The area I live in gets plenty of rain but unfortunately it all falls in a small number of storms. Although this is okay as far as replenishing the water-table it causes problems with a closed water system and is not sufficient to provide water for plants. The other problem is it causes flooding in a pond that does not have a full time in-out water supply such as a stream or spring.
My ponds normal water level only allows for about 3 inches of extra water to flow in before it breaches the sides and the downhill levy.
My solution was to dig a reservoir about 2 feet away from the pond and install flood/overflow that drains the excess water from the pond and storm runoff into the reservoir. The reservoir also collects the rain that falls into it naturally from the rainfall and when the reservoir reaches capacity it drains to a spillway.
The circulation pump in the reservoir has a valve that lets you divert its flow back into the pond for refilling it in between rainfalls due to evaporation or when doing a partial water change in the pond.
The biggest advantages to this system is number one you conserve your water supply and number two you don't waste the rain that does fall.
Due to the fact that the water comes from rain and from the pond overflow system you end up with water that is much closer to your ponds chemistry making it less of a shock to the ecosystem when it is introduced to your pond.
The system is very easy to make and only requires some PVC pipe, a pump & something to line the reservoir with. The size of the reservoir will depend on the size of your pond and the percentage of that amount that evaporates and/or the amount you normally use when doing a purge or partial water change.
I recommend you build it as small of a diameter as possible while still not going too deep to exceed your pumps height capacity.
Mine is about 4 feet across and 5 feet deep and is plenty of water for me to do my maintenance and still have some left for emergencies.
I have included a picture below and if you are interested I can email you the schematics, instructions and materials needed list that includes info for several alternative material choices (pipe size, liner material etc)
If you want the plans just ask and I will glad to send them to you and for the quickest response or to ask a question directly and securely you can use my websites support service portal to easily and quickly submit your question/request/comment (link included)