I believe that in this context this would be an amount of water that a particular party is entitled to purchase and/or take from another party (from the amount of water to which the other party holds a right) under the terms of a formal agreement and at the descretion of the purchasing or taking party (thus optional). It's thus a reserve in a certain sense, in that it can be used if the amount of water available to the taking party from its own right(s) is insufficient to meet its needs. In the context of your question, it might be that the options are negotiated with respect to a publicly held water right.

As for the measurement and control aspects (Überwachung), that probably relates to monitoring the quantity of water and/or flow drawn when the option is exercised.

Some examples from the US context:

Choices Article - The Evolving Western Water Markets
... benefiting not only the owner of a water right, but also the public at ...A specific type of leasing — the option agreement — is gaining currency in ...
www.choicesmagazine.org/ 2005-1/environment/2005-1-12.htm

[PDF] The Evolving Western Water Markets
... owner of a water right, but also the public at large. Public ... An option agreement negotiated. in advance of the water year helps the ...
www.choicesmagazine.org/2005-1/environment/2005-1-12.pdf