COMPARING TECHNOLOGIES FOR POINT-SCALE MEASUREMENT OF NEAR-SURFACE SOIL WATER CONTENT

Near-surface soil water content, within the upper few cm, affects many physical and biological processes. For soils topped by biological soil crusts (BSC), which are found in nearly all arid environments, water content influences physiological processes and thus has a role in biotic carbon dynamics. Therefore, focusing measurement technologies at the upper few cm of soil material allows us to better understand, model and predict the impact of soil water status on BSC function, particularly carbon cycling. In this study, we compared vertically-installed thermal probes, specifically dual-probe heat pulse (DPHP; sensor length 3 cm), to horizontally-installed, frequency domain reflectometer (FDR; sensor length 20 cm) probes. The probes were installed in close proximity in two research plot that differed by a temperature treatment (i.e., equal amounts of water addition). The study was conducted near Moab, Utah and extended ~160 days, covering the April – September time period that included the winter dry down and several summer monsoon events. A total of 10 precipitation events were found significant enough for both technologies to record increases in soil water content. However, while results showed no difference in peak water content values between plots (i.e., temperature treatment was not significant), aspects of the overall water content behavior measured between DPHP and FDR technologies did show significant differences. On the one hand, peak water content measured with the DPHP was between 3-4 times larger than the values measured with the FDR. On the other hand, drydown rates measured with DPHP and FDR technologies were similar, as were the abilities of the two technologies to detect small precipitation events—in this case, the technologies appeared to effectively measure precipitation depths down to ~2 mm. Both technologies are non-destructive, a distinct advantage over the physical sampling and laboratory analyses of BSC water content that is often reported in the literature. In summary, photosynthetic rate calculations that depend on accurate water content measurements could be influenced by the measurement technology being used. Our results show that the DPHP technology can provide the needed future measurements to help model the role of BSC in arid ecosystem C dynamics.