Burned forests impact water supplies

Wildland fires enhanced annual river flow in western regions with a warm temperate or humid continental climate.

“Wildland fires enhanced annual river flow in the western regions with a warm temperate or humid continental climate.”

Prescribed burns in the subtropical Southeast did not significantly alter river flow.

“In contrast, prescribed burns in the subtropical Southeast did not significantly alter river flow.”

• Also stated in: § Introduction · plain

  “In contrast, prescribed burns in the subtropical Southeast did not significantly alter river flow.”

Annual river flow changed in 32 of 168 studied locations where more than 19% of the basin area was burned.

“We show that annual river flow changed in 32 locations, where more than 19% of the basin area was burned.”

• Also stated in: § Introduction · plain

  “We show that of the 168 locations studied, wildland fires affected annual river flow in 32 locations, where more than 19% of the basin area was burned.”

Wildland fires increased annual river flow most in the semi-arid Lower Colorado region, despite frequent droughts there.

“Wildland fires increased annual river flow most in the semi-arid Lower Colorado region, in spite of frequent droughts in this region.”

The relative influence of the change in monthly precipitation variance was greater for the West (9.5%) than for the East (4.9%).

“The relative influence of the change in monthly precipitation variance was greater for the West (9.5%) compared to the East (4.9%) (Supplementary Fig. 1b cf. Supplementary Fig. 1c).”

In the Eastern CONUS, the proportion of the watershed with unvegetated land cover (barren land and urban area) had some influence on river flow (4.1% and 2.3%, respectively), secondary to climate factors (51% and 4.9% for annual precipitation and monthly precipitation variance, respectively).

“By contrast, in the Eastern CONUS, the proportion of the watershed with unvegetated land cover (barren land and urban area) had some influence on river flow (4.1% and 2.3%, respectively), secondary to climate factors (51% and 4.9% for annual precipitation and monthly precipitation variance, respectively) (Supplementary Fig. 1b cf. Supplementary Fig. 1c).”

Wildland fire that burned 19% or more of the watershed area generally increased river flow.

“Wildland fire that burned 19% or more of the watershed area generally increased river flow.”

A change (breakpoint) in dQ/dP was detected in the same 13 watersheds plus 23 additional watersheds, all in the Western CONUS.

“however a change (breakpoint) in (dQ/dP) was detected in the same watersheds plus 23 additional watersheds (all in the Western CONUS)”

Prescribed burning had only a limited effect on river flow in the subtropical Southeast.

“For example, prescribed (controlled) burning, conducted to reduce accumulated fuels such as dead vegetation and fallen branches, had only a limited effect in the subtropical Southeast.”

For the CONUS as a whole, river flow declined by a median of −5.9 mm (−5.7%) in fire-affected locations.

“For the CONUS as a whole, river flow declined (median −5.9 mm or −5.7%) in fire-affected locations”

Climate elasticity of dQ could be determined for six more burned watersheds.

“Finally, climate elasticity of dQ could be determined for six more watersheds (Supplementary Fig. 6c c.f. Supplementary Fig. 6d).”

In the Lower Colorado region, wildfires typically burned 15.4% of the watershed area.

“where wildfires typically burned 15.4% of the watershed area”

The BAR for rivers in the Western US was higher (median +9.6%, 116 rivers) than for smaller river systems east of the Mississippi River.

“This is less than the BAR for rivers in the Western US (median of +9.6%, 116 rivers).”

In the Lower Colorado region, river flow increased (+9.9 mm or +25.6%) notwithstanding a declining precipitation trend (−30.6 mm or −6.0%).

“This was the common response in the Lower Colorado region (river flow +9.9 mm or +25.6% vs. precipitation −30.6 mm or −6.0%)”

The smaller river systems east of the Mississippi River (52 rivers) had the lowest BAR, typically below 5%.

“The smaller river systems east of the Mississippi River (52 rivers) had the lowest BAR, typically below 5% (Fig. 2).”

The greatest decrease in river flow was observed in burned watersheds in the Great Basin (median −45.4 mm or −37.1%), Rio Grande (−15.6 mm or −29.8%), Texas-Gulf (−10.7 mm or −25.4%), and California (−38.4 mm or −18.4%) regions.

