A lumped unconfined aquifer model has been developed and interactively coupled to a land surface scheme [Yeh and Eltahir, this issue]. Here, we address the issue of the representation of sub-grid variability of water table depths (WTD). A statistical-dynamical (SD) approach is used to account for the effects of the unresolved sub-grid variability of WTD in the grid-scale groundwater runoff. The dynamic probability distribution function (PDF) of WTD is specified as a two-parameter Gamma distribution based on observations. The grid-scale groundwater rating-curve (i.e., aquifer storage-discharge relationship) is derived statistically by integrating a point groundwater runoff model with respect to the PDF of WTD. Next, a mosaic approach is utilized to account for the effects of sub-grid variability of WTD in the grid-scale groundwater recharge. A grid-cell is categorized into different sub-grids based on the PDF of WTD. The grid-scale hydrologic fluxes are computed by averaging all the sub-grid fluxes weighted by their fractions. This new methodology combines the strengths of the SD approach and the mosaic approach. The results of model testing in Illinois from 1984-1994 indicate that the simulated hydrologic variables (soil saturation and WTD) and fluxes (evaporation, runoff, and groundwater recharge) agree well with the observations. Due to the paucity of the large-scale observations on WTD, the development of a practical parameter estimation procedure is indispensable before the global implementation of the developed LSXGW in climate models.



  Yeh, P.  J.-F. and E. A. B. Eltahir, 2005. Representation of water table dynamics in a land surface scheme. Part II. Subgrid variability, Journal of Climate, 18(12): 1881-1901.a_jcli05a_files/2005%20Yeh%20Eltahir%20partII%20subgrid%20var%20JClimate.pdfa_jcli05a_files/2005%20Yeh%20Eltahir%20partII%20subgrid%20var%20JClimate.pdfshapeimage_8_link_0shapeimage_8_link_1
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