I would like to know if the emissivity of soils in the thermal infrared region (8 µm - 12 µm) are affected by acidic, basic or neutral soil types. I am trying to take thermal infrared measurements of three (Sandy, Clay and Loam) different soil types in the lab, however I am interested to know if pH has any effect on the emissivity. My aim is to emulate real soil types found in Yellowstone National Park which have acidic, basic and neutral soil types in and around geothermal areas. Since I do not have access to the actual soil types in Yellowstone, I am trying to use proxy soils for thermal measurements. However I was wondering whether I need to also consider the pH values of soils that I am using for the analysis. Requesting subject matter experts to shed some light on the discussion.
Dear James,
principles you find in:
http://www.sciencedirect.com/science/article/pii/003442577990035X
http://www.uv.es/~ucg/articulos/2009/Publications_2009_5.pdf
The issue of influence of both, emissivity and kinetic temperature, is discussed in
http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=317447&url=http%3A%2F%2Fieeexplore.ieee.org%2Fxpls%2Fabs_all.jsp%3Farnumber%3D317447
while the pH comes in where dependence on soil moisture is discussed:
http://onlinelibrary.wiley.com/doi/10.1029/2007JF000749/full
Best wishes, Kees
pH may effect the Emissivity, but it also depends on the sensitivity of the sensors which you are using and the pH range of your study area. Other way you can use radar image to get a proxy of soil type.
Thanks Prof. Stigter for the links. I have been following some of these papers for quite some time now. Therefore, I am not new to the principles and the temperature/emissivity separation issues. In any case I will go through the papers in detail.
Surajit, thanks for your comments. However, the question remains still open, how does pH effect the emissivity? Do you have any hypothesis, papers etc. which can point in the right direction? What do you mean by the sensitivity of the sensors? Do you mean the sensitivity of the radiometer? or the sensitivity of the pH measuring instrument? The pH range of my study area is quite complex. Think of Yellowstone National Park Geothermal areas.
The FTIR which I am using in the lab is quite sensitive therefore I don't think it is an issue while measuring reflectance/emissivity. For remote sensing datasets, I am using ASTER and Landsat TIRS. I don't think we have any other multispectral satellite sensors better than these at the moment based on spatial and spectral resolutions.
I have thought about the question and I think when there is a change in pH it may be a completely different soil type. I don't know whether it is possible to have a same soil type with different pH. I guess a soil scientist can provide some useful inputs.
Soil type is historically determined and so is its pH.
It can be changed by additions to make it more suitable for certain crops/plants.
See https://en.wikipedia.org/wiki/Soil_pH
Best regards, Kees
This is an interesting Wikipedia link. Thanks for the link, Prof. Stigter.
I never thought of this until now. It seems pH of soils can be changed both artificially and naturally. I am wondering now whether artificially changing the pH of each soil type (sandy, loam, clay) that I am using for the experiment will make any sense because it is difficult to simulate the Yellowstone geothermal soil types using these proxy soils. What is the alternative that you suggest? As I mentioned earlier, I don't have access to the actual soil types in Yellowstone.
In my view, it is not so much pH that changes your thermal properties but the water content. You should not be anxious about emissivity but about the (near) surface temperature being very dependent on water content, changing its conductivity and other thermal properties.
Best regards, Kees
Thanks Prof. Stigter for your reply. I know that I have to take into account both the emissivity and surface temperature (skin temperature) if I want to accurately estimate the surface emitted radiance [Planck's law]. So far I have conducted experiments by measuring the emissivity based on variations in soil moisture content. I found that as moisture content increases the emissivity also increases which also compares well with existing literature. However, I am not sure how to prepare an experimental setup to simultaneously measure the skin temperature while measuring the corresponding emissivity associated with moisture content in the soil.
If your equipment is able to measure the skin temperature with an infrared thermometer that is independently calibrated with known surface temperatures of a black body and you can simultaneously measure the emissivity, would that not do?
See attached paper.
Best regards, Kees
Thanks Prof. for the comments and the paper. I think what you suggest can be done. I am using the attached FTIR Instrument for the experiments. However, I am wondering whether this instrument supports TIR measurements of soil samples when they are artificially heated up. Since temperature and emissivity both have an effect on emitted radiance, I would have liked to dynamically vary both variables simultaneously and see the resultant emitted radiance. I thought to fully saturate the soils and take TIR measurements as the soil sample dries out. Simultaneously, I can take the skin temperature using an infrared thermometer. But I don't know if it is possible to artificially heat up the soil sample beginning from a saturated state and measure both the variables?
http://www.mdpi.com/1424-8220/11/11/10981
My instrumental knowledge is no longer enough up to date. Your most tricky situation is between the whole surface being moist and when the drying front is going to withdraw into the ground. There is a period with dry and wet patches on the surface, the latter one still evaporating maximally, the former evaporating from within the soil with an additional resistance related to the depth of drying soil. You will have a very inhomogeneous situation. But this is only after the surface has been completely wet, so immediately after rains. I wish you success with finding a solution.
Best regards, Kees
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