Update h_boundary_conditions.rst

hai, I added a little bit to it, hope you like it.

source/h_boundary_conditions.rst

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 .. _boundary_conditions:
 
-Boundary conditions
-===================
+Open boundary conditions
+========================
 
-Boundary conditions define the conditions beyond the boundary of the model and allow water to flow into or out of the model domain.
+At open boundaries, one needs to define the conditions beyond the boundary of the model. This will allow allow water to flow into or out of the model domain. These conditions can of course change during the course of a simulation, so for boundary condition types, a time serie can be defined. Boundary conditions are generally defined far away of the region of interest, as one want to study the natural flow, and not the behaviour of the boundary conditions. It is important to choose the location of a boundary condition carefully. Locations, where the flow and the bathymetry uniform are simple, generally give better results. Also, be careful in case of various grid resolutions at the boundary location. Especially, for the water level and discharge boundaries, because for those types, the boundary cells are treated independent of its resolution.
 
 Domains
 -------
 
-Boundary conditions can be used for 1D flow, 2D surface flow and 2D groundwater flow.
+Boundary conditions need to be defined at the open boundaries in the 1D, 2D surface and 2D groundwater domain. 
 
 Types
 -----
 
-A boundary condition must define one variable, so that the computational core can calculate the flow based on the values of the neighbouring node or flowline. You can define one of the following variables in a boundary condition:
+There are various types of boundaries, which enforce the in- and outgoing discharges. 3Di supports the following boundary types:
 
-* Water level, a user defines a water level (time series). This value is fixed at the boundary cell (1D domain) or for all cells along the boundary (2D surface and groundwater domains).
+* Water level, this type fixates the water level and enforces water to flow in or out of the domain to for fill this condition. 
 
-* Flow velocity, a user defines the flow velocity (time series). This value is fixed at the boundary cell (1D domain) or for all cells along the boundary (2D surface).
+* Velocity, this type prescribes the flow velocity. Depending on the local depth at the boundary, the in- or out-going flux is defined. Take care of the sign of the value, as the sign of the velocity determines its direction. 
 
-* Discharge, a user defines the discharge (time series). This value is fixed at the boundary cell (1D domain) or for all cells along the boundary (2D surface and groundwater domains). Please note, the total amount of water is the sum of the discharge over all boundary flow lines.
+* Discharge, this type prescribes the discharge, it defines the flux in/out of the model domain. Generally, this is the boundary condition type one uses for upstream boundaries. Keep in mind the sign of the value, as it is linked to the flow direction. Note, that this value is defined for each boundary flow line in all model domains. However, concerning the 2D (surface and groundwater domains), the total incoming discharge is the sum of the discharge over all boundary flow lines.
 
-* Water level gradient (Sommerfeld boundary)
+* Water level gradient (Sommerfeld boundary), this type prescribes the water level gradient at the boundary. This will try to get the system to an equilibrium condition. Use this boundary condition, when expects a system that is in (near) equilibrium state.
 
-* Total discharge, a user defines the total discharge for a flowpath in 2D domain (surface and groundwater layers). 3Di will then distribute the total discharge to the boundary cells that fall within the cross-section of the flowpath depending on the resistance of each cell to the flow. This resistance depends on the wet area, friction, and vegetation of the cell for surface water, and wet area and hydraulic conductivity of the cell if interflow is included. For the ground water layer, the distribution of the discharge is determined based on the wet area and hydraulic conductivity of the soil for each cell. 
+* Total discharge, a user defines the total discharge for a boundary in the 2D domains (surface and groundwater layers). 3Di will distribute the total discharge over the boundary cells. For 2D surface water, the distribution of the discharge over the full edge is based on a so-called conveyance approach. It uses information of the wet area, roughness, and vegetation of the cell. When an interflow layer is present in the model, the distribution is also depending on the wet area and hydraulic conductivity there. In case of a groundwater boundary condition, the distribution of the discharge is based on the wet area and hydraulic conductivity.  
 
-Note: In case of using different refinement levels at the boundary of the domain, the user should be aware of the difference in water level gradient for boundary type Water level. This stems from the cell size difference that might lead to different gradient, more notably in steep areas.
 
 Time series
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