Source and Sink terms
Mass conservation is one of the two fundamental laws to which a 3Di simulation is bounded. This is locally, per computational cell valid and globally in the simulation domain. There are various scenarios possible to add and extract water to a simulations. Some of these are linked in 3Di to specific physical processes, others are more generic in order to give some freedom to the users.
Here, laterals, surface sources and sinks and leakage will be discussed. They all act as sources and sinks, but differ in their application. However, they share two major characteristics; i) water can always be added ii) there can never be water extracted which is not there. These characteristics seem trivial, but especially the second one is computationally not always trivial. 3Di will limit the extraction in case of very limited water levels. Therefore, just before the cell will become dry, the extraction discharge will be less, than the users input. This, to guarantee stability and capture reality better 1. This is valid for all types of sources and sinks that are discussed in this section.
A Surface-Subsurface Model; balancing speed, accuracy and reality, Stelling and Volp (to be published)