7.2 Agricultural Burns
| Category ID | Description | EIC |
|---|---|---|
| 315 | Prunings | 67066002620000 |
| 316 | Field Crops | 67066202620000 |
| 317 | Weed Burning | 67066802000000 |
| 318 | Range Improvement Burning | 67066402000000 |
| 319 | Forest Management | 67066602000000 |
Introduction
This document outlines the methodology for estimating greenhouse gas (GHG) emissions from agricultural and prescribed burning activities in the San Francisco Bay Area (SFBA). These activities are a source of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) emissions. Emissions are reported in specific categories based on the types of materials burned:
Agricultural Burns
- Category 315 (Prunings) includes the burning of pruned branches, twigs, and other residual biomass generated from the maintenance and attrition of orchards, vineyards, and cane fruits.
- Category 316 (Field Crops) includes the burning of residue left after harvesting crops such as wheat, rice, and corn and for the purpose of establishing an agricultural crop in a location that formerly contained another type of agricultural crop, or on previously uncultivated land.
- Category 317 (Weed Abatement) includes burning activities conducted to remove weeds, stubble, and straw and to control growth of vegetation in irrigation ditches and canals.
Prescribed Fires
- Category 318 (Range Improvement) includes controlled fires to enhance the productivity of rangelands. This includes marsh management and the removal of shrubs, small trees, or other vegetation to promote forage growth for livestock.
- Category 319 (Forest Management) includes prescribed fires conducted to reduce fuel loads, prevent wildfires, and maintain ecosystem health. This category includes wildland vegetation management and the burning of deadwood, underbrush, and other forest debris.
Methodology
This section describes how GHG emissions are estimated for agricultural and prescribed burning categories. These categories are considered area source categories since they cover emission sources that are not directly permitted by the BAAQMD (Air District), so emissions are not systematically or annually cataloged and/or reported. The methodology used to calculate emissions for the base year(s) of agricultural and prescribed burning area sources is as follows:
Base Year(s) Emissionscounty,pollutant =
Activity Data × Emission Factorpollutant × Control Factorpollutant × Fractioncounty × GWPpollutant
Where:
- Base Year: is a year for which activity / throughput data is available (or can be derived or estimated)
- Activity Data: is the total regional throughput or activity data for applicable base year(s).
- Emission Factorpollutant: is a factor that allocates an amount of emissions, in mass, of a particular pollutant by unit of activity data.
- Control Factorpollutant : is a fractional ratio (between 0 and 1) that captures the estimated reduction in emissions as a result of Air District rules and regulations.
- Fractioncounty : is the fraction of total regional emissions (between 0 and 1) estimated to be allocated to a particular county.
- GWPpollutant is the Global Warming Potential of a particular GHG pollutant. The current version of the GHG emissions inventory incorporates the global warming potential (GWP) reported in the Fifth Assessment report of the Intergovernmental Panel for Climate Change (IPCC, 2014). The GWPs for the three principal GHGs are 1 for carbon dioxide (CO2), 34 for methane (CH4), and 298 for nitrous oxide (N2O), when calculated on a 100-year basis with climate-carbon feedback included.
Once base year emissions are determined, historical backcasting and forecasting of emissions relative to the base year emissions are estimated using growth profiles as follows:
Current Year Emissions county = Base Year(s) Emission county × Growth Factor
More details on activity data/throughput, county distribution, emission factors and controls are provided in the following subsections.
Activity Data / Throughput
The Air District’s Regulation 5 requires that scheduled burns be permitted and coordinated with the Air District, and that detailed burn reports, which include information such as the burning event that occurred, a description of the material burned, the corresponding burning category etc., are annually submitted to CARB (BAAQMD, 2019a; BAAQMD, 2019b). Activity data for agricultural and prescribed burning events within BAAQMD jurisdiction are compiled from annual burn reports provided by the Air District’s Compliance & Enforcement Division as part of Regulation 5 reporting requirements (BAAQMD, 2024). The burning categories defined in the annual burn reports are assigned to one of the five agricultural and prescribed burn categories, as shown below. Burn categories are assigned by Compliance & Enforcement based on definitions on burn materials in Regulation 5.
