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The "boezem" in Dutch water management

The boezemstelsel is a Dutch reservoir system into which a polder evacuates excess water upward, and from which water may be drawn.

With a prescribed but variable surface level, the boezem system is a buffer between the polder and the open water of a river or the sea. And maintenance of strict levels is necessary in the polder. The rich organic nature of much of western Netherlands' soil means that if it's exposed to oxygen, it will decompose — causing the land to subside. (And it has — this is the reason that delta land is now under sea-level.)

A boezem is usually shared by several polders, and often consists of a network of diverse bodies of water — canals, lakes, ditches and/or waterways. The commonality between boezems is their variable (but closely-regulated) water-level — and their function as reservoir in the evacuation of excess water.

At release, whether by pump or by sluice, the flow must be controlled for quality, quantity, and speed.

Electric pumps — formerly windmills, steam and then diesel engines — elevate water from the polder floor to the boezem. The boezem holds the water until agreed measurements allow its release — usually into a river. Volume, speed, and quality of the output must be considered.

The water in the boezem can also be allowed back into the polder in the management of ground water there.

And, of course, agriculture always needs water. In a drought, the boezem can stabilize the supply.

Mostly, though, a boezem holds water pumped out of the polder and releases it (by sluice or, if necessary, further pumping) into the wider ecosystem on its way to sea.

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* "Boezem" also means "bosom," or "breast." This can lead to some search difficulties. I know, for example, that the average cup size of Dutch and Belgian women is a "C."

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