Just click on this website link to learn about the "water cycle" and how we get our water supply:
http://www.epa.gov/safewater/kids/flash/flash_watercycle.html

Where does our water come from?

The City of Garland and many surrounding cities, including
Rowlett and Sachse, receive their water supply from the
North Texas Municipal Water District (NTMWD) using
water resources from Lake Lavon and Cooper Lake.

You can visit Lake Lavon's own website to learn more about
the lake, where it is located, how large it is, and other
interesting information.

http://www.swf-wc.usace.army.mil/lavon/

Why do we sometimes have
water use restrictions?

When we do not receive rain for long periods of time, we have what is
known as a "drought". Sometimes it will rain heavily in one place and hardly at all in another.
So, even though you received a lot of rain at your house, it may not
have rained at the lakes that supply our water. The amount of water
we have in our supply lakes is part of what tells us when we need to
have water use restrictions.

Due to lack of rain or high demands on water use (like in the summer when everyone
needs to water their yards), the NTMWD and the City of Garland may have to ask
people to cut back on the amount of water they use everyday.

Water conservation. A little goes a long way.