Articles | Volume 16, issue 2
Research article
09 Feb 2016
Research article |  | 09 Feb 2016

Comparison of different methods for the in situ measurement of forest litter moisture content

C. Schunk, B. Ruth, M. Leuchner, C. Wastl, and A. Menzel

Abstract. Dead fine fuel (e.g., litter) moisture content is an important parameter for both forest fire and ecological applications as it is related to ignitability, fire behavior and soil respiration. Real-time availability of this value would thus be a great benefit to fire risk management and prevention. However, the comprehensive literature review in this paper shows that there is no easy-to-use method for automated measurements available. This study investigates the applicability of four different sensor types (permittivity and electrical resistance measuring principles) for this measurement. Comparisons were made to manual gravimetric reference measurements carried out almost daily for one fire season and overall agreement was good (highly significant correlations with 0.792 <  =  r <  =  0.947, p  <  0.001). Standard deviations within sensor types were linearly correlated to daily sensor mean values; however, above a certain threshold they became irregular, which may be linked to exceedance of the working ranges. Thus, measurements with irregular standard deviations were considered unusable and relationships between gravimetric and automatic measurements of all individual sensors were compared only for useable periods. A large drift in these relationships became obvious from drought to drought period. This drift may be related to installation effects or settling and decomposition of the litter layer throughout the fire season. Because of the drift and the in situ calibration necessary, it cannot be recommended to use the methods presented here for monitoring purposes and thus operational hazard management. However, they may be interesting for scientific studies when some manual fuel moisture measurements are made anyway. Additionally, a number of potential methodological improvements are suggested.

Short summary
Dead fine fuel (e.g. litter) moisture is a key parameter for wildfire and ecological applications, as it is related to ignitability, fire behavior and soil respiration. For example, fire ignition is highly likely at moisture contents below 10 %, yet nearly impossible above 25 %. Thus, fine fuel moisture measurements are very desirable; however, there is no easy-to-use automated technique available to date. This study investigates the applicability of different sensor types for this measurement.
Final-revised paper