Figure 10.3 Effective head as difference between reservoir water level and tailwater level
The Tailwater table specifies tailwater level as a function of release from the reservoir.
To include a Tailwater table, enable ‘Use Tailwater table’ and click 
to browse to an existing table or to create a new table.
Note that a discharge-dependent tailwater level necessitates iterations in the solution of the water allocation problem (demand becomes a function of tailwater level, which depends on reservoir release, which finally depends on demand). Thus, for precise results, often a daily simulation time step should be chosen in connection with hydropower simulations.
Use downstream release from reservoir
Besides the flow through the turbines other releases through spillways and gates may contribute to the tailwater level. If this is the case, enable ‘Use downstream release from reservoir’, and this flow together with the flow through the turbines will be used to estimate the tailwater level in the Tailwater Table.
In cascades of reservoirs, tailwater elevation for a hydropower station can be determined by water level in the next reservoir downstream rather than by discharge from the supplying (upstream) reservoir. If this is the case for a particular model, enable the ‘Has backwater from reservoir below’ option.
