Representing Preferential Flow through Variably-Saturated Soils with
Surface Ponding in a Large-Scale Land Surface Model over the
Conterminous US
Abstract
Most land surface models (LSMs) do not explicitly represent surface
ponding, infiltration of ponded water, or the soil macropore effects on
infiltration, percolation, and groundwater recharge. In this study, we
implement a dual-permeability model (DPM) based on the mixed-form
Richards’ equation, which solves pressure head continuously across
unsaturated and saturated zones while conserves mass, into the Noah-MP
LSM to represent slow flow through soil matrix and rapid flow through
macropore networks. The model explicitly computes surface ponding depth,
infiltration of ponded water, and runoff beyond a ponding threshold
(infiltration-excess runoff) by switching the atmospheric boundary
condition between head and flux boundary conditions. The new model also
provides two optional soil water retention models of Van Genuchten (VG)
and Brooks-Corey (BC). Model experiments over the conterminous US
indicate that 1) surface ponded water and its runoff contribute
substantially to seasonal variations in total water storage and peak
flows in wet regions with low soil permeability (e.g., the Lower
Mississippi River and surrounding regions), 2) the VG model produces
drier topsoil with less soil surface evaporation than does the BC model
with the Clapp-Hornberger parameters, especially during droughts and in
dry regions, better matching remote sensing soil moisture, and 3) DMP
produces more runoff with increased subsurface runoff, thereby improving
the modeling skill at monthly scale over all subbasins of the
Mississippi River, especially for low flow events. This study also
highlights the importance of consistent representations of soil and
plant hydraulics in Earth System Models to modeling ecosystem drought
resilience.