填空题 Today, I want to discuss underground water. We wouldn't like to take out more than naturally comes into it. The {{U}} {{U}} 1 {{/U}} {{/U}}is that if you only take as much out as comes in, you're not going to {{U}} {{U}} 2 {{/U}} {{/U}}the amount of water that stores {{U}} {{U}} 3 {{/U}} {{/U}}there. Right? Wrong. That's the {{U}} {{U}} 4 {{/U}} {{/U}}called safe yield. We can {{U}} {{U}} 5 {{/U}} {{/U}}as much water out as naturally flows back in. And the recharge {{U}} {{U}} 6 {{/U}} {{/U}}doesn't change. So the {{U}} {{U}} 7 {{/U}} {{/U}}is we've reduced the amount of water that stores in the underground system. If you keep doing that long, if you {{U}} {{U}} 8 {{/U}} {{/U}}as much water out as naturally comes in, {{U}} {{U}} 9 {{/U}} {{/U}}the underground water level will {{U}} {{U}} 10 {{/U}} {{/U}}. In the underground systems there are natural discharge points. Well, a drop of water {{U}} {{U}} 11 {{/U}} {{/U}}can mean those discharge points will {{U}} {{U}} 12 {{/U}} {{/U}}dry up. Sustainability and safe yield are {{U}} {{U}} 13 {{/U}} {{/U}}, because what sustainability means is that it's sustainable for all systems that {{U}} {{U}} 14 {{/U}} {{/U}}on the water, for the people who use it, and for {{U}} {{U}} 15 {{/U}} {{/U}}water to the {{U}} {{U}} 16 {{/U}} {{/U}}, like some streams. So, if we are using a safe yield {{U}} {{U}} 17 {{/U}} {{/U}}, we're only {{U}} {{U}} 18 {{/U}} {{/U}}what we take out with what gets recharged, but don't forget water also flows out. Then the underground amount gradually gets reduced and that is going to lead to another problem, the {{U}} {{U}} 19 {{/U}} {{/U}}and streams are going to {{U}} {{U}} 20 {{/U}} {{/U}}up.