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@ARTICLE{Zhang:840115,
      author       = {Zhang, Hongjuan and Kurtz, Wolfgang and Kollet, Stefan and
                      Vereecken, Harry and Hendricks-Franssen, Harrie-Jan},
      title        = {{C}omparison of different assimilation methodologies of
                      groundwater levels to improve predictions of root zone soil
                      moisture with an integrated terrestrial system model},
      journal      = {Advances in water resources},
      volume       = {111},
      issn         = {0309-1708},
      address      = {Amsterdam [u.a.]},
      publisher    = {Elsevier Science},
      reportid     = {FZJ-2017-07678},
      pages        = {224-238},
      year         = {2018},
      abstract     = {The linkage between root zone soil moisture and groundwater
                      is either neglected or simplified in most land surface
                      models. The fully-coupled subsurface-land surface model
                      TerrSysMP including variably saturated groundwater dynamics
                      is used in this work. We test and compare five data
                      assimilation methodologies for assimilating groundwater
                      level data via the ensemble Kalman filter (EnKF) to improve
                      root zone soil moisture estimation with TerrSysMP.
                      Groundwater level data are assimilated in the form of
                      pressure head or soil moisture (set equal to porosity in the
                      saturated zone) to update state vectors. In the five
                      assimilation methodologies, the state vector contains either
                      (i) pressure head, or (ii) log-transformed pressure head, or
                      (iii) soil moisture, or (iv) pressure head for the saturated
                      zone only, or (v) a combination of pressure head and soil
                      moisture, pressure head for the saturated zone and soil
                      moisture for the unsaturated zone. These methodologies are
                      evaluated in synthetic experiments which are performed for
                      different climate conditions, soil types and plant
                      functional types to simulate various root zone soil moisture
                      distributions and groundwater levels. The results
                      demonstrate that EnKF cannot properly handle strongly skewed
                      pressure distributions which are caused by extreme negative
                      pressure heads in the unsaturated zone during dry periods.
                      This problem can only be alleviated by methodology (iii),
                      (iv) and (v). The last approach gives the best results and
                      avoids unphysical updates related to strongly skewed
                      pressure heads in the unsaturated zone. If groundwater level
                      data are assimilated by methodology (iii), EnKF fails to
                      update the state vector containing the soil moisture values
                      if for (almost) all the realizations the observation does
                      not bring significant new information. Synthetic experiments
                      for the joint assimilation of groundwater levels and surface
                      soil moisture support methodology (v) and show great
                      potential for improving the representation of root zone soil
                      moisture.},
      cin          = {IBG-3},
      ddc          = {550},
      cid          = {I:(DE-Juel1)IBG-3-20101118},
      pnm          = {255 - Terrestrial Systems: From Observation to Prediction
                      (POF3-255)},
      pid          = {G:(DE-HGF)POF3-255},
      typ          = {PUB:(DE-HGF)16},
      UT           = {WOS:000418592800016},
      doi          = {10.1016/j.advwatres.2017.11.003},
      url          = {https://juser.fz-juelich.de/record/840115},
}