How do forest fires affect rain run-off?

Question

The rains are doing all sorts of things to the landscape in Waterton-Glacier National Park this summer. All of the trails in the park appear to serve as water ways for rain runoff, and the rain appears to be doing more running overground than soaking into anything. The forest floor is covered with charcoal, which is getting washed into the creeks and lakes, making the waterfalls change colour after a rain, and washing out pathways.

Besides producing dark waterfalls that smell like a bucket of water poured into a campfire, do forest fires do anything else to affect water run off? Is there something about having charcoal on the ground that impedes the soil's ability to absorb rain water?

^{Forest fire runoff after a heavy hail and rain storm, the water was clear just before the rains: Before the fires this waterfall used to turn pink after heavy rains.}

Answer 1

Basically it increases the runoff in a number of ways, less soil cover in the form of vegetation, ash blocking the water from being absorbed, and in some cases a hydrophobic water coating can be left behind.

Loss of vegetation exposes soil to erosion; water runoff may increase and cause flooding; sediments may move downstream and damage houses or fill reservoirs putting endangered species and community water supplies at risk.

After the Fire, U.S. FOREST SERVICE

The fire burns the litter layer and part of the humus layer. Therefore stones on the surface become destabilized. In mountain areas it is often the case that a stone starts rolling during a fire. The worst consequences of a fire however occur after the fire: the lack of cover from ground and tree vegetation allows rain drops to hit the soil directly. Due to this soil structures are destroyed and erosion and washing out take place (Fig. 1). The bare soil dries out quicker and its surface becomes impermeable. The rain tends to flow off the surface eroding rills and gullies into the soil. (Marxer 2003).

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The impermiability of the soil depends on the temperature of the fire (higher temperatures mean greater destruction of the soil structure) and from the production of ash (greater quantities of fire material and more intensive fire produce more ash (Fig. 2)), which then physically blocks the soil pores (Letey 2001).

Erosion and surface drainage after forest fires

In severe, slow-moving fires, the combustion of vegetative materials creates a gas that penetrates the soil profile. As the soil cools, this gas condenses and forms a waxy coating. This causes the soil to repel water – a phenomena called hydrophobicity. This hydrophobic condition increases the rate of water runoff. Percolation of water into the soil profile is reduced, making it difficult for seeds to germinate and for the roots of surviving plants to obtain moisture.

Hydrophobic soils do not form in every instance. Factors contributing to their formation are: a thick layer of litter before the fire; a severe slow-moving surface and crown fire;

Soil Erosion Control after Wildfire

For more information

• Increased Runoff and Floods Following Wildfire
• Assessing Impacts of Fire and Post-fire Mitigation on Runoff and Erosion from Rangelands
• What to expect after a wildfire.

Answer 2

Yes

Charlie's answer is excellent, but to append some technical information from my background as a civil engineer may be helpful.

When evaluating stormwater impacts for a site, there's generally 3 key criteria: runoff rate, water quality, and groundwater recharge.

Runoff rate is what it sounds like and is a function of the time of concentration, which refers to how long it takes a drop of water to travel from the most hydraulically distant point in a watershed (meaning what takes the most time) to some common low point (i.e. an inlet, a river, or some other receiving water body). The time of concentration is heavily impacted by the surface condition that the water passes over. Moving through thick grasses means that a drop of water has to keep changing it's path and thus has a longer time of concentration. Conversely, were water passing over an area of bare ground without any grass or other vegetation (such as after a fire), it will move much more quickly. The time difference can be drastic, some sites I've worked to develop have had the time of concentration reduced from 45 minutes to just 6 minutes; this means that water is hitting the receiving water body much faster than normal and thus causing much higher peak flows unless we design control basins to delay that flow.

The next criteria, water quality, is obviously impacted as you've demonstrated with your picture of a waterfall running black. Typically, water quality is improved by letting it infiltrate into the ground, slowing it down to permit sediment and fines to settle out, or some other method, but none of those techniques are available when water's moving fast with a high sediment load, hence, waterfalls that smell like extinguished campfires.

The last criteria, groundwater recharge, also suffers for most of the same reasons that the other two do: The water is just moving too fast to practically infiltrate into the ground. It's possible that the charcoal fines are impact the soil media to make it less permeable, especially if aspects of the fire 'compact' the fines into the media, such as vaporizing into the soil media and condensing therein as described in Charlie's answer.

In addition to all these issues, the reduced time of concentration means that the moving water has a lot of energy, which will increase the likelihood of ruts and gullies forming.