Regulation 5 Burn Category | Assigned Category |
Orchard Pruning & Attrition | 315 (Prunings) |
Flood Debris | 315 (Prunings) or 316 (Field Crops) based on material burned |
Disease & Pest | 315 (Prunings) or 316 (Field Crops) based on material burned |
Crop Replacement | 316 (Field Crops) |
Irrigation Ditches | 317 (Weed Abatement) |
Stubble | 317 (Weed Abatement) |
Range Management | 318 (Range Improvement) |
Marsh Management | 318 (Range Improvement) |
Forest Management | 319 (Forest Management) |
Wildland Vegetation Management | 319 (Forest Management) |
The amount of material burned is reported in various units, including volume (in cubic yards), weight (in tons), or acreage. Burn volume or weight is reported to the Air District by the requestor asking for the permit as part of Regulation 5 requirements. To ensure consistency in calculations, the following conversion methods are applied to ensure that all activity data are expressed in terms of weight:
- Volume to Weight Conversion: For materials reported in cubic yards, the volume is converted to weight using the EPA’s volume-to-weight conversion factors (USEPA, 2016) for “Prunings & Trimmings” and “Branches & Stumps” (127 lb/yd³).
- Acreage to Weight Conversion: For materials reported by acreage, the burned area is converted to weight using fuel loading factors specific to vegetation types, as outlined in the CARB agricultural and prescribed burning methodology, Appendix B (CARB, 2020). Fuel loading factors for each Regulation 5 burn category are summarized in the table below and are determined using an equally weighted average of factors from commonly burned materials within each category.
Regulation 5 Burn Category | Fuel Loading (ton/acre) | Basis of Fuel Loading |
Orchard Pruning & Attrition | 1.7 | Average of Almond, Apple, Apricot, Avocado, Bean/Pea, Date Palm, Fig, Grape, Nectarine, Olive, Peach, Pear, Prune, and Walnut |
Flood Debris | 1.9375 | Average of Orchard Pruning & Attrition and Crop Replacement |
Disease & Pest | 1.9375 | Average of Orchard Pruning & Attrition and Crop Replacement |
Crop Replacement | 2.175 | Average of Alfalfa, Barley, Corn, Oats, Rice, Safflower, Sorghum, and Wheat |
Irrigation Ditches | 2.175 | Average of Alfalfa, Barley, Corn, Oats, Rice, Safflower, Sorghum, and Wheat |
Stubble | 2.175 | Average of Alfalfa, Barley, Corn, Oats, Rice, Safflower, Sorghum, and Wheat |
Range Management | 2.688 | Average of Grassland, Pasture |
Marsh Management | 2.688 | Average of Grassland, Pasture |
Wildland Vegetation Management | 1 | Fuel loading factor for ponderosa pine |
Forest Management | 1 | Fuel loading factor for ponderosa pine |
County Distribution / Fractions
The county distribution is derived from the total weight of material burned in each county as reported by burn permit requestors to the District as part of Regulation 5 requirements. Requestors must report the latitude and longitude coordinates for where the burn will occur, which is then assigned to the county where the burn will occur. Since the amount of material burned in each county varies annually, the county distribution will also change from year to year. For Solano and Sonoma counties, where portions of the county are outside the Air District’s jurisdiction, the information available is specific to the SFBA portion of these counties. The table below shows the county distribution for agricultural and prescribed burning categories for the base year 2022.
Emission Factors
CO2 and CH4 emission factors for agricultural and prescribed burning are derived from Jenkins et al. (1996), specifically Tables 4.1.1 through 4.1.8, which provide data for eight different vegetation or crop types: barley, corn, rice, wheat, almond, walnut, Douglas fir, and ponderosa pine. These emission factors are based on controlled studies conducted in wind tunnels and are originally presented as percentage mass of dry fuel. N2O emission factors are sourced from CARB’s GHG inventory (CARB, 2022) for the same eight materials, also in units of percentage of dry fuel. To convert the emission factors from percentage of dry fuel to units of pounds per ton of raw material burned (lbs/ton), the moisture content of the material was used to convert from dry mass to total mass, and the resulting value was multiplied by 2000 lbs/ton. The moisture content is defined in Jenkins et al. (1996) for each material type in the study. All parameters used to derive emission factors in lb/ton of raw material are summarized in the table below.
