Abstract
Forest duff (fermentation and humus) water content is an important parameter for fire risk prediction and water resource management. However, accurate determination of forest duff water content is difficult due to its loose structure. This study evaluates the feasibility of a standing wave ratio (SWR) sensor to accurately determine the forest duff water content. The performance of this sensor was tested on fermentation and humus with eight different compaction levels. Meanwhile, a commercialized time domain reflectometry (TDR) was employed for comparison. Calibration results showed that there were strong linear relationships between the volumetric water content (θV) and the SWR sensor readings (VSWR) at different compaction classes for both fermentation and humus samples. The sensor readings of both SWR and TDR underestimated the forest duff water content at low compacted levels, proving that the compaction of forest duff could significantly affect the measurement accuracy of both sensors. Experimental data also showed that the accuracy of the SWR sensor was higher than that of TDR according to the root mean square error (RMSE). Furthermore, low cost is another important advantage of the SWR sensor in comparison with TDR. This low-cost SWR sensor performs well in loose materials and is feasible for evaluating the water content of forest duff. In addition, the results indicate that decomposition of the forest duff should be taken into account for continuous and long-term water content measurement.
Highlights
The forest floor plays a significant role in forest hydrological processes by affecting water and energy transfer between the sub-canopy atmosphere and the mineral soil [1,2,3]
In order to keep better linear relationships between volumetric water content determined In order to keep better linear relationships between volumetric water content determined gravimetrically and standing wave ratio (SWR) output voltage (VSWR), the eight different compaction levels of gravimetrically (θ V ) and SWR output voltage (VSWR ), the eight different compaction levels of fermentation and humus samples were divided into three compaction classes: incompact, slightly fermentation and humus samples were divided into three compaction classes: incompact, slightly compacted and strongly compacted
This study evaluated a moisture sensor based on the principle of standing wave ratio to determine forest duff water content under laboratory conditions
Summary
The forest floor plays a significant role in forest hydrological processes by affecting water and energy transfer between the sub-canopy atmosphere and the mineral soil [1,2,3]. It can be divided into the surface litter layer (Oi), with freshly fallen plant residue, the sub-surface fermentation layer (Oe), with partially decomposed but still recognizable organic material, and the underlying humus layer (Oa), with well decomposed organic matter [4,5]. In forest ecology, forest duff moisture strongly impacts leaf litter decomposition rates, which affects carbon cycling and microbial activity [11]
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