The cosmic-ray method of soil moisture measurement is still developing and there is more scientific research required to refine the method. The current status along with the science enabled by the data it provides is described in the COSMOS-UK User Guide available from the Resources page.
Here is a selection of relevant publications explaining the method:
Andreasen, M., K. H. Jensen, M. Zreda, D. Desilets, H. Bogena, and M. C. Looms (2016), Modeling cosmic ray neutron field measurements, Water Resour. Res., 52, 6451–6471, doi:10.1002/2015WR018236.
Baatz, R., H. R. Bogena, H.-J. Hendricks Franssen, J. A. Huisman, W. Qu, C. Montzka, and H. Vereecken. 2014. Calibration of a catchment scale cosmic-ray probe network: A comparison of three parameterization methods, J. Hydrol., 516, 231–244, doi:10.1016/ j.jhydrol.2014.02.026.
Bogena, H. R., Huisman, J. A., Güntner, A., Hübner, C., Kusche, J., Jonard, F., Vey, S. and Vereecken, H. 2015. Emerging methods for noninvasive sensing of soil moisture dynamics from field to catchment scale: a review. WIREs Water, 2: 635–647. doi:10.1002/wat2.1097.
Cooper, H. M., Bennett, E., Blake, J., Blyth, E., Boorman, D., Cooper, E., Evans, J., Fry, M., Jenkins, A., Morrison, R., Rylett, D., Stanley, S., Szczykulska, M., Trill, E., Antoniou, V., Askquith-Ellis, A., Ball, L., Brooks, M., Clarke, M. A., Cowan, N., Cumming, A., Farrand, P., Hitt, O., Lord, W., Scarlett, P., Swain, O., Thornton, J., Warwick, A., and Winterbourn, B. 2021. COSMOS-UK: national soil moisture and hydrometeorology data for environmental science research. Earth Syst. Sci. Data, 13, 1737–1757, doi: 10.5194/essd-13-1737-2021.
Desilets D, Zreda M, Ferré TPA. 2010. Nature’s neutron probe: land surface hydrology at an elusive scale with cosmic rays. Water Resources Research 46: W11505. DOI:10.1029/2009WR008726.
Evans J. G., H. C. Ward, J. R. Blake, E. J. Hewitt, R. Morrison, M. Fry, L. A. Ball, L. C. Doughty, J. W. Libre, O. E. Hitt, D. Rylett, R. J. Ellis, A. C. Warwick, M. Brooks, M. A. Parkes, G. M. H. Wright, A. C. Singer, D. B. Boorman, A. Jenkins. 2016. "Soil water content in southern England derived from a cosmic-ray soil moisture observing system – COSMOS-UK". Hydrol. Process., doi: 10.1002/hyp.10929.
Franz TE. 2012. Installation and calibration of the cosmic-ray solar moisture probe. pp: 12.
Franz TE, Zreda M, Rosolem R, Ferre TPA. 2013. A universal calibration function for determination of soil moisture with cosmic-ray neutrons. Hydrology and Earth System Sciences 17: 453–460. DOI:10.5194/hess- 17-453-2013.
Fry, M. J.; Evans, J.; Ward, H.; Blake, J.; Ball, L.; and Doughty, L. 2014. "The Provision Of Data From The COSMOS-UK Soil Moisture Monitoring Network". International Conference on Hydroinformatics. Paper 56. http://academicworks.cuny.edu/cc_conf_hic/56.
Kohli, M., M. Schron, M. Zreda, U. Schmidt, P. Dietrich, and S. Zacharias. 2015. Footprint characteristics revised for field-scale soil moisture monitoring with cosmic-ray neutrons, Water Resour. Res., 51(7), 5772–5790, doi:10.1002/2015WR017169.
Zreda, M., D. Desilets, T. P. A. Ferr_e, and R. L. Scott. 2008. Measuring soil moisture content non-invasively at intermediate spatial scale using cosmic-ray neutrons, Geophys. Res. Lett., 35, L21402, doi:10.1029/2008GL035655.
Zreda M, Shuttleworth W, Zeng X, Zweck C, Desilets D, Franz T, Rosolem R. 2012. COSMOS: the cosmic-ray soil moisture observing system. Hydrology and Earth System Sciences 16: 4079–4099.