Crop Type | Moisture Content | % of dry fuel mass | Emission Factor (lb/ton) | ||||
CO2 | CH4 | N2O | CO2 | CH4 | N2O | ||
Barley | 6.9% | 117% | 0.247% | 2E-6% | 23.43 | 0.049 | 4.00E-07 |
Corn | 8.6% | 131% | 0.175% | 2E-6% | 26.25 | 0.035 | 4.00E-07 |
Rice | 8.6% | 116% | 0.072% | 2E-6% | 23.22 | 0.014 | 4.00E-07 |
Wheat | 7.3% | 119% | 0.182% | 2E-6% | 23.88 | 0.036 | 4.00E-07 |
Almond | 18.3% | 183% | 0.117% | 2E-6% | 36.59 | 0.023 | 3.99E-07 |
Douglas fir | 30.0% | 224% | 0.190% | 2E-6% | 44.61 | 0.038 | 3.99E-07 |
Ponderosa pine | 24.5% | 214% | 0.133% | 2E-6% | 42.62 | 0.027 | 3.99E-07 |
Walnut | 33.1% | 164% | 0.164% | 2E-6% | 32.75 | 0.033 | 3.99E-07 |
Emission factors are assigned to each burning event based on the most similar type(s) of material from the eight vegetation or crop types. This ensures that calculated emissions accurately reflect the representative characteristics of the burned biomass, even when an exact match is unavailable. For burning events where no specific materials are defined, default vegetation or crop types are used to derive aggregate emission factors for each category. These default factors are shown in the table below for matching burning categories to emission factors presented in Jenkins et al. (1996). Each crop type is weighed equally in the averaging of the emission factor. The table below summarizes emission factors assigned to each burn category based on representative crop types.
Category | Emission Factor Basis | Emission Factor (lb/ton) | ||
CO2 | CH4 | N2O | ||
Prunings | Average of Almond, Walnut | 34.67 | 0.028 | 3.99E-07 |
Field Crops | Average of Barley, Corn, Rice, Wheat, Almond, Walnut | 27.69 | 0.032 | 3.99E-07 |
Weed Abatement | Average of Barley, Corn, Rice, Wheat | 24.20 | 0.034 | 4E-07 |
Range Improvement | Average of Barley, Corn, Rice, Wheat | 24.20 | 0.034 | 4E-07 |
Forest Management | Average of Douglas fir, Ponderosa pine | 43.62 | 0.032 | 3.99E-07 |
Control Factors / Emission Controls
Although no control factors are formally applied in the emissions calculations, BAAQMD Regulation 5 – Open Burning (BAAQMD, 2019a) impacts the activity (and therefore emissions) from agricultural and prescribed burning by imposing restrictions and conditions that limit the frequency, size, and types of permissible burns. In addition, all agricultural and prescribed burns are required to obtain a permit from the Air District. Requirements for agricultural and prescribed burns were originally included as an amendment to Regulation 5 in 1990, but electronic reporting data became available in 2005. Key elements of Regulation 5 that influence GHG emissions from agricultural and prescribed burning are summarized in the corresponding staff report (BAAQMD, 2019b) and include:
- A prohibition on the burning of certain materials, such as plastics, garbage, and treated wood. Allowed materials are limited to those originally present at the burning site. This restriction prevents the release of toxic air contaminants and ensures that only natural biomass contributes to emissions.
- A limit on the tonnage, volume, or acreage of material burned on any given day and at any specified site to values set by the Air District and CARB. These limits are determined by daily meteorological and air quality conditions to minimize adverse impacts on regional air quality and reduce short-term emission spikes.
- A restriction on burning by time of day and at specific times of the year based on the material burned. For example, orchard pruning may only be burned from November to April. These seasonal restrictions align with weather patterns that facilitate better dispersion and reduce the likelihood of contributing to elevated particulate matter levels during critical air quality periods.
- A requirement for burn management plans for large-scale prescribed burns. These plans must outline specific strategies to minimize emissions, including fuel moisture considerations, ignition techniques, and contingency measures to adapt to changing weather conditions.
- Mandatory reporting and record-keeping for all permitted burns, ensuring compliance with Regulation 5 and allowing for improved emissions tracking and analysis. This electronic reporting system, in place since 2005, provides a more accurate historical record of open burning activities and supports long-term air quality planning efforts.
- Provisions allowing the Air District to declare No-Burn Days when meteorological conditions would exacerbate air pollution. Burn bans are issued based on factors such as wind patterns, temperature inversions, and regional pollutant concentrations, reducing emissions from open burning during periods of poor dispersion.
