Effects of Forest Biomass Use on Watershed

Processes in the Western United States

William J. Elliot

As society looks to our nation’s forests as sources of energy, there is a risk of increased runoff and erosion. This report gives an overview of watershed processes,

discusses the impacts of biomass removal on those processes, provides some guidelines to minimize adverse impacts, and describes an approach for estimating

the effects of biomass removal on soil erosion.

Keywords: energy, wood fuel, timber harvest

With increasing energy costs, society is looking at alternative energy sources. One potential energy source that iscurrently underused in the United States is forest biomass (Rummer et al. 2003). However, if forests are subject to a

significant increase in biomass removal for energy production, then

it is possible that hydrologic resources may be adversely affected.

This report describes forest hydrologic processes, looks at the

impacts of biomass use on those processes, and then describes an

approach for watershed analysis to quantify those impacts. Although

many of the principles discussed in this article apply to all forests,

this article focuses on forest watershed processes common in the

western United States.

Hydrologic Processes

Hydrologic processes in forested ecosystems of the western

United States are complex (Figure 1) (Luce 1995). Climate, topography, geology, soils, and vegetation all interact to affect runoff and

erosion rates. Erosion influences onsite forest soil productivity

(Page-Dumroese et al. 2000) and offsite water quality (Elliot and

Audin 2006). These processes are driven by natural disturbances

such as wildfire and severe weather that can lead to flooding, landslides, debris flows, etc. Human disturbances that accelerate erosion

include intentional and accidental fires, timber harvest, grazing,

road and building construction, and other land uses.

Hydrologic processes can be divided into vertical water movement (including precipitation, snow accumulation and melt, infiltration, and deep seepage) and lateral flow (Figure 2) (Fangmeier et

al. 2006). Once water has entered the soil, it can move laterally

either as shallow lateral flow through the forest duff layer or through

surface soil horizons and macropores. It may also move laterally as

groundwater. Groundwater impacts may be limited at the hillslope

scale, but they become important in larger watersheds.

Infiltration rates depend on soil properties, such as the soil surface cover and soil water content. If the soil is not saturated, then the

infiltration rate is a function of water content and soil properties,

such as noncapillary porosity, texture, bulk density, water repellency, and surface cover. If the soil is saturated, then the infiltration

rate will be limited by the saturated hydraulic conductivity of the

soil (Luce 1995).

Forest vegetation can play a significant role in forest hydrology

(Hubbart 2007). Evapotranspiration varies with age and condition

of vegetation, with very young and mature forests tending to have

lower transpiration rates, whereas healthy growing forests have the

greatest rates. In forests where snow dominates hydrology, trees have

several significant influences on snow hydrology. During the winter,

trees can intercept a significant amount of snowfall. Hubbart (2007)

measured up to about 200 mm of snow water equivalent less under

a canopy than in a clearing in northern Idaho. Troendle et al. (2006)

stated, “In coniferous forests in the cold snow zone, one generally

can expect that 25–35% of the winter snowpack will be intercepted

Manuscript received August 13, 2008; accepted September 24, 2009.

William J. Elliot (welliot@fs.fed.us), US Forest Service, Rocky Mountain Research Station, Moscow Forestry Sciences Laboratory, 1221 South Main, Moscow, ID 83843. The author

acknowledges his indebtedness to Dr. Deborah Page-Dumroese, David Hall, and Louise Ashmun from the Rocky Mountain Research Station, who reviewed the manuscript prior to

submission, and the three anonymous reviewers, all of whom provided valuable suggestions for improving this article. This paper was written and prepared by a US government employee

on official time, and therefore it is in the public domain and not subject to copyright.

This article uses metric units; the applicable conversion factors are: millimeter (mm): 1 mm ! 0.039 in.; square kilometers (km2

): 1 km2 ! 0.3861 mi2; hectares (ha): 1 ha !

2.47 ac; metric ton (Mg, megagram, or metric tonne): 1 Mg ! 2204 lb.

Figure 1. Diagram showing complex interactions that drive forest

watershed processes.