“The greatest decrease in flow was observed in burned watersheds in the Great Basin (median of −45.4 mm or −37.1%), Rio Grande (−15.6 mm or −29.8%), Texas-Gulf (−10.7 mm or −25.4%) and California (−38.4 mm or −18.4%) regions.”

At the CONUS scale, river flow was mostly influenced by precipitation change (47.4%), change in monthly precipitation variance (8.3%), pre-fire monthly precipitation variance (4.5%), and post-fire monthly precipitation variance (3.3%).

“At the CONUS scale, river flow was mostly influenced by precipitation change (47.4%), change in monthly precipitation variance (8.3%), pre-fire monthly precipitation variance (4.5%) and post-fire monthly precipitation variance (3.3%) (Supplementary Fig. 1a).”

For the ensemble of CONUS watersheds affected by a fire of any given size or extent (BAR ≥ 1%), precipitation characteristics had the greatest influence on 5-year mean annual river flow.

“For the ensemble of watersheds in the CONUS affected by a fire of any given size or extent (BAR ≥ 1%), precipitation characteristics had the greatest influence on 5-year mean annual river flow (Supplementary Fig. 1a).”

Large watersheds in the drought-prone Western US experienced the largest uncontrolled fires.

“Large watersheds in the drought-prone Western US experienced the largest uncontrolled fires.”

Observed changes in 5-year post-fire river flow coincided with local changes in precipitation and burn patterns.

“Our analysis of flow records from 168 CONUS rivers shows that observed changes in 5-year post-fire river flow coincided with local changes in precipitation and burn patterns (Supplementary Table 1).”

The impact of wildland fire on river flow was related to the proportion of the watershed affected by moderate-to-high burn severity (relative influences 2.2% and 2.8%, respectively) for a minimum of 3.8% and 6.5%, respectively.

“This impact was related to the proportion of the watershed affected by moderate-to-high burn severity (relative influences 2.2% and 2.8%, respectively) for a minimum of 3.8% and 6.5%, respectively (Supplementary Fig. 2).”

The greater portion (65.5%) of the upstream burned area for rivers with a notable increase in river flow (>10%) in the Lower Colorado, Pacific Northwest, and Texas-Gulf regions was characterized by low burn severity.

“Interestingly, the greater portion (65.5%) of the upstream burned area for rivers with a notable increase in river flow (>10%) in the Lower Colorado, Pacific Northwest and Texas-Gulf regions, was characterized by low burn severity.”

Not all wildland fires affected annual river flow.

“Not all wildland fires affected annual river flow.”

CEM2, with parameters for P and PET, yielded the best fit for ten watersheds at lower elevation.

“CEM2 with parameters for P and PET yielded the best fit for ten watersheds at lower elevation.”

The greatest decrease in precipitation was recorded for the Rio Grande (−100 mm or −13.1%), California (−35.4 mm or −4.6%), and Mid-Atlantic (−59.5 mm or −4.3%) regions.

“The greatest decrease in precipitation was also recorded for the Rio Grande (−100 mm or −13.1%), California (−35.4 mm or −4.6%), and Mid Atlantic (−59.5 mm or −4.3%) regions.”

Wildland fire increased annual river flow in the humid subtropical Texas-Gulf region and in Mississippi.

“Similarly, wildland fire increased the annual river flow in the humid subtropical Texas-Gulf region, and in Mississippi, where rapid vegetation growth produces massive amounts of fuels susceptible to fire.”

The largest fire-induced increase in river flow occurred in the Lower Colorado, Pacific Northwest, and Texas-Gulf regions.

“We detected the largest fire-induced increase in river flow in the Lower Colorado, Pacific Northwest, and Texas-Gulf regions.”

In northern California (Csb climate type), river flow declined post-fire mainly as a result of short-term drought.

“For example, in northern California (Csb climate type), river flow declined post-fire mainly as a result of short-term drought (Fig. 1a cf. Fig. 1b).”

In southern California and the Pacific Northwest, increases in river flow corresponded with high severity of wildland fires.

“In southern California and the Pacific Northwest, in particular, increases in river flow corresponded with a high severity of wildland fires.”

The fire-induced increase in annual river flow was especially pronounced throughout the Pacific Northwest and in the semi-arid Lower Colorado region where wildfires burned large portions of headwater catchments.

