Canadian Forest Service Publications

Water balance of a burned and unburned forested boreal peatland. 2013. Thompson, D.K.; Benscoter, B.W.; Waddington, J.M. Hydrological Processes 28(24):5954-5964.

Year: 2013

Issued by: Northern Forestry Centre

Catalog ID: 35506

Language: English

Availability: PDF (request by e-mail)

Available from the Journal's Web site.
DOI: 10.1002/hyp.10074

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We examined the water balance of a forested ombrotrophic peatland and adjacent burned peatland in the boreal plain of western Canada over a 3-year period. Complete combustion of foliage and fine branches dramatically increased shortwave radiation inputs to the peat surface while halting all tree transpiration at the burned site. End-of-winter snowpack was 7–25% higher at the burned site likely due to decreased ablation from the tree canopy at the unburned site. Shrub regrowth at the burned site was rapid post-fire, and shading by the shrub canopy in the burned site approached that of the unburned site within 3 years after fire. Site-averaged surface resistance to evaporation was not different between sites, though surface resistance in hollows was lower in the burned site. Water loss at both burned and unburned sites is largely driven by surface evaporative losses. Evaporation at the burned site marginally exceeded the sum of pre-fire transpiration and interception at the unburned site, suggesting that evapotranspiration during the growing season was 20–40mm greater at the burned peatland. Although the net change in water storage during the growing season was largely unchanged by fire, the lack of low-density surface peat in the burned site appears to have decreased specific yield, leading to greater water table decline at the burned site despite similar net change in storage.

Plain Language Summary

We compared the water balance in burned and unburned forested peatlands near Slave Lake, Alberta, and then created a model of the water table below the surface of the bogs. In most non-wetland areas, the death of trees after severe wildfire increases runoff or stream flow. After a wildfire in peatlands, however, any excess water stays in place and causes changes in the water table. In the burned peatland, we found that the water table declined to a lower level below the surface, especially later in the summer. This change in the water table was probably related to an increase in surface evaporation, because more wind and sunlight were reaching the surface of the peatland after the trees were burned away. In non-wetlands, water loss from soils is dominated by trees uptaking water through their roots, but in forested peatlands, this activity is reduced compared to evaporation directly from the soil surface. As a result, after wildfire the increase in surface evaporation trumped the loss of tree water uptake, and we observed a decrease in the water table. A lower water table could slow down moss regrowth or enhance decomposition of peat above the water table.