Estimating the Biomass of Hand-Piled Fuels for Smoke Management Planning

Project Description

Dimensions, volume, and biomass were measured for 121 hand-constructed piles composed primarily of coniferous (n=61) and shrub/hardwood (n=60) material at sites in Washington and California. Equations using pile dimensions, shape, and type allow users to accurately estimate the biomass of hand piles.

Purpose and Scope

Land managers and air quality regulators needed a tool to accurately and efficiently estimate the biomass of hand-piled fuels as pile burning becomes a more widespread and common method for treating high fire hazard areas with heavy surface fuels. Our objective was to quantify the relationships between pile composition, pile size (dimensions and volume), and pile biomass by measuring and weighing hand-constructed piles.

Methods

Different vegetation types (i.e., conifer, shrub, and hardwood), shapes, and pile sizes were sampled to develop equations for estimating the volume and biomass of hand piles. The field portion of this study was concentrated in forest and woodland types in the Western United States. Stands with hand piles were selected in Washington and California with the assistance of local and regional fire and fuels managers. Our intention in selecting study sites and pile types was that the results of this study would have utility throughout the West where surface fuels are being treated with the use of hand piling and burning.

Within stands, piles were selected using a random walk procedure. The nearest pile that was 10 m at a random azimuth from a pre-selected starting point was chosen, with each successive pile located 10 m at a random azimuth from the last measured pile. Once located, pile volume was measured using two methods: geometric volume and surface shape volume. For estimates of geometric pile volume, dimensions required to compute the volume of one of seven specific geometric shapes were measured, and the appropriate volume formula was employed.

For estimates of surface shape volume, we mapped the contours of the pile surface using an angle gauge and level system. A series of level lines were projected from the center to the edge of the pile in 30° increments and measurements of the vertical offset (nearest 3 cm) from the level line were taken at 15 cm intervals in the horizontal from the pile center. This method allowed us to compute a three-dimensional coordinate for systematically located points on the surface of the pile, from which volume was estimated using a triangular irregular network (TIN). For the purposes of this study, we considered the TIN-derived shapes and volumes to be the best representation of the true volume of the pile.

Following dimension and surface measurements, piles were deconstructed and sorted into species and size class groups (<2.5, 2.5-7.6, and >7.6 cm diameter). Species and size class groups of separated piles were weighed in the field with a precision hanging scale (nearest 10 g). Moisture content subsamples were collected for each category for each pile to convert field-measured weight to oven-dry weight.

Volume, biomass, and composition data were synthesized and used to calculate physical properties, including packing ratio (the ratio of solid material volume to total pile volume) and bulk density (the ratio of pile biomass to total pile volume). Ordinary least squares regression was used to develop equations to (1) estimate true volume from dimension measurements and shape assignments (i.e., from geometric volume) and (2) estimate biomass from true volume for different pile types (i.e., conifer, shrub/hardwood, etc.).

T-tests were used to test for differences in regression slopes between pile types. These regression equations are being encoded in a web-based calculator that will allow users to accurately estimate volume and biomass of hand-constructed piles for use in determining potential emissions impacts from burning of these piled fuels.

Key Findings

• In general, hand piles are of a modest size (overall mean volume=3.01 m3; overall mean biomass=156.63 kg); the largest pile we measured was 14.47 m3, and the heaviest pile weighed 672.14 kg. Piles composed primarily of coniferous material tended to have higher bulk density, in Figure 5.
• Overhead plan view (left) and cross-section view (right) of pile surface measurement methodology. 0° 30° 60° 90° 120° 150° 180° 240° 210° 270° 300° 330° 15 cm Vertical offset 0° 30° 60° 90° 120° 150° 180° 240° 210° 270° 300° 330° 0° 30° 60° 90° 120° 150° 180° 240° 210° 270° 300° 330° 15 cm Vertical offset
• Figure 6. Example TIN lattice for a typical pile at the Naches, Washington, field site. This pile was 0.73 m high and 1.86 m wide; the “true” volume of this pile was 1.68 m3

Piles composed primarily of coniferous material tended to have higher bulk density, in large part owing to the greater percentage of large (>7.6 cm diameter) woody particles in the pile. While there were differences in overall size, of the shrub and hardwood piles we measured there was little difference in the bulk density and the size distribution of the fuel particles. On average pile volume determined using pile dimensions and geometric formulas (geometric volume) overestimated true pile volume.

Summary data for sampled hand-constructed piles appear in table 3

Project Deliverables

Biomass and volume estimation equations were being programmed into a web-based calculator to allow users to estimate either value from pile dimensions.