I am carrying out a lab experiment to see the effect of dry and wet soil on Emitted Radiance. The results are anticipated to help develop an understanding of how soil moisture influences Geothermal anomaly detection in remotely sensed data. To determine the amount of moisture in a soil sample, standard gravimetric method will be followed and the ratio of water content to dry soil will be recorded in %. For the associated Spectral measurements a Fourier Transform Infrared (FT-IR) Spectrometer ( 2.5 to 16 µm ) will be used to retrieve the directional-hemispherical reflectance values and subsequently converted into emissivity using Kirchoff's law. Using emissivity and temperature the Emitted Radiance can be calculated for each soil type in dry and wet states using Planck's black body equation. Emissivity measurements are crucial for accurate land surface temperature estimation.
However, setting-up the experiment requires some careful considerations. For example, it is challenging to keep soil sample homogeneously wet at least as long as the measurements are taken before the top part of the soil dries up. One of the suggestions is to take the spectral and soil moisture measurements quickly to have a good estimation of the soil moisture at the top layer. Since the TIR measurements are restricted only to the top few micrometers/sub millimeters of the soil surface or otherwise known as the skin level, it is essential that the soil moisture measurements correspond to the TIR measurements.
Are there any alternative ways to do the same experiment? What other challenges can be expected?
The Time domain refractometry (TDR) is the best way to measure soil wetness since it measures in-situ wetness.
Thanks Wim for your question. I am planning to fill the petri dish ( 1.5 cm height by 6 cm diameter) with my soil sample for the measurements. So the thickness of the soil sample will not cross the height of the petri dish.
I would like to add that the soils were collected from different parts of Netherlands but my original study areas are located in Yellowstone, US and Kenya, East Africa, both being Geothermal Areas. The soils themselves are not the focus of my study since every soil has different characteristics, but the point that I am trying to prove is to see the influence of soil moisture on emitted radiance and to deduce how it impacts geothermal anomalies. So the soils are just representatives, as a proof for the hypothesis. Following are the soils which were collected, Sandy Loam, Loam and Silty Loam with Low Organic Matter. The idea is to use low, medium and high soil moisture content and take the measurements. No measurements were taken in situ and the soils are already disturbed and brought to the lab for measurements.
Mohammad, I have heard about the TDR. But I am not taking any in-situ wetness measurements. Can this be used in the lab for taking SM measurements in a petri dish filled with a soil sample? But I guess then the instrument will have to just penetrate the top surface of the soil to only a few millimeters or sub-millimeters. How can I do that? Is there any other instrument which can measure SM independently for the top part of a soil?
James, I think it is possible to measure with TDR sensors. TDR has few sensors to put in soil sample and it is possible to measure by using data logger. please see the attached file.
You are emphasising "Laboratory conditions"; however, your eventual soil data has to be actually taken in the field...i.e. actual field soil conditions.. so, why not depend on remotely sensed detection data as obtained from the NASA platforms: SMAP/SMOS/SAR for soil moisture; GRACE for ground water changes; AQUA for water cycle interactions; Landsat/ASTER/TIROS for TIR... As obvious, lots of possible input.
Thanks Dr. Khawlie for your suggestion. You are right. That is why I am also doing a remote sensing satellite data analysis using TRMM, GPM, SMAP, MODIS LST/LSE, ASTER Surface Kinetic Temperature and Surface Emissivity products. I did not mention it but I am actually searching for satellite data immediately after a precipitation event to get an idea of the soil moisture and emissivity states and how they are varying through time until they finally dry out. The difficult part is to get both day and night time cloud free satellite images with high resolution immediately after a precipitation event and later in the thermal infrared wavelength region.
The best I could manage after scanning through the entire 15 years of ASTER's 90m TIR pixel size Images' database is just one image of a day time which could be useful but nevertheless not sufficient. Therefore I had to go down to 1 km pixel size with MODIS LST/LSE. With SMAP, I get 9 km pixels. With MODIS the temporal resolution is better but the spatial aspect is not so great. So you see, there is a compromise. I have to work with what is available. I have to look for homogeneous areas of 1 sq.km to make a credible analysis of the states of soil moisture and their associated LSE/LST variations. The geothermal features are generally formed in clusters ranging from some cms to some sq. kms. and the in situ surface temperatures range from > 10°C to < 100°C. Satellite Images alone cannot answer all the research questions that I have. Collecting soil samples and taking spectral measurements from my original study areas in Yellowstone and Africa would have been an ideal condition but such possibilities do not exist for my research therefore I had to plan alternative methods. The alternative was to take dry and wet soil TIR measurements in the lab. This may not be the best but I think it can give a proxy answer for my research question. Probably if I get some more funding when I do my PhD I will make a plan to visit my study areas and take SM & TIR In situ measurements.
Thanks Dr. Sedghi-Asl for the paper. I think the TDR will be quite useful for me if I use it in the field. But it seems the conventional gravimetric method is working just fine for the moment. I took some wet soil reflectance measurements in the lab today with the FTIR. The main concern before taking the spectral measurement was the usage of Nitrogen (N2) gas which is used to purge out the inside of the FTIR system from Carbon Dioxide (CO2) and water vapor (H20) gas which influences the reflectance spectrum. If N2 were to dry up the top part of the wet soil while the spectral measurements were being taken then the measurements may not have been representative. Therefore, the measurements were taken both with and without using the N2 gas to see whether there are any large differences between the reflectance measurements of the same wet soil sample (Sandy Loam). I have attached the reflectance plots here.
If N2 had dried up the soil sample while taking the spectral measurements, the green color plot would have shown a higher reflectance but with the same pattern as shown by the red color plot. Fortunately, the differences are quite subtle. Does anybody know why N2 gas does not dry up the soil sample when allowed to flow at 100 litres/hr at 5 psi? I have also attached the link to the FTIR paper which is the same spectrometer being used for this experiment.
http://www.mdpi.com/1424-8220/11/11/10981
Actually I am not familiar with the above-mentioned method. There is a traditional method to measure soil moisture. If you have soil sample with known volume (total volume) and total mass (wet mass) then you can dry it in Owen during 24 hours and then you should measure its weight. Now by calculating the (wet mass- dry mass)/dry mass=mass water content. this is the most commonly-used method.
Yes Dr. Sedghi-Asl that is the same method I am following to estimate Soil Moisture Content (SMC), known as the Gravimetric method. I guess there are other names for the same method. The FT-IR Spectrometer is used for spectral measurements in the infrared region. So ultimately what I am trying to achieve is to measure SMC from Field Capacity to a completely dry state of a Soil and corresponding Radiant energy. A graph which gives the relationship between Emitted Radiance at different SMC states for different soils.
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