12 WEST. J. APPL. FOR. 25(1) 2010 ABSTRACT
and lost to the atmosphere by some combination of sublimation and

evaporation.” In a compilation of studies in the Pacific Northwest,

where rain is more common in the winter, interception losses ranged

from 10 to 30% (Moore and Wondzell 2005) and approximately

20% in northwestern California in another study (Reid and Lewis

2007). Frequently, this intercepted snow sublimates (changes from

solid water to water vapor) and never reaches the ground. Thus, the

snow available to melt, infiltrate, or run off can be reduced by up to

200 mm per year in mature forests. Trees also can affect the distribution of snow, with greatest accumulations of snow on the edges of

clearings, and under windy conditions, the least accumulation of

snow is found in the middle of large clearings. In the spring, trees on

southerly facing slopes can shade snow on the ground, delaying the

snowmelt from early spring (February to March) until late spring

(April to May) (Troendle et al. 2006, Hubbart 2007). Early melt

rates driven by solar energy tend to be slower than later melt rates

driven by warm air temperatures. A final major effect of vegetation

on hydrologic processes is that it generates surface residue cover. In

forests, this is generally referred to as duff, the layer of organic

material made up of needles, leaves, branches, and larger organic

debris (tree trunks and large branches) at different stages of decomposition (Robichaud and Miller 1999). This layer plays a major role

in enhancing infiltration, reducing surface runoff, reducing soil

evaporation, and contributing to lateral flow (Luce 1995, Pannkuk

and Robichaud 2003).

In watersheds greater than 5 km2

, hillslope processes become less

important (Ziemer and Lisle 1998). Runoff in mountain forested

terrain is dominated by lateral flow and groundwater because of high

infiltration capacities and rapid lateral flow (Dun et al. 2009). High

flows are influenced mainly by weather, including precipitation

amounts, intensities, and durations, and snowmelt rates. Most high

flows are associated with prolonged periods of rainfall and/or a

combination of rain on snow, which saturate soils, so that the only

place the water can go is into the stream network (e.g., McClelland

et al. 1997).

Impacts of Biomass Use

Biomass removal can cause considerable disturbance within a

watershed. Logging traffic can cause soil compaction, particularly if

soils are wet, and duff disturbance, exposing mineral soil (Powers et

al. 2004, Robichaud et al. 2007). These disturbances contribute to

reduced infiltration and increased surface erosion. Harvesting trees

for biomass has the potential for greater onsite impacts than common with current forest harvesting practices. For example, mechanical harvesting of many small-diameter trees per hectare requires

machine traffic over much of the area. In one northern California

study, about 1⁄3 of the harvested area was highly disturbed, 1⁄3 moderately disturbed, and only 1⁄3 relatively undisturbed (Nakamura

2004).

Compaction

If woody biomass is removed using wheeled or tracked equipment when soils are wet and soil strength is low, soil compaction is

likely to occur (Johnson et al. 2007). Moderate and high disturbance from harvesting can measurably increase soil bulk density and

soil strength, but it is not clear what impact that increase will have on

tree growth. Powers et al. (2004) have reported preliminary results

from a long-term soil productivity study indicating that soil compaction increases bulk density and soil strength on loam soils but has

only slight effects on biomass productivity over 10 years. Compacted soils may take many decades to recover to undisturbed conditions (Froehlich et al. 1985). Processes that reduce compaction

include wetting and drying in soils that are high in clay and freezing

and thawing in climates where temperatures drop below freezing

before there is a snow cover. Growth of plant roots can open up

passages that become macropores when the root dies. Compacted

soils tend to resist root penetration, however, slowing this form of

recovery. In most forest environments, none of these processes occur

quickly, and it is not uncommon to observe compaction many decades after the forest operation that created the compaction (Alexander and Poff 1985). Compaction reduces infiltration rates (Johnson and Beschta 1980, Cafferata 1983), and in all but sandy soils, it

reduces the amount of soil water available for plant growth (Zou et

al. 2000). These problems are most detrimental in drier forests,

where lack of water restricts forest growth during part of the growing

season.