“This was especially the case throughout the Pacific Northwest, where fires occurred in areas with a Mediterranean or humid continental climate, and in the semi-arid Lower Colorado region where wildfires burned large portions of headwater catchments (Fig. 2).”

dQ in the year following wildland fire was significant in 13 of 43 burned watersheds, all in the Western CONUS.

“dQ in the year following wildland fire was significant in 13 of 43 burned watersheds (all in the Western CONUS) (Supplementary Fig. 6a)”

• AssumptionThe authors state this as an assumption: River flow disturbance was associated with the fire, so that a detected change point within one year following the fire indicates fire-associated disturbance.

River flow in watersheds burned for at least 1% of their drainage area generally declined in nine water resource regions: Mid-Atlantic, Tennessee, Great Basin, South Atlantic-Gulf, California, Great Lakes, Rio Grande, Texas-Gulf, and Missouri.

“River flow in watersheds burned for at least 1% of their drainage area (burned area ratio, or BAR ≥ 1%), generally declined in nine water resource regions.”

The most extreme responses occurred in mid-elevation parts of the Pacific Northwest (Dsb) and Lower Colorado (BSk) regions, where wildland fire increased flow >100%.

“We detected the most extreme responses in mid-elevation parts of the Pacific Northwest (Dsb) and Lower Colorado (BSk) regions, where wildland fire increased flow >100% (Fig. 1c).”

The one-parameter (dP) CEM1 was the best model for four watersheds with the steepest terrain, although not necessarily at high elevation.

“The one-parameter (dP) CEM1 was the best model for four watersheds with the steepest terrain, although not necessarily at high elevation.”

One-half of the fires affected less than 6% of the area of a gaged watershed.

“One-half of the fires affected less than 6% of the area of a gaged watershed.”

CEM3, with parameters for dP and change in monthly precipitation variance, was the best model for most watersheds (n = 11).

“CEM3 with parameters for dP and (change in monthly precipitation variance) was the best model for most watersheds (n = 11).”

With some exceptions, wildland fire and climate contributions to flow increase in the lower Missouri region were comparable (<10%; Dfa).

“With some exceptions, wildland fire and climate contributions to flow increase in the lower Missouri region were comparable (<10%; Dfa).”

River flow predominantly increased in six water resource regions: Lower Mississippi, Upper Colorado, Lower Colorado, Ohio, Pacific Northwest, and Arkansas-White-Red River.

“In contrast, river flow predominantly increased in six other regions, viz., Lower Mississippi (Cfa), Upper Colorado (Dfb), Lower Colorado (BSk), Ohio (Cfa), Pacific Northwest (Csb, Dsb, Dfb), and Arkansas-White-Red River (BSk, Cfa, Dfa).”

For the CONUS as a whole, precipitation declined by a median of −23.1 mm (−2.3%) in fire-affected locations.

“in accordance with a negative precipitation trend (median −23.1 mm or −2.3%) (Supplementary Table 1).”

The predominantly humid watersheds for which CEM3 was best (lower quartile PQ25 > 656 mm) were located in most regions above 300 m.

“These predominantly humid watersheds (a lower quartile PQ25 > 656 mm) were located in most regions above 300 m.”

Areas affected by high severity burning had a consistently greater influence on river flow change (up to 4.0% for BAR ≥ 25%) than areas affected by moderate burning.

“Areas affected by high severity burning had a consistently greater influence on river flow change (up to 4.0% for BAR ≥ 25%) than areas affected by moderate burning.”

Despite uncertainty in burn severity classification thresholds, especially for moderate and high burn severity classes, the influence on river flow was consistently greater when these classes were accounted for individually.

“Despite the uncertainty in burn severity classification thresholds, especially for moderate and high burn severity classes21, we report a consistently greater influence on river flow when these classes were accounted for individually.”

• cites [21] Kolden, C. A., Smith, A. M. & Abatzoglou, J. T. Limitations and utilisation of monitoring trends in burn severity products for assessing wildfire severity in the USA. Int. J. Wildland Fire 24, 1023–1028 (2015). sourceNot checked yet

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Fire impacts on river flow were most evident in four Arizona rivers (Lower Colorado) with a high BAR (>39%), increasing flow by more than +128%.

“Fire impacts on river flow were most evident in four Arizona rivers (Lower Colorado) with a high BAR (>39%), increasing flow by more than +128% (Fig. 1c).”