These regulatory measures collectively work to limit the overall emissions impact of agricultural and prescribed burning by reducing the amount of material burned, preventing combustion of high-emission waste materials, and ensuring that burns are conducted under conditions that minimize their effects on air quality.
Historical Emissions / Backcast
For years 1990-2004, total agricultural production by county, obtained from the U.S. Department of Agriculture (USDA, 2024a), is used to develop the backcast profile for all categories under the assumption that increased agricultural production leads to increased burning. Annual burn reports from Regulation 5 reporting are used to develop the historical emissions for years 2005-2021. For each year, the same methods outlined in previous sections of this document are used to estimate emissions for each category.
Future Projections / Growth Factor
Similar to the development of the backcast profile, the forecast profile is developed using USDA agricultural projections from 2023-2033 (USDA, 2024b). In lieu of specific forecast profiles for California, the projection for fruit, nut, and vegetable production for the United States is used. This specific projection is selected because according to USDA’s historical agricultural production by county, the majority of crop production by weight in the Bay Area is fruits, nuts, or vegetables. Furthermore, the California Department of Agriculture estimates that three-quarters of fruits and nuts and one-third of vegetables grown in the United States are grown in California. The forecast profile is extrapolated out to 2050 using a regression line fit to the 2023-2033 projection values.
Sample Calculations
The table below shows an example calculation for calculating base year 2022 CH4 emissions from prunings (category 315) in Contra Costa County. Emissions are reported in metric tons of CO2 equivalents (MTCO2eq) per year.
Step 1 | Annual material burned |
| |
Step 2 | Conversion to weight burned | 0.0635 | |
Step 3 | Total weight burned | = 104 tons = 199.25 ton fuel/year | |
Step 4 | Type of material burned; | Grapevines, orchard trees; | |
Almond | Walnut | ||
Step 5 | Fuel moisture content | 18.3% | 33.1% |
Step 6 | CH4 Emission Factors (percentage of dry fuel) | 0.117% | 0.164% |
Step 7 | Convert Emission Factors (lb/ton fuel) | (1-0.183) × 0.00117 ton/ton fuel × 2000 lb/ton = 1.91 lb/ton fuel | (1-0.331) × 0.00164 ton/ton fuel × 2000 lb/ton = 2.19 lb/ton fuel |
Step 8 | Calculate Average Emission Factor (lb/ton fuel) | (1.91 + 2.19) ÷ 2 = 2.05 lb/ton fuel | |
Step 9 | Calculate Emissions (tons/year) | 199.25 ton fuel/year × 2.05 lb/ton fuel × 1/2000 ton/lb = 0.204 tons/year | |
Step 10 | Global Warming Potential for CH4 | 34 | |
Step 11 | Calculate Emissions using GWP (MTCO2eq/year) | 0.204 tons/year × 34 × 0.907 MT/tons = 6.30 MTCO2eq/year | |
Assessment of Methodology
Routine updates to the agricultural and prescribed burning GHG inventory include incorporating the most recent Regulation 5 annual burn reports for each year not covered in the previous iteration and updating GWPs for each greenhouse gas to match the most recently published IPCC assessment report. Additional updates to the inventory for base year 2022 include assigning emission factors and converting activity data from volume or acreage to weight based on actual vegetation or crop type rather than using aggregate values.
Base Year | Revision | Reference |
2022 |
|
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2015 |
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2011 |
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Emissions
The table below shows the total GHG emissions by pollutant in MTCO2eq for agricultural and prescribed burning categories.
| ID | Description | CH4 | CO2_bio | N2O | Total |
|---|---|---|---|---|---|
| 319 | Forest Management | 147.3 | 5099.9 | 0.0 | 5247.2 |
| 315 | Prunings | 39.9 | 1723.7 | 0.0 | 1763.6 |
| 318 | Range Improvement Burning | 83.1 | 1158.6 | 0.0 | 1241.7 |
| 316 | Field Crops | 7.8 | 297.9 | 0.0 | 305.7 |
| 317 | Weed Burning | 0.5 | 7.7 | 0.0 | 8.2 |
Summary of Base Year 2022 Emissions
Agricultural and prescribed burning events produce CO2, CH4, and N2O emissions based on the type and amount of material burned. Emissions are initially calculated in pounds and then converted to MTCO2eq.