Duff Integrity

Forest duff is not a static material but is rather a layer that is

constantly decomposing and being replenished by needles, leaves,

and branches from forest vegetation (Luce 1995, Robichaud and

Miller 1999). As woody biomass is removed, there will be fewer trees

to generate organic material to replenish duff material that is lost

through decomposition. This is particularly true in warmer climates, where decomposition may exceed replenishment rates, but it

is less common in cooler, drier parts of the western United States.

Under some conditions, the duff cover will decline through decomposition for several years until vegetation regeneration is sufficient to

restore the duff cover to what it was before the disturbance. In

addition, the physical process of removing biomass can displace the

duff layer, leaving bare mineral soil exposed to the erosive forces of

wind, rain, and overland flow.

Infiltration

Compaction and loss (or large-scale displacement) of duff will

reduce soil infiltration rates, leading to increased surface runoff and

hillslope erosion (Robichaud et al. 1993, 2007). In some climates,

frozen soils can also significantly decrease infiltration rates, particularly if the insulating duff layer has been disturbed by mechanical

operations. Generally, only a small fraction of runoff from undisturbed forests is surface runoff (Sloan and Moore 1984). Where

surface runoff does occur, it is frequently associated with areas that

are heavily disturbed, such as roads and skid trails. Small changes in

Figure 2. Dominant hydrologic processes on a forest hillslope.

WEST. J. APPL. FOR. 25(1) 2010 13
infiltration are unlikely to cause any major changes in surface runoff

rates. Any increase in surface runoff will likely be accompanied by a

decrease in lateral flow (Crabtree 2007), minimizing the effects of

changes in infiltration on runoff (Figure 2). These changes in infiltration can, however, change lag times and increase instantaneous

peak discharges for small storms in small watersheds (Sendek 1985).

Surface Erosion

Compaction, loss of surface cover, and a small increase in surface

runoff can lead to a significant increase in erosion (Robichaud et al.

1993, Pannkuk and Robichaud 2003). Splash erosion and runoff

are minimal when surface cover protects mineral soil from raindrop

impact. Raindrop splash erosion, rill erosion, and gully erosion are

all directly affected by surface runoff rates (Fangmeier et al. 2006).

Wildfire

Wildfire is common in western watersheds, occurring at intervals

ranging from 20 to more than 300 years (McDonald et al. 2000). In

many forest watersheds, the greatest source of sediment is associated

with erosion following wildfire (Elliot 2006, Lavine et al. 2006).

Peak runoff rates and erosion rates following wildfire are 10 to 1,000

times greater than from undisturbed forests (Elliot 2006, Lavine et

al. 2006, Robichaud et al. 2007). One of the benefits of harvesting

biomass is that the severity or frequency of wildfire may be reduced

(Elliot 2006, Graham et al. 2007).

Roads

After wildfire, the second greatest source of sediment in most

forest watersheds is the road network (MacDonald et al. 2004, Elliot

2006). A road network is necessary to remove biomass. Many forest

roads were built to lower standards in the last century and are now

covered with grass and shrubs. Forest roads are highly compacted by

design, and they retain this compaction for decades (Luce 1997,

Foltz et al. 2009). The impact of overgrown “legacy” roads on erosion may be minimal or significant, depending on site-specific conditions. In northern California and the Pacific Northwest, legacy

roads subjected to large storm events can produce substantial sediment delivery to stream channels from mass failures associated with

poorly constructed road fills, as well as catastrophic failure of unmaintained stream crossings (Elliot et al. 1994, Cafferata and Spittler 1998, Madej 2001, Gerhardt and Strohmeyer 2008). Another

concern with roads is that they can concentrate flow on a hillside and

cause channel initiation and erosion where previously there had

been no channel (Elliot and Tysdal 1999). Unmaintained roads may

also cause localized increases in surface runoff. If overgrown roads

are opened up to allow access for removing biomass for fuel, surface

erosion rates from these roads will likely increase by a factor of 100

(Foltz et al. 2009).

