Submitted to: Vadose Zone Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/18/2016
Publication Date: 6/17/2016
Citation: Seyfried, M.S., Link, T., Marks, D.G., Murdock, M.D. 2016. Soil temperature variability in complex terrain measured using fiber-optic distributed temperature sensing. Vadose Zone Journal. 15(6). doi: 10.2136/vzj2015.09.0128.
Interpretive Summary: Soil temperature controls many important soil processes, including germination and carbon release to the atmosphere. It is understood that soil temperature can vary across the landscape for a variety of reasons, such as due to slope and aspect, but there is little or no data defining how variable it is. This is important because it determines how well we can describe soil temperature which, in turn, determines how can quantify those processes of interest. We used a new instrument, fiber-optic DTS, that measures temperature along a fiber optic cable every meter on a specified time step. We found that soil temperature is highly variable in a single subwatershed. In fact, temperature differences under different vegetative cover types are greater than those between similar cover types separated by a 1000 m elevation difference. We also found that soil temperatures were relatively homogeneous within those vegetative cover types. These results demonstrate the value of subdividing areas in complex terrain based on land cover units.
Technical Abstract: Soil temperature (Ts) exerts critical controls on hydrologic and biogeochemical processes but magnitude and nature of Ts variability in a landscape setting are rarely documented. Fiber optic distributed temperature sensing systems (FO-DTS) potentially measure Ts at high density over a large extent. A fiber optic cable 771 m long was installed at a depth of 10 cm in contrasting landscape units (LU’s) defined by vegetative cover at Upper Sheep Creek (USC) in the Reynolds Creek Experimental Watershed (RCEW) and Critical Zone Observatory (RC CZO) in Idaho, USA. The purpose was to evaluate the applicability of FO-DTS in remote settings and to determine how Ts varies in complex terrain. Measurement accuracy was similar to other field instruments (± 0.4°C) and Ts changes of approximately 0.05°C at a spatial scale of 1 m were resolved with occasional calibration and an ambient temperature range of 50° C. Differences in solar inputs among LU’s were strongly modified by surface conditions. During spatially continuous snow cover, Ts was practically homogeneous across LU’s. In the absence of snow cover, Ts was highly variable among LU’s, with a standard deviation (SD) greater than 8°C, and relatively uniform (SD < 1.5 °C) within LU’s. The variation in mean annual soil temperature (MAST) of 5.1 °C among LU’s was greater than the MAST difference of 4.4 °C associated with a 910 m elevation difference within the RCEW. In this environment, effective simulation Ts requires representation of relatively small-scale (<20 m) LU’s due to the deterministic spatial variability of Ts.