Edge effects (Water Overlay)
When the Water Module calculates the flow of water, it can "freely" flow across the entirety of the map, until it reaches the edge of the project area. Water cannot flow out of the project area at the edge, nor can any water flow in from outside the project area. The edge behaves as an impenetrable barrier which water cannot pass, and in fact can bounce back from. Both the lack of natural water flow from the cells beyond the edge, as well as the behavior of water hitting the edge and being forced to flow back are known as edge effects.
All hydrological models require some forethought on the existance of edge effects when performing hydrological calculations and analyses. Depending on your use-case, you can consider doing any or all of the following to account for edge effects.
Note that there is no clear-cut single method for perfectly resolving these artifacts. For every situation and use-case you will need to make your own judgement call as to which solution is most appropriate.
Larger project area
By creating a larger project area, the water which flows out of your area of interest does not immediately reflect back in. Instead, it has the room to flow further, continuing onwards to the edge. Additionally, when dealing with water sources such as rain, water from just outside the area of interest can flow in naturally as well, creating a more truthful representation of the amount and movement of the water in your model.
The degree to which this lessens the edge effect depends on the increase in size and the impulse of the water.
By adding boundary conditions, in the form of inlets, breaches, or even just very deep water terrain or a ditch, it's possible to define in greater detail what happens with water flowing into (and potentially out of) the edge. The boundary conditions would allow water which reaches the edge to be removed from the hydrological model, preventing water from flowing back into your area of interest. At the same time, it's possible to define specific points as inlets which add water to the model, also allowing for water to flow into a project area. This can be relevant when modelling a steadily flowing river.
The degree to which this lessens the edge effects depends on the amount of boundary conditions added to the project, their capacities relative to the amount of water they are to process, and their exact placement.
It's not necessarily so that all edge effects are an issue in all use-cases or all calculations. The results of some calculations may already be so inaccurate (for example, because the dataset used to configure the model is very crude), or the level of detail required may not be so high, that the effect of the edge does not have a significant impact on the accuracy of the results. In these cases, it is important to be aware of the presence of such an artifact.