MAGNET “automatically” captures the effects of major drainage features (lakes, streams, rivers and wetlands) that control prevailing flow patterns under natural conditions. This is done through intelligent use of the aforementioned high-resolution Digital Elevation Models (DEMs) that provide aquifer top information essentially “everywhere” on earth.
The default treatment in MAGNET is to calculate, during the simulation process, instances where the groundwater head exceeds the land surface elevation, i.e., where gaining rivers / lakes / wetlands are found, allowing groundwater to leave the aquifer as a sink of water (i.e., groundwater is lost as surface seepage). This approach captures the spatially-distributed aquifer drainage effects of groundwater to gaining surface water bodies (groundwater-fed lakes, streams, wetlands, and springs) as part of the robust solution process. This is possible because the surface water stages (elevations) are embedded in the high-resolution DEM datasets used for assigning the aquifer top elevations.
The ease with which water can drain from the aquifer to the surface is controlled by a parameter called Surface Drain Leakancy, a factor representing the hydraulic conductivity per unit thickness of the land surface. The leakancy can be adjusted or fine-tuned to better reproduce the observed surface water network, e.g., if the leakance is too low, the flooded area will be large, and vice versa. The input field for this parameter is provided in the Aquifer Attributes tab of the Domain Attributes menu (see Figure 13).
If you wish to “turn off” treating the entire land surface as a drain, and instead represent surface water bodies using conceptual features (zones and lines – see Section 5.3), you can set the Surface Drain Leakancy to zero.
Losing surface water bodies (where the groundwater head is less than the surface water body’s stages), are not represented in this default treatment, but MAGNET users can add zone or line features to the model as lakes or streams to allow two-way exchange of water and/or stages that vary with time. MAGNET also includes an Import Shapefile tool to add many streams/rivers, lakes and wetland features in this way, or users can automatically extract surface water features (lakes and streams) from the MAGNET Data Center and add them as conceptual features to the model.