow rates have been reported in the region over 6-7 years. (13).

Various studies have revealed that small forest fires do not have measurable effects on the peak flow rates that will occur with precipitation after the fire. However, in some studies, it has been observed that the hydrological effect of medium and large-scale and long-term fires is much more. It was concluded that short-term heavy rains falling in this region after the fire may produce peak flow rates 5 to 8 times greater than previously observed in normal forest lands (14,5).

Evaluation

Forest fires affect the aquatic ecosystem, water supply and treatment systems, public health provinces. It can significantly threaten water resources with e-related consequences. However, these threats vary considerably according to geographical regions under certain effects. The effects produced by one fire may have different characteristics from the effects produced by another fire. For these reasons, it is not easy to predict the effects of forest fires on various water resources and hydrological cycle locally and regionally.

However, it is known that climate change and forest fires affect water resources and local hydrological conditions to various extents. Considering the increasing forest fires both in the world and in our country, institutions related to water or soil resources will need to be able to predict these effects more and more.

References

1.Costanza, R.; d’Arge, R.; de Groot, R.; Farberk, S.; Grasso, M.; Hannon, B.; Limburg, K.; Naeem, S.; O’Neill, R. V.; Paruelo, J.; Raskin, R. G.; Suttonkk, P.; van den Belt, M. The value of the world’s ecosystem services and natural capital Nature 1997, 387 (6630) 253– 260

2. Committee on Hydrologic Impacts of Forest Management. Hydrologic Effects of a Changing Forest Landscape; The National Academies Press: Washington, D.C., 2008; p 157.

3. Vose, J. M.; Sun, G.; Ford, C. R.; Bredemeier, M.; Otsuki, K.; Wei, X.; Zhang, Z.; Zhang, L. Forest ecohydrological research in the 21st century: What are the critical needs? Ecohydrology 2011, 4 (2) 146– 158

4.Shakesby, R. A.; Doerr, S. H. Wildfire as a hydrological and geomorphological agent Earth-Sci. Rev. 2006, 74 (3–4) 269– 307

5.Neary, D. G.; Gottfried, G. J.; Ffolliott, P. F. In Post-Wildfire Watershed Flood Responses, 2nd International Wildland Fire Ecology and Fire Management Congress and 5th Symposium on Fire Forest Meteorology, Orlando, FL, November 16–20, 2003; American Meterological Society: Boston, MA, 2003; p 7.

6.Burles, K.; Boon, S. Snowmelt energy balance in a burned forest plot, Crowsnest Pass, Alberta, Canada Hydrol. Processes 2011, 25 (19) 3012– 3029

7.Doerr, S. H.; Woods, S. W.; Martin, D. A.; Casimiro, M. “Natural background” soil water repellency in conifer forests of the north-western USA: Its prediction and relationship to wildfire occurrence J. Hydrol. 2009, 371 (1–4) 12– 21

8.Huffman, E. L.; MacDonald, L. H.; Stednick, J. D. Strength and persistence of fire-induced soil hydrophobicity under ponderosa and lodgepole pine, Colorado front range Hydrol. Processes 2001, 15 (15) 2877– 2892

9.Shakesby, R. A.; Doerr, S. H.; Walsh, R. P. D. Erosional impacts of soil hydrophobicity: Current problems and future research directions J. Hydrol. 2001, 231–232 (S1) 178– 191

10.Mataix-Solera, J.; Cerda, A.; Arcenegui, V.; Jordan, A.; Zavala, L. M. Fire effects on soil aggregation: A review Earth-Sci. Rev. 2011, 109 (1–2) 44– 60

11.Ebel, B. A.; Moody, J. A. Rethinking infiltration in wildfire-affected soils Hydrol. Processes 2013, 27 (10) 1510– 1514

12.Cerdà, A.; Doerr, S. H. The effect of ash and needle cover on surface runoff and erosion in the immediate post-fire period Catena 2008, 74 (3) 256– 263

13.Moody, J. A.; Martin, D. A. Initial hydrologic and geomorphic response following a wildfire in the Colorado Front Range Earth Surf. Processes Landforms 2001, 26 (10) 1049– 1070

14.Moody, J. A.; Martin, D. A. Post-fire rainfall intensity-peak discharge relations for three mountainous watersheds in the western USA Hydrol. Processes 2001, 15 (15) 2981– 2993