Seguridad y Certificación CSI

The ground heat flux and the thermal regime of the soil constitute a consider- able challenge for both measurements and models and significant uncertainties must be accepted (see Sect. 4.1.3). This may seem surprising in first place, considering the simple physical law of conductive heat transfer, that is largely applicable in permafrost soils (e.g. Kane et al., 2001). However, the uncertainty is much more caused by the small-scale heterogeneity of the soil parameters than by the actual description of the process of heat transfer into the soil. In this thesis, two different approaches to determine the ground heat flux have been followed. The assets of the conductive method (see Sect. 2.3.2) are the ex- cellent temporal resolution of the obtained fluxes and the relative simplicity of the installation: as a record of three temperatures between the surface and ap- proximately 0.3 m depth is sufficient, it is feasible to apply the method at a large number of sites. However, the technique is limited to temperatures, where freez- ing or thawing of soil water does not occur, so that it is not possible to compile an annual budget based entirely on the conductive method. Furthermore, the method requires rapid temperature changes within the monitored soil domain, which may not occur during winter. A strong source of uncertainty is the deter- mination of the volumetric heat capacity chfrom soil samples, which is required

to calculate the thermal conductivity from the thermal diffusivity dh. Taking

the average from many soil samples collected in the vicinity of the temperature profile is not ideal, as the value may not be representative for the temperature profile due to the small-scale heterogeneity of the soil properties. Furthermore, the heat capacity can change over time, e.g. after rainfall events, which is diffi- cult to capture with soil samples. A significant accuracy improvement could be achieved by in-situ measurements of the heat capacity by heated-needle probes, which have been successfully applied for unfrozen and fully frozen soil (Putko- nen, 2003; Overduin et al., 2006; Ochsner et al., 2006). Ochsner and Baker (2008) optimize the technique to be applicable close to a freeze front and are

able to derive ground heat fluxes during the time of soil freezing and thawing. Thus, it seems possible to combine the conduction method with heated needle probes to facilitate monitoring of ground heat fluxes during the entire year. In the calorimetric method (see Sect. 2.3.1), the volumetric heat capacity is a major source of uncertainty, too. The problem is even more severe, as it must be determined over a profile up to the depth, where the annual temperature cycle is negligible. As it is unpractical to install heated-needle probes deeply within the permanently frozen ground, a soil core must be obtained by drilling a borehole to determine the heat capacity in laboratory measurements. Despite of these limitations, the calorimetric method is more suitable than the conduc- tive method to establish an annual budget of the internal energy content or long-term averages of the ground-heat flux, respectively.

In addition to the volumetric heat capacities, a record of temperature and volu- metric soil water content is required up to the depth, where annual temperature cycle become insignificant. While it is feasible to drill a borehole, in which the temperature is recorded, the determination of the content of unfrozen water is more difficult. Time-Domain-Reflectometry (TDR) can be installed within the active layer (Boike et al., 2003b), where a hole can be dug to insert the TDR probes. However, the soil water content can also change below the depth of annual thaw due to the freezing characteristics of the soil. Instrumenting the permanently frozen ground with TDR would be an extremely arduous task, which would also involve a considerable destruction of the site.

Non-invasive radar techniques can provide averages of the soil water content over larger volumes with high accuracy, as they are required by the calorimetric method. Two schemes are conceivable:

1. Multi-channel ground penetrating radar (GPR) can determine the soil water content of the thawed zone by evaluating the travel times of multi- ple radar signals reflected from the freeze-thaw interface (Gerhards et al., 2008). Multi-channel GPR has been successfully applied at permafrost sites by Wollschl¨ager et al. (2010) and Westermann et al. (2010), who determine both the thaw depth and the soil water content of the thawed zone over transects of several 100 m length. For permafrost sites that fea- ture soils with a low content of unfrozen water at subzero temperatures, a combination of temperature records from one or multiple boreholes in conjunction with multi-channel GPR measurements could e.g. deliver the annual minimum and maximum content of internal energy of the soil col- umn, which would be a near-ideal monitoring scheme for the soil thermal conditions: a radar survey conducted just before the onset of refreezing in fall could provide the soil water content and thus the latent heat content of the thawed zone at the end of the thaw season. The annual maximum content of internal energy is then calculated from the latent heat content and the temperature profile data (Westermann et al., 2010). The annual minimum of internal energy during winter can directly be derived from temperature measurements in the fully frozen soil. It must be emphasized that such a scheme could still produce meaningful results when the soil contains a non-negligible content of unfrozen water at subzero tempera- tures, but a reduced accuracy must be tolerated.

content by evaluating the travel time of a radar signal, that is emitted at depth d1 in one borehole and received at depth d2 in another borehole a

few meters next to the first hole. Other than multi-channel GPR, it holds potential to measure the content of unfrozen soil water below the freeze- thaw interface. By recording travel times at a number of depths (d1,

d2), a tomography of the soil water content between the two boreholes

can be performed. While cross-borehole radar has not yet been employed in permafrost monitoring, its potential for hydrological applications and mapping the vertical distribution of the soil water content has become manifest in non-permafrost regions (e.g Binley et al., 2001; Olsson et al., 2006).

Finally, the potential of heat flux plates is briefly discussed, as they are widely applied to determine the ground heat fluxes in studies on the surface energy bud- get (Foken, 2008a). Heat flux plates consist of thin plate with known thermal conductivity, across which the temperature gradient is measured. The method is satisfactory, if the thermal conductivities of the heat flux plate and the sur- rounding soil material are equal. If not, the assumption of 1D-heat transfer is clearly violated, and the heat flux through the plate is not the same as in the surrounding soil. When the soil freezes, the thermal conductivity can change considerably, so that a number of heat flux plates with different thermal conduc- tivities would be required to successfully determine the ground heat flux. While heat flux plates are appealing in their simplicity, significant and unacceptable er- rors in the determination of the ground heat flux have been documented (Sauer et al., 2003; Ochsner et al., 2006), so that great care must be warranted when applying the method.

In some permafrost areas, it might be feasible to determine ground heat fluxes by measuring temperature gradients across permanent layers of pure ice with known thermal conductivity, which would thus act as a natural in-situ heat flux plate. As these ice layers naturally occur below the depth of annual thaw, ad- ditional techniques, e.g. the calorimetric method, must be employed to obtain the ground heat flux at the surface.