# Uncontrollable flow

A hydropower producer has direct control over several variables in the hydropower system, such as the amount of water to discharge through each turbine in each hour and how large the opening of a gate in a tunnel should be. Other variables are only controllable implicitly, such as the amount of water contained in a [](reservoir) or the time it takes for water to travel down a long [](river). The natural flow in (partially) regulated rivers and spillage over dams in reservoirs can be steered by timing the discharge of water in the controllable parts of the system. The relationship between these variables are modelled in SHOP, which is crucial for not violating environmental constraints and for timing the production in different parts of the system to the markets.