Data source:
 
   Pressure exerted by water contained in the soil, snow, and canopy 
   from GEOS-IT numerical model, land product "total water store in land
   reservoirs" (TWLAND) dataset. GEOS-IT is developed at the Global 
   Modelling  Assimilation Office at NASA.

   The model has resolution over longitude 0.625 deg, over latitude 
   0.5 deg, at has time resolution 1 hours. However, time resolution
   is reduced to three hours for loading computation. Two out of
   three epochs are ignored.

   The land water storage pressure is smoothed and re-gridded to 
   a grid with latitude/longitude resolution 10800 over latitude
   and 5400 over longitude (~3.7 km). For a point that land-water mask 
   is greater than zero, i.e. not a total water, the pressure is computed 
   by convolving the water pressure at the original grid with an exponential
   decay function e^{-d/r} where d is the distance from the new grid 
   point at the original grid point, r is a smoothing parameter chosen to be 
   1/2 of the original grid resolution over latitude.
   
   The dataset at the new grid from 1998.01.01 through 2025.01.01 was 
   fitted to an empirical model that includes the mean pressure and 
   harmonic variations at each grid cell at the following frequencies: 

   Mean  
   SA    1.990968752920D-07  ! Long period  annual
   SSA   3.982127698995D-07  ! Long period  semi-annual
   PI1   7.232384890619D-05  ! Diurnal
   P1    7.252294578148D-05  ! Diurnal
   S1    7.272206167609D-05  ! Diurnal 
   K1    7.292115855138D-05  ! Diurnal
   PSI1  7.312025542667D-05  ! Semi-diurnal
   2T2   1.450459105823D-04  ! Semi-diurnal
   T2    1.452450074576D-04  ! Semi-diurnal
   S2    1.454441043329D-04  ! Semi-diurnal
   R2    1.456432012082D-04  ! Semi-diurnal
   K2    1.458423171028D-04  ! Semi-diurnal
   U3    2.177679627494D-04  ! Ter-diurnal
   T3    2.179670596247D-04  ! Ter-diurnal
   S3    2.181661850283D-04  ! Ter-diurnal
   R3    2.183652533752D-04  ! Ter-diurnal
   K3    2.185643502505D-04  ! Ter-diurnal
   U4    2.904900529562D-04  ! Four-times-a-day
   T4    2.906891498303D-04  ! Four-times-a-day
   S4    2.908882467044D-04  ! Four-times-a-day (cosine term only)

   The parameters of the model: The mean, sine, and cosine amplitudes of 
   harmonic variations were computed for the time range of 27 years 
   [1998.01.01, 2025.01.01]. 

   Finally, the contribution of the mean, relevant discontinuities, 
   diurnal, semi-diurnal, ter-diurnal, and four-times-a-day surface 
   variations were removed from the surface pressure field exerted 
   by land water storage. Annual and semi-annual variations are kept 
   with the data. 
References:

   Petrov, L., The International Mass Loading Service, 
      Proceedings of the Reference Frames for Applications in 
      Geosciences Symposium, held in Luxembourg in October 2014, 2015. 
      DOI: 10.1007/1345_2015_218, http://arxiv.org/abs/1503.00191

   Petrov L., J.-P. Boy, "Study of the atmospheric
      pressure loading signal in VLBI observations", Journal of
      Geophysical Research, 10.1029/2003JB002500, vol. 109,
      No. B03405, 2004.

   Rienecker, M.M., Suarez M.J., Gelaro R., Todling R., Bacmeister J., 
      Liu E., Bosilovich M. G., Schubert S. D., Takacs L., Kim G.-K., 
      Bloom S., Chen J., Collins D., Conaty A., da Silva A., et al. 
      MERRA ---- NASA's Modern-Era Retrospective Analysis for Research 
      and Applications, J. Climate, 24, 3624--3648, 2011.

   Reichle, R.~H., Koster~R.D, De Lannoy~G.J.M., Forman~B.A., Liu~Q., 
      Mahanama~S.P.P, Tour\'e~A., Assessment and Enhancement of MERRA 
      Land Surface Hydrology Estimates, J. Climate, 24, 6322--6338, 
      2011.