Areas with low severity burning impacts were too small and their influence on river flow fell below that of a random variable introduced into the analysis for comparison, even for BAR ≥ 25%.

“Areas with low severity burning impacts were too small and their influence on river flow fell below that of a random variable introduced into the analysis for comparison, even for BAR ≥ 25%.”

Large wildland fires enhanced annual river flow for at least 5 years, even in areas affected by recurring drought.

“Large wildland fires enhanced annual river flow for at least 5 years, even in areas affected by recurring drought (Fig. 1).”

Absolute values of precipitation (including snowfall) had a greater influence (2.4%) on river flow in Western watersheds.

“Furthermore, absolute values of precipitation (which includes snowfall) (2.4%) also had a greater influence.”

In relatively large, low-altitude (<500 m) watersheds such as those in the Texas-Gulf region (BAR = 2.3%) and the South Atlantic-Gulf Region (3.1%), smaller prescribed burns are more common than elsewhere in the US.

“This was notably the case in relatively large, low-altitude (<500 m) watersheds like those in the Texas-Gulf region (BAR = 2.3%), and in the South Atlantic-Gulf Region (3.1%), where smaller prescribed burns are more common than elsewhere in the US.”

In many other rivers with upstream areas affected by fire, changes in annual river flow associated with wildland fires were less than +15% (median for CONUS).

“In many other rivers with upstream areas affected by a fire, changes in annual river flow associated with wildland fires were less than +15% (median for CONUS).”

River flow generally increases in the months following wildland fires.

“River flow generally increases in the months following wildland fires8”

• cites [8] Bart, R. R. A regional estimate of post-fire streamflow change in California. Water Resour. Res. 52, 1465–1478 (2016). sourceNot checked yet

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Patterns and trends of post-fire river flow are expected to vary considerably between CONUS regions depending on fire characteristics, hydroclimate, forest cover, and topography.

“Nonetheless, patterns and trends of post-fire river flow are expected to vary considerably between CONUS regions, depending on fire characteristics, hydroclimate, forest cover, and topography11,12.”

• cites [11] Moody, J. A. et al. Current research issues related to post-wildfire runoff and erosion processes. Earth Sci. Rev. 122, 10–37 (2013). sourceNot checked yet

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• cites [12] Hallema, D. W. et al. Assessment of wildland fire impacts on watershed annual water yield: analytical framework and case studies in the United States. Ecohydrology 10, e1794 (2017). sourceNot checked yet

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Increased river flow can be sustained over multiple years after fires in New Mexico and Colorado.

“recent studies in New Mexico and Colorado also show that increased river flow can be sustained over multiple years3,9.”

• cites [3] Wine, M. L. & Cadol, D. Hydrologic effects of large southwestern USA wildfires significantly increase regional water supply: fact or fiction? Environ. Res. Lett. 11, 085006 (2016). sourceNot checked yet

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• cites [9] Ebel, B. A., Rengers, F. K. & Tucker, G. E. Observed and simulated hydrologic response for a first-order catchment during extreme rainfall 3 years after wildfire disturbance. Water Resour. Res. 52, 9367–9389 (2016). sourceNot checked yet

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Headwater forests supply more than 50% of water consumed in the contiguous US via streams and rivers.

“Headwater forests supply more than 50% of water consumed in the contiguous US (CONUS)5 via streams and rivers”

• cites [5] Brown, T. C., Hobbins, M. T. & Ramirez, J. Spatial distribution of water supply in the coterminous United States. J. Am. Water Resour. Assoc. 44, 1474–1487 (2008). sourceNot checked yet

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The role of burn severity is more complex because burn patterns tend to alter the path along which precipitation is transferred to the river.

“But, the role of burn severity is more complex because burn patterns tend to alter the path along which precipitation is transferred to the river25.”

• cites [25] Moody, J. A. et al. Relations between soil hydraulic properties and burn severity. Int. J. Wildland Fire 25, 279–293 (2016). sourceNot checked yet

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The greater overall importance of precipitation variables in Western watersheds with higher elevation and steeper topography is explained by a more extreme climate with higher interannual and seasonal variability.

“The greater overall importance of precipitation variables in these Western watersheds with higher elevation and steeper topography is explained by a more extreme climate characterized by higher interannual and seasonal variability.”

