Surface water and groundwater are one resource
Groundwater and surface water are connected parts of the same system. Surface water is often where groundwater daylights. The modeling question is not whether they interact, but how much surface-water detail is needed for the decision.
Level 1: fully coupled SW-GW
The most rigorous approach is to model surface water and groundwater together. This is physically general, but computationally intensive and data demanding. It introduces more parameters and calibration burden, and can increase uncertainty when the data are insufficient to constrain the coupled system.
Level 2: DEM surface as drain — emergent surface water
The opposite approach is not to impose mapped surface-water features at all. Instead, groundwater discharges where simulated head intersects the land surface. A DEM surface acts as a one-way head-dependent drain. This is elegant and powerful for seeps, wetlands, shallow water bodies, gaining streams, lakes, and groundwater-dependent ecosystems, especially where features are small, unmapped, fluctuating, or not known ahead of time.
Most surface water bodies function as drains much of the time. Modeling the DEM surface as drainage can capture a large amount of surface-water drainage effect without prescribing every stream and wetland.
Important limitation of DEM-as-drain
A drain only removes water. It does not represent losing streams, losing lakes, riverbank filtration, or situations where surface water recharges groundwater. If modeling RBF near a major river, the river cannot be represented merely as a one-way drain.
Level 3: mapped hydrography as boundary conditions
The third approach is to impose mapped streams and lakes as boundary conditions linked to IGW-NET hydrography. These may be represented as one-way drains or two-way flux boundaries. This is more general than DEM-as-drain and is important for major rivers, lakes, and managed surface-water systems.
Stage uncertainty and infinite-source risk
Mapped river stage is rarely measured everywhere in a network. It is often derived from DEM elevation. If streams are small and the DEM is coarse, stage can be significantly wrong, especially in headwaters, complex topography, cliffs, discontinuities, and narrow valleys. When a surface-water boundary prescribes head, it can behave like an infinite source or sink. That is often inappropriate for shallow wetlands, small headwater streams, and fragile groundwater-dependent ecosystems.
For small, shallow, or uncertain surface-water features, a drain representation is often safer than prescribing an infinite source of water.