The tables below show the contribution of agricultural and prescribed burning GHG emissions to the overall regional total and to the Agriculture sector total.
Contribution of Agricultural Burns Emissions by Sector| Subsector | Sector | Subsector GHG Emissions (MMTCO2eq) | Sector GHG Emissions (MMTCO2eq) | % of Sector |
|---|---|---|---|---|
| Agricultural Burns | Agriculture | 0.000 | 1.26 | 0.02% |
Contribution of Agricultural Burns Emissions to Regional Total
| Subsector | Subsector GHG Emissions (MMTCO2eq) | Regional Total GHG Emissions (MMTCO2eq) | % of Regional Total |
|---|---|---|---|
| Agricultural Burns | 0.000 | 65.68 | 0.000% |
Trends
The time series chart below shows the emission trends for agricultural and prescribed burning categories.
Summary of Trends
Emissions and activity associated with agricultural and prescribed burning are difficult to predict and depend on a variety of factors such as total annual agricultural production, specific types and amounts of crops grown, and changes in burning techniques based on seasonality and weather. As a best estimate, agricultural production is used as the basis for backcasting and forecasting agricultural and prescribed burning emissions and activity.
On average, the forecast profile shows a 0.4% increase in agricultural and prescribed burning per year. This is consistent with the BAAQMD staff report for Regulation 5 (BAAQMD, 2019b), which predicts a slight increase in agricultural and prescribed burning over time. Similar to the backcast profile, the forecast profile is normalized based on the average annual agricultural production by county from 2005-2022 and applied to the average emission rate by county for the same time period.
Uncertainties
Activity data reported as part of Regulation 5 requirements is self-reported and may not always be accurate due to underreporting, overestimation, or misclassification of burn types. Additionally, records may be incomplete or contain errors in spatial or temporal assignment.
Jenkins et al. (1996) remains the most up to date study for GHG emission factors for agricultural and prescribed burning. This resource is outdated and only provides emission factors for eight different types of vegetation or crops. Actual agricultural and prescribed burning events rarely match these eight types. Emission factors are assigned as a best estimate based on the most similar type or types of vegetation.
The emission factors in Jenkins et al. (1996) are based on controlled studies conducted in wind tunnels. Actual emission factors and combustion efficiency will vary based on open-field conditions such as temperature, wind speed, and humidity. Furthermore, the materials burned in Jenkins et al. (1996) are individual samples and actual moisture content and composition will vary in real-world conditions.
Conversion of material volume or acreage to weight assumes constant density (EPA, 2016) and fuel loading (CARB, 2020). Actual density and fuel loading depends on factors that are not tracked in the Air District burn reports such as variability in material burned, moisture content, arrangement and stacking of material, and fragmentation and size distribution of material.
For Range Improvement and Forest Management burns, geographic layers containing the actual vegetation cover can be combined with the actual location of the burns to obtain a better estimate of the type of vegetation burned. This method is part of CARB’s most recent methodology updates (CARB, 2023a; CARB, 2023b) along with using the First Order Fire Effects Model (FOFEM; Lutes, 2020) to estimate criteria air pollutant and greenhouse gas emissions. FOFEM addresses many of the uncertainties mentioned above by calculating the mass of fuel consumed in flaming and smoldering phases of combustion and corresponding emissions based on fuel moisture condition. Future regional inventories will use updated CARB methods to estimate agricultural and prescribed burning emissions from Range Improvement and Forest Management or directly incorporate CARB’s emission estimates for the Bay Area.
The backcast and forecast profiles are developed assuming that an increase in agricultural production results in greater amounts of material consumed in agricultural and prescribed burning. While this may be true in some cases related to prunings, crop replacement, and weed abatement, it is not a direct correlation as the primary purpose of prescribed burning is typically focused on forest management and wildfire mitigation, and not solely for agricultural needs. As seen from the backcast profile from 2005-2021 based on actual data, agricultural and prescribed burning rates are difficult to predict from year to year and depend on several factors in addition to agricultural production as described in the Summary of Trends section. In lieu of a more accurate representation of agricultural and prescribed burning forecasting, agricultural production is used as a best estimate for agricultural and prescribed burning activity.
Contact
Author: Yuan Du
Reviewer: Abhinav Guha
Last Update: 07/21/2025
References
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