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Flood and Drain is a method of controlling the water flow within an aquaponics system. growbeds filled with a growing media such as gravel or expanded clay pellets are repeatedly flooded and drained so that most or all media is immersed. The flooding should occur frequently enough that the media remains wet at all times. Typically, flooding is stopped an inch or 2-3 centimeters below the surface of the media. Draining may be complete, or may leave an inch or 2-3 centimeters of water in the bottom of the growbed. The flooding and draining provides water, oxygen and nutrients to the plants in the growbed. Additionally, the growing media becomes a giant biological filter, housing the bacteria that break ammonia down into nitrite and then nitrate.

[edit] Traditional Flood and Drain

Traditional flooding and draining of a growbed involves water flowing into a growbed at a greater rate than outflow, with a continuous drain. The pump is switched on, and when the water level has reached the desired height in the growbed, the pump is stopped. Since the outflow is continuous, after the inflow is stopped, the water level in the growbed begins to fall. This type of Flood and Drain requires the pump flow rate (inflow) to be higher than the outflow of the drain pipe and also requires the pump to be cycled (switched on and off). Often a pump is used that has a much higher flow rate than the outlet, so a stand pipe is used which has small holes at the bottom to let a system drain down and the top is wide open to allow the excess flow of overflow out of the system from the top; aka "ebb and flow".  Some are of the opinion that this can be hard on the pump, but also, controlling the pump cycle requires a timer or float switch. Tuning the cycling will require adjusting timing/float levels, adjusting the flow of the pump, and using different pipe diameters.

[edit] Flood and Drain with an Autosiphon

The next step in the evolution of Flood and Drain came about by trying to reverse the process, having a smaller pump running continuously and a drain pipe larger so that the outflow from the GB is greater than the inflow. Somewhere along the line it has become known as “autosiphoning”. The draining process is initiated when the water level in the growbed reaches a certain height, but timers or float switches are not used. Instead, the drain is constructed as a siphon, so that once the siphon is started, the water in the growbed drains until air is taken up into the siphon. Tuning the cycling will require adjusting the flow of the pump, and using different diameter hoses or pipes to construct the autosiphon.

[edit] Flood and Drain with other Mechanisms

There is work being done by more adventuresome aquaponics hobbyists to use car door actuators to control the flooding and draining of growbeds.