Nelson Model 1- and 10-hr Fuel Moisture Estimation Methods
Ralph M. Nelson (2000) developed a fuel moisture model for estimating the diurnal fuel moisture changes in a 10-hr NFDRS fuel stick. Requiring hourly observations, it produces a more dynamic estimate that better reflects changes in precipitation, humidity and sunshine. 2016 NFDRS uses this methodology.
Called SimpleFFMC, 1-hr Fuel Moisture Estimation Tables have been calibrated for the southeastern US by W. Matt Jolly (2016) and a web-app is available for online users at http://www.wfas.net/ffmc/
Fosberg Model 1-hr Fuel Moisture Estimation Methods
Michael A. Fosberg and John E. Deeming (1971) documented procedures for estimating 1- and 10-hour Timelag Fuel Moistures. This methodology, along with seasonal adjustment tables, were integrated into Richard Rothermel’s (1983) tools and methods for surface fire behavior predictions. 78/88 NFDRS use this.
Day Time Estimation Procedure:
- Using Table A, determine Reference Fuel Moisture (RFM) % from intersection of temperature & relative humidity. Record this RFM percentage.
- Select Table B, C, or D to adjust RFM for local conditions by finding current month in table title.
- Is the fine fuel more than 50% shaded by canopies and clouds? If yes, use bottom (shaded) portion of table. If no, use top (Exposed) portion of table.
- Determine the appropriate row based on aspect and slope.
- Determine the appropriate column based on time of day & elevation of area of concern when compared to the wx site elevation. Use (A)bove if the fire is 1-2000’ above your location, (B)elow if the fire is 1-2000’ below you, and (L)evel if the fire is within 1000’ above or below you.
- Obtain the 1-hr Moisture Content Correction (%) from the intersection of row & column.
- Add the resulting 1-hr Moisture Content Correction (%) to the Reference Fuel Moisture (%)
Night Time Estimates of 1-hr Fuel Moisture
Published Reference Fuel Moisture and Correction Tables for Night Time Conditions are not included here based on recommendation from Pat Andrews at the Missoula Fire Lab. She recommends:
- Estimate Dry Bulb Temperature and RH for the location of interest
- Use Table A to estimate the Reference Fuel Moisture
- Use the appropriate 1-hr Moisture Content Correction Table based on the time of the year
- Obtain the correction for 0800, shaded conditions, and appropriate aspect from that table and add it to the Reference Fuel Moisture to estimate 1-hr moisture content for night time conditions.
Table A. Reference Fuel Moisture
Table B. 1-hr Fuel Moisture Corrections, May-June-July
Table C. 1-hr Fuel Moisture Corrections, Feb-Mar-Apr & Aug-Sep-Oct
Table D. 1-hr Fuel Moisture Corrections, Nov-Dec-Jan
10hr, 100-hr and 1000-hr Fuel Moisture Content
10-hr and 100-hr Fuel Moisture may be estimated in the following ways and applied along with the Fosberg fuel moistures in surface fire behavior predictions. 1000-hr fuel moisture is not usually needed for fire behavior calculations.
- After estimating 1-hr moisture content, 10-hr and 100-hr fuel moisture content can be estimated by adding incremental amounts (e.g. adding 1-2% for 10-hr and 2-4% for 100-hr).
- Using a local RAWS station or the Geographic Area’s Predictive Service summaries, 78/88 NFDRS moisture content estimates or forecast values that utilize the Fosberg Model may be available for each of these fuel categories.
- The National Fuel Moisture Database may have sampling locations near your setting that have estimates for these fuel moistures (http://www.wfas.net/index.php/national-fuel-moisture-database-moisture-drought-103 ).
In NFDRS, if danger rating calculations are suspended in the dormant season, default dormant fuel moistures are provided for 100-hr (10%-25%) and 1000-hr (15%-30%) fuel moistures when calculations are restarted in the spring. Default values are established with climate class designation for the location.
Fuel Moisture Conditioning in US Spatial Fire Growth Models
Spatial analyses in WFDSS, FARSITE and FLAMMAP use historic and forecast weather to estimate current and future fuel moistures. At this writing in 2017, initial dead fuel moistures in deterministic analyses default to estimates from the Fosberg model while conditioning weather uses the Nelson model to adjust 1-hr and 10-hr fuel moisture content over 1 to several days. Use 7 days or less.
FSPro draws its fuel moistures in the ERC table from the NFDRS system and fuel model that produces its ERC distribution. As of this writing in 2017, it uses 78 Fuel Model G and the Fosberg model for all dead fuel moistures.