Some watersheds have high road densities (kilometers of road per

square kilometer of watershed), a legacy of past logging practices.

Road removal may be considered to offset the effects of removing

biomass. For example, removing a kilometer of road may offset the

increase in erosion from several hundred hectares of forest that was

harvested for biomass. Road removal studies have shown that road

treatments can reduce the long-term sediment production from

decommissioned roads but that there will be short-term sediment

impacts due to channel adjustments following crossing removal

(Madej 2001, Keppeler et al. 2007, Foltz et al. 2008).

Water Yield

Runoff from forested watersheds is increasingly needed for societal uses such as irrigation, domestic and industrial consumption,

fisheries, and other wildlife conservation. Removing trees reduces

rain and snow interception and evapotranspiration, making more

water available for other uses (Troendle et al. 2006, Hubbart 2007).

In the case of evapotranspiration, the benefits will likely last a few

years, as younger forests can have higher evapotranspiration rates

than mature forests (Keppeler and Ziemer 1990, Moore and

Wondzell 2005). In snow-dominated watersheds, canopy removal

may decrease snow interception for several decades (Troendle et al.

2006). Differences in interception between logged and unlogged

areas are likely to explain the majority of the observed increases in

larger winter peak flows, when transpiration is at its annual minimum. Forest canopy removal can also influence rain-on-snow floods

in some areas. For example, Ziemer and Lisle (1998) and Moore and

Wondzell (2005) both report that clearcutting exacerbated some

rain-on-snow floods in northern California.

Increased runoff from increased snowpack is not seen every year,

however. The increased flows generally occur in wet years, when

there are likely more than adequate flows in streams and rivers

already. In dry years, when water may be limited, increases in runoff

associated with timber removal are unlikely. There is also some

concern that altering flow regimes from forests could alter channel

shapes and adversely affect the aquatic environment (Elliot and

Audin 2006). In addition, in larger watersheds, Ziemer (1987) and

Hubbart (2007) both reported that total water yield increases resulting from management were small and likely not measurable.

Table 1. Typical best management practices to follow to minimize

impacts of biomass use.

Practices

Onsite practices

Minimize mineral soil exposure

Minimize turnarounds with equipment

Use designated skid trails

Use harvesters with longer booms

Maintain undisturbed buffers along streams

Avoid working when soils are wet

Winter logging may work

Avoid dragging logs

Lift ends with skidders

Use grapple skidders or forwarders

Minimize the amount of traffic

Mitigate skid trails

Install frequent water bars

Cover trails with slash

Use low ground pressure equipment

Bigger tires or tracks

Note: Smaller vehicles make more trips

Road network management

Remove unwanted road segments, especially culverts and stream crossings

Outslope whenever safe with frequent dips

Keep insloped ditches stable with coarse gravel or vegetation

Install cross drains or ditch relief culverts where there is adequate vegetated

buffer available between the road and the stream

Consider the fate of all road drainage

Install ditch relief culverts about 50 ft before stream crossings

Use gravel (or asphalt) within 50 ft of live water crossings

Avoid ruts

Close roads when wet

Use gravel

Blade regularly

Monitor culverts to prevent blockage and diversion

Locate roads to minimize sediment delivery to streams

14 WEST. J. APPL. FOR. 25(1) 2010
Summary of Guidelines

There are two general guidelines for minimizing the watershed

impacts of removing biomass. Managers should seek to minimize

duff layer and soil disturbance, and the road network should be well

maintained and kept to a minimum. Best management practices

(BMPs) have been developed by most state and federal agencies to

follow these guidelines (Seyedbagheri 1996, Keller and Sherar

2003). A partial list of these BMPs is provided in Table 1.

