Hi Farid,

First a general observation. Soil moisture content does not change very fast, it takes some days before it starts responding to changes in precipitation for example. The inertia for changes in soil moisture is rather high. Hence, to my opinion it won't make a big difference when you perform you comparison (validation) during night or day. Have a look at your TDR data over a depth profile of say 50 cm.. How much do they differ for night and day?

And if you really want to convince yourself take the measurement data which correspond in time exactly with the data you simulated. I guess you simulated soil moisture content data with modeled ones no?

Cheers,

Frank

... but if you consider satellite soil moisture data, depending on the measurement sensor (e.g, active versus passive microwave) and the algorithm, usually you can find in the papers theoretical considerations for which night-time (or day-time) measurements are better.

However, in one of our papers on RSE (https://www.researchgate.net/publication/229237881_Soil_moisture_estimation_through_ASCAT_and_AMSR-E_sensors_An_intercomparison_and_validation_study_across_Europe?ev=prf_pub), we compared ascending (day-time) and descending (night-time) passes from the passive microwave AMSR-E sensor by using ground observations as benchmark. We found that ascending passes provide better results (see Figure 3) even though, theoretically, we expected the opposite.

Anyhow, this is still an open scientific question to be investigated!

Article Soil moisture estimation through ASCAT and AMSR-E sensors: A...

Hi Luca,

Do you obtain different reponses from the AMSR_E as well over water surfaces in ascending and descending modes?

If you do, I guess the AMSR-E delivers biased data depending on the direction it makes its orbit. Ever checked that out?

I didn't because I use thermal inertia to estimate soil moisture content. Check out my papers on that topic, here on ResearchGate. Compared to passive microwave methods I can go to 1 km spatial resolution, and we don't observe a difference in SMC estimations between day or night. Our TDR validation probes don't idnicate differences between day or night either. So, what's up with passive (or active) microwave SMC estimation?

Cheers,

Frank

Hi Frank,

we didn't check if there is a bias related to the orbit with AMSR-E, but I have to admit I'm not a remote sensing expert.

As hydrologist, I am a data user... and indeed I'm very curious to check the potential of using thermal inertia for soil moisture monitoring from space. I am well aware of your paper on RSE (Verstraeten et al., 2006).

Did you create a soil moisture dataset from thermal inertia for some areas in Europe, hopefully in Italy? In this case, we might check with ground measurements and hydrological applications the reliability of this dataset, it would be highly interesting for us.

Thanking you in advance,

L

Hi Luca,

The largest SMC dataset we made using therma inertia was in China. To be exact, for the most Northwestern province, the Xinjiang province, which represents still about a surface area of 200,000 km², which would represent a big chunk of Europe.

My institute could have engaged in the operational production of global SMC, but they did not take or have the opportunity. Funding from ESA,or EUMETSAT is still lacking to produce SMC with thermal inertia data. I guess these organisations rather like to see their microwave systems being used for the purpose of SMC extraction.

For Europe there is indeed a large SMC dataset which has been produced with an active microwave system. I am not so sure about the quality of this large datset. It is difficult to validate a product with pixels of about 40x40 km. Hence microwave SMC spatial resolution is pretty poor when compared to the 1x1 km obtained with thermal sensors (thermal inertia approach).

It is disappointing, because SMC is an important variable for many applications. I hope that ESA or EUMETSAT change their mind at some point in time and finance a programme for the opertational production of an SMC dataset for Europe based on thermal inertia. It will certainly be interesting when this dataset would be compared with data obtained with active or passive microwave systems.

I hope that my institute keeps watching the ESA calls for SMC. Maybe one day a call appears on SMC extraction using thermal inertia, using one of the many thermal sensors of ESA and the archived data form these sensors. Might give firework when compared with microwave SMC.

Cheers,

Frank

Dear Frank

This contrast between Luca's results and yours must be investigated. However, this thermal Inertia method seems very interesting. when most researches insist on using microwave methods with poor resolutions, 1*1 Km resolution seems very promising. I am not familiar with this method, but it must have its own advantages and disadvantages over microwave method. Could you introduce some papers to get the general idea of thermal inertia method.

Best wishes,

Farid

I also suggest this paper for a nice intercomparison among the methods:

Hain, C.R., W.T. Crow, J.R. Mecikalski, M.C. Anderson and T. Holmes, "An intercomparison of available soil moisture estimates from thermal-infrared and passive microwave remote sensing and land-surface modeling," Journal of Geophysical Research - Atmospheres, 116, D15107, doi:10.1029/2011JD0156332012, 2011.

Hi Ffarid,

The following two papers might be revealing:

Goto

https://www.researchgate.net/profile/Frank_Veroustraete/publications/?pubType=article&ev=prf_pubs_art

and next to:

Article: Soil moisture content retrieval based on apparent thermal inertia for Xinjiang province in China. Frank Veroustraete, Qin Li, Willem W. Verstraeten, Xi Chen, Anming Bao, Qinghan Dong, Tieu Liu, Patrick Willems. International Journal of Remote Sensing 06/2012; 33(12):3870-3885. · 1.14 Impact Factor

and to

Article: Soil moisture retrieval using thermal inertia, determined with visible and thermal spaceborne data, validated for European forests. Verstraeten W.W.,, Veroustraete F.,, Vande Sande C.J.,, Grootaers I.,, Feyen J. Remote Sensing of Environment 01/2006; 101:299-314. · 5.10 Impact Factor.

Cheers,

Frank

Dear Luca

Can you give me a brief explanation why should night SMC data give better results, theoretically?

Farid

Dear Frank

Do you have any suggestion regarding Thermal Inertia Method that suits for a PhD thesis?

Best Wishes,

Farid

From our paper on RSE, thanks to the suggestions from Amsterdam guys working on the retrieval of soil moisture from passive microwave sensors from several years, it reads:

"A preliminary analysis was carried out for the selection of the best orbit to be used for the AMSR-E soil moisture products. Figure 3 shows the R-values averaged over all the comparisons between the 21 site-specific data sets for a 5 cm layer depth and the three AMSR-E soil moisture algorithms (LPRM, NASA and PRI) split up according to the orbit: ascending, descending and ascending plus descending. Overall, it is quite clear that data from ascending passes provide higher correlations with site-specific data (even though the differences in performance could be considered not very significant). The same results were obtained for the soil moisture anomalies (not shown for sake of brevity). These results are in accordance with Loew et al. (2009) who employed AMSR-E data over Europe but, on the other hand, are in contrast to many other AMSR-E studies (Rudiger et. al. 2009; Wagner et. al., 2007; Gruhier et. al 2010; Draper et. al 2009; Liu et. al., 2011) that have used descending overpass (01:30 am). In fact, it is well-known that over night time the negative effect related to the difference between the surface and the canopy temperature is reduced. At the same time, the ascending passes have the positive effect that during the day the vegetation is more transparent, being dryer around peak temperatures of the day, and, therefore, the quality of the ascending passes might be better at certain vegetation densities. Considering the results of Figure 3, the ascending passes seem to be more accurate over Europe, but more in-depth investigations are needed to clarify this interesting and important issue. "

Hi Farid,

As a method to be applied in a PhD thesis I would suggest to apply the method as described in the two papers I cited in the response here above with validations in China and Europe. I have a paper on the validation of LST in China as well. That is evidently crucial for a good estimate of thermal inertia. You can find it on my papers in ResearchGate as well.

Cheers,

Frank

Dear Frank

How can I be in touch with you? Can I have your email in case I had any question?

Cheers,

Farid