Coupling Approaches for Surface Water and Groundwater Interactions inWatershed Modeling

摘要

In the core of an integrated watershed model is the coupling among surface waterand subsurface water flows. Mathematical, there are two approaches ofsurface/subsurface coupling based on the physical nature of the interface: continuousor discontinuous assumption. Physically, only the continuous case exists in nature.However, when a far less permeable layer exists at the interface, the layer may beremoved for computational efficiency. Under such circumstances, the discontinuoussimplification may be justified. Numerically, there are three strategies of couplingbetween surface water and groundwater water: time-lagged, iterative, andsimultaneous solutions. Current literature is dominated by the discontinuityassumption with the simultaneous solution strategy. Since modelers often resort tothe simplest, fastest schemes in practical applications, it is desirable to quantifypotential errors and the performance specific to each coupling scheme. This paperevaluates these coupling schemes in watershed modeling with WASH123D.Numerical experiments are used to compare the performance of each couplingapproach and strategy for different types of surface water and groundwaterinteractions. These experiments are done in terms of errors in state variables (e.g.,water depth and pressure head) and fluxes (e.g. infiltration/seepage rate). It is foundthat different coupling approaches and strategies are justifiable for only the specificflow problem of physical setting of interfaces and the specific scale of time andspace. Therefore, for practicality and for accurate and efficient simulations, awatershed simulator should include various options of mathematical approaches andnumerical strategies. However, the time-lagged strategy should be avoided since itgenerally produces too much error in solutions in fluxes, thus causing problem ofmass conservation across the interface.