Burn impacts of moderate-to-high severity affected river flow in watersheds with fires exceeding BARt, mainly because vegetation mortality increases runoff.

“Burn impacts of moderate-to-high severity affected the river flow in watersheds with fires exceeding BARt, mainly because vegetation mortality increases runoff.”

• AssumptionRests on an assumption: Vegetation mortality increases runoff.

The outcomes are consistent with existing theories suggesting that multi-year (<10 years) increases in post-fire water yield can increase municipal water supplies during times of water scarcity.

“The outcomes are consistent with existing theories, suggesting that multi-year (i.e., <10 years) increases in post-fire water yield can increase municipal water supplies during times of water scarcity4.”

• cites [4] Martin, D. A. At the nexus of fire, water and society. Philos. Trans. R. Soc. B 371, 20150172 (2016). sourceNot checked yet

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Our results corroborate recent evidence of significant increases in river flow following wildland fires in arid watersheds such as New Mexico and Colorado.

“Our results corroborate with recent evidence of significant increases in river flow following wildland fires in arid watersheds, such as New Mexico and Colorado9,16,22.”

• cites [9] Ebel, B. A., Rengers, F. K. & Tucker, G. E. Observed and simulated hydrologic response for a first-order catchment during extreme rainfall 3 years after wildfire disturbance. Water Resour. Res. 52, 9367–9389 (2016). sourceNot checked yet

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• cites [16] Robinne, F. N. et al. A spatial evaluation of global wildfire-water risks to human and natural systems. Sci. Total Environ. 610, 1193–1206 (2018). sourceNot checked yet

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• cites [22] Buma, B. & Livneh, B. Key landscape and biotic indicators of watersheds sensitivity to forest disturbance identified using remote sensing and historical hydrography data. Environ. Res. Lett. 12, 074028 (2017). sourceNot checked yet

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Given the small size of the vast majority of wildland fires compared to the drainage area of affected watersheds, the influence of fire on river flow in the CONUS (<1%) was typically overshadowed by climate, topography, and land cover.

“Given the small size of the vast majority of wildland fires compared to the drainage area of affected watershed, the influence of fire on river flow in the CONUS (<1%) was typically overshadowed by climate, topography and land cover.”

Findings invariably point to the relative size of a fire as a factor limiting impacts on water supply.

“Findings invariably point to the relative size of a fire as a factor limiting impacts on water supply.”

The critical relative burned area threshold found (BARt = 19%) is very close to the 20% threshold traditionally reported in forest harvest studies.

“This explains why the critical relative burned area threshold we found (BARt = 19%), is very close to the 20% threshold traditionally reported in forest harvest studies26.”

• cites [26] Stednick, J. Monitoring the effects of timber harvest on annual water yield. J. Hydrol. 176, 79–95 (1996). sourceNot checked yet

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Fire effects on river flow were not always immediately evident in recorded flow data because severe weather events and short-term climate variations offset or masked the fire impacts.

“Fire effects on river flow were not always immediately evident in the recorded flow data, due to severe weather events (e.g., post-burn drought or flooding) and variations in the short term (5-year) climate that offset or masked the fire impacts.”

The post-fire hydrological impacts often depend on chance post-fire weather events.

“However, the post-fire hydrological impacts often depend on chance post-fire weather events.”

The impacts of wildland fire on water quality and quantity at the regional scale are poorly understood.

“The impacts of wildland fire on water quality and quantity at the regional scale are poorly understood18”

• cites [18] McLaughlin, D. L., Kaplan, D. A. & Cohen, M. J. Managing forests for increased regional water yield in the Southeastern US Coastal Plain. J. Am. Water Resour. Assoc. 49, 953–965 (2013). sourceNot checked yet

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The plenary set of burned watersheds was sufficiently large to allow identification of environmental variables impacting dQ in the Eastern CONUS (n = 52) and Western CONUS (n = 110) individually.

“The plenary set was sufficiently large to allow identification of environmental variables impacting dQ in the Eastern CONUS (n = 52) and Western CONUS (n = 110) individually.”

The total impact of wildland fire on river flow was determined with greater accuracy by filtering out the complex climate variability effects established by the gradient boosting machine.

“Disturbance in Q or Q/P aside, the total impact of wildland fire on Q was determined with greater accuracy by filtering out more the complex climate variability effects established by the GBM (Fig. 4).”