An Iterative Procedure For Computing Water Surface Profiles Of Return Flows In Groundwater Modeling.

Pablo. A. Cello, Leticia B. Rodríguez


This communication describes the application of a methodology designed to
improve the representation of water surface profiles along drainage canals within the frame
of regional groundwater modeling. It is an iterative procedure that combines the use of two
public domain computational codes, MODFLOW and HEC-RAS. MODFLOW is a quasi-threedimensional
finite difference model to simulate groundwater flow. The model possesses a
modular structure, each module representing a feature of the hydrologic system such as
return flow to drains, stream-aquifer interactions, recharge, etc. In spite of its versatility,
modeling water surface profiles in drainage canals presents some limitations. The Drains
Module available with MODFLOW simulates groundwater flow to drain canals as a linear
function of the difference between the aquifer hydraulic head and the drain hydraulic head.
The main disadvantage of this module is that considers a static representation of water
surface profiles along drains. Therefore, the proposed methodology uses HEC-RAS, a 1-D
code for surface water calculations, to iteratively estimate hydraulic profiles along drains in
order to improve the aquifer/drain interaction process. The procedure was applied to the
grounwater/surface water system of the Choele Choel Island, Río Negro, Argentina. Although
more testing is needed, preliminary results show the feasibility of the approach. Smooth and
realistic hydraulic profiles along drains were obtained while backwater effects were clearly

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