Science Report
12/10/2014
Juho Vehvilainen
On EVA 8 Crew Engineer Victor Luo and the writer of this report increased the spatial coverage of soil moisture measurements on surrounding areas of the MDRS. These measurements were done at two different sites that were not visited earlier. See more about the project from science report published on 2nd of December.
Originally, the plan of the soil moisture project was to monitor the dynamics of water in soil. However, as there was no change in prevailing weather (read: no rain), it was decided that the project should concentrate more on the spatial variability of soil moisture. The next step in the project will be correlating the collected measurements with satellite data, but that will have to happen after our rotation. The particular satellite for this will be European Space Agency’s Soil Moisture and Ocean Salinity mission, shortly SMOS. More often than not the data from SMOS is rather hard to interpret, but I hope that here, around the MDRS, this won’t be the case, since there is not much vegetation, between the satellite sensor and soil surface. Vegetation has water in it, which makes the satellite to observe the water in trees, plants etc. rather than in soil.
Here I’d like to talk a bit about the theory in which our sensor is based. An UK based company Deltaco manufactures our specific sensor, ThetaProbe ML2x, but the same principles are valid for a multitude of soil moisture sensors. The basis of the measurement is the dielectric constant (the real part), which is a property of material and affects the way electrical field behaves in matter. This is not the same as electrical conductivity, just to be clear. The dielectric constant varies from 1 for air to 81 for water. For dry mineral soil the dielectric constant is around 3-4. The significant difference between dry soil and free (=not frozen) water allows us to detect water in soils.
In practice the sensor measures voltage difference inside its electronics, which is kind of a complicated process and out of scope of this report. This value is transformed to a reading of dielectric constant. When the dielectric constant is acquired, an equation including soil specific calibration constants is utilized to calculate the volumetric water content in soil. It means the ratio between the volume of water present and the total volume of the sample. The end product is either the dielectric constant or the volumetric water content depending on which context the values will be used. For example when correlating with microwave satellite observations, the dielectric constant is used.
The fieldwork part for the soil moisture project is now finished. As mentioned earlier, the next step is to correlate the results with satellite observations.