Operationalization of SoilDTS
Introduction
In the last ten years Distributed fiber optic Temperature Sensing, often shortened to DTS, has
been developed as a useful tool for all sorts of hydrological research. Within this wide span of topics several studies on soil thermal properties and soil moisture using DTS are performed, so-called SoilDTS. SoilDTS is split in two methods: PassiveDTS, where the response of soil temperatures to the diurnal signal in Rn is measured; and ActiveDTS where a heat pulse is applied to the cables using the electrical resistance of its steel armoring.
Methods
From March to October 2011 three fiber optic cables at 1, 6 and 11 centimeters are plowed in a grass plot in Delft, The Netherlands. From March to May 2011 PassiveDTS measurements are conducted and in September 2011 a four day ActiveDTS campaign took place. For the passive measurements an inversion method is used to calculate diffusivity from soil temperature using the thermal diffusion equation. With a decagon KD2 probe a relation between thermal conductivity and soil moisture is derived from which soil heat flux (SHF) is calculated. During the ActiveDTS campaign 30 minute heat pulses of 6 W/m are applied to the steel armoring of the cable. The response to this heat pulse measured in that same cable is used to calculate thermal conductivity which leads to SHF profiles. SHF estimates are finally compared to other parameters of the surface energy balance.
Results
- The PassiveDTS method gives accurate estimates of surface heat flux profiles. From the thermal conductivity vs. soil moisture content relation also soil water content can be calculated. These calculated values are consistent with measured values from an in-situ EC5 moisture probe.
- The relation between thermal conductivity (K) and soil moisture is essential for the success of the PassiveDTS method. This relation is very hard to derive from existing models like Johansen75 or Campbell but in this research it is shown that deriving this relation manually is fairly simple and rewarding when accurate K estimates are preferred.
- The four day ActiveDTS campaign gave very promising results for this method. From the temperature response to the applied heat pulses, thermal conductivity is calculated using the single-needle-heat-pulse-probe method used in commercially available single-needle-heat-pulse-probes like the Hukseflux TP02. One crucial aspect is the determination of the exact amount of heat applied to the soil. The measured temperature inside the cable can be different from the temperature on the outside of the cable.
- The SHF estimates are sufficiently accurate to use in energy balance closure calculation. This is valuable for evaporation calculations or satellite reference measurements for instance used in agricultural research.
Conclusion and recommendations
SoilDTS is a very promising and useful tool to determine and monitor soil thermal properties and soil moisture. PassiveDTS is still dependent on accurate reference measurements of thermal conductivity and soil moisture. ActiveDTS can be a tool that is capable of deriving thermal conductivity without extensive reference measurements. Further research on the effect of cable insulation on the applied heat pulse to the surrounding soil is necessary but can definitely be rewarding.
Student: J.H.A.M. Jansen
Committee: Prof.dr.ir. N.C. van de Giesen, dr.ir. S.C. Steele-Dunne, prof.dr. M. Menenti
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