An Approach to Watershed Analysis

In some cases, forest managers may be required to carry out a

watershed impact analysis before proceeding with a biomass removal

project. Federal agencies are required by law to complete such an

analysis, and state or local jurisdictions may require such a plan for

private land owners. This analysis should compare the watershed

impacts of biomass removal to naturally occurring processes. There

are a number of different tools available for such an analysis (Elliot

and Audin 2006). For example, Rocky Mountain Research Station

(RMRS) (2005) has developed one simplified method to provide a

framework for such an analysis. This method incorporates four

steps. The first step in a watershed analysis is to estimate the erosion

associated with natural processes. There are generally two conditions to consider for erosion from natural processes: the undisturbed

condition and the erosion associated with wildfire. Generally, erosion associated with an undisturbed condition is minimal. An “average” annual effect of wildfire can be estimated by dividing the

estimated erosion the year following wildfire by the estimated number of years between wildfires in that forest (Elliot 2006). For example, a manager may estimate wildfire erosion to be 10 t/ha, but as it

only occurs once every 100 years, the “average” annual erosion is 0.1

Mg/ha. The erosion from an undisturbed forest can be estimated

and added to the average wildfire erosion value to estimate wildfire

contribution to the natural erosion rate. For example, an undisturbed forest erosion rate may be about 0.05 Mg/ha per year, so the

total natural erosion rate is 0.1 " 0.05 ! 0.15 Mg/ha per year. In

some areas, the natural rate may need to be increased to account for

other natural processes, including soil creep, mass wastage, and bank

erosion (Reid 2007). This number is sometimes referred to as the

background erosion rate.

The second step in this type of watershed analysis is to consider

the effects of the road network on current sediment delivery and the

risk for future mass failure. This may be for two different conditions,

the current condition with low-use or unmaintained roads, and the

use condition, with maintained or upgraded roads. Some roads may

be considered permanent features because of public demand, and

they might be included with the background erosion rate.

The third step in this analysis is to consider the erosion associated

with biomass removal from harvesting and any postharvest treatment activities. These erosion rates are divided by the frequency of

the activity to compare with the background value.

The final step is to synthesize the results. All erosion estimates

should be in the same units, such as tonnes from the watershed,

tonnes per square kilometer, or tons per hectare. The effect of removing biomass for fuel may be a reduction of a wildfire risk and in

the estimated erosion rate following the fire. Table 2 provides an

example of synthesis. The US Forest Service Rocky Mountain Research Station has developed an online erosion prediction tool to

conduct this type of synthesis (RMRS 2005). Additional sources of

sediment from landslides, gullying, or channel erosion may be appropriate to include in some watersheds. Additional features, such as

wetlands, lakes, other areas of deposition, water intakes, and fisheries resources, may be prominent in other watersheds, and the impacts of sedimentation on these beneficial uses of water need to be

considered (Elliot and Audin 2006).

The typical values shown in Table 2 indicate that there may be a

net decrease in sediment generation associated with biomass removal because the interval between severe wildfires is increased and

the burn severity is estimated to be less. Recent observations have

noted that wildfire intensity is likely to be reduced in areas that have

had fuel reduction treatments compared with undisturbed forest

with high fuel loads (Nakamura 2004, Graham et al. 2007, Murphy

et al. 2007). The example in Table 2 is for a single hillslope only. A

similar analysis is needed for all or a sample of all of the hillslopes

that are to be accessed for biomass removal. There also are watershed

analysis tools available that can assist with larger area analyses or in

analyzing cumulative effects of biomass removal spanning a number

of years on sites dispersed throughout a large watershed (Elliot and

Audin 2006). The example presented in Table 2 is for sediment

only. Managers may wish to carry out similar analyses for impacts of

fuel management on runoff, wildlife, forest health, or recreation

resources.

Summary

As society looks to our nation’s forests as sources of energy, there

is a risk of increased runoff and erosion. Compacting and displacing

soil in forests and increased traffic on forest roads may increase

runoff amounts and rates and soil displacement. Following well-established management practices can minimize these risks. When

evaluating the risks of erosion associated with biomass use, managers

should also consider erosion risk associated with wildfire, as that risk

will decline as forest fuels are removed.

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Table 2. Example of the erosion part of a watershed analysis.

Background Erosion (Mg/ha) Biomass removal Erosion (Mg/ha)

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