Sometimes while modeling, we might have both discharge (flow) boundary on the upstream (source) and downstream (sink). IN these cases, the initial condition of water surface elevation is very crucial. If we have flow boundary at the upstream and elevation boundary at the downstream then the initial water surface elevation doesn’t matter a lot because the model will adjust automatically with the change in elevation on the downstream. However, it is not the same with flow boundary at both upstream and downstream. Two different initial conditions would give totally different results. I want to know what is the best procedure to get the correct initial surface elevation while using both discharge boundaries.
This is quite a common issue with hydrologic modeling. The initial water level will help set the initial volume of the reservoir. It is normal practice to set the initial WL to the measured value. To achieve mass balance you need a good conceptual model – that is a model that is physically meaningful, and accounts for evaporation, precipitation, groundwater refill and loss, errors in inflow and outflow (commonly up to 10%). While the calculated mass balance accumulation may not be zero at small intervals over the longer term water mass should balance.
As you say, you could fix the problem of WL by setting an open downstream boundary but this probably won’t reflect physical reality. You may want to use this approach if you just want to look at hydraulics in system.