Tham khảo tài liệu 'the official guide to the new toefl ibt part 21', ngoại ngữ, toefl - ielts - toeic phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Professor OK. So we wouldn t want to take out any more than naturally comes into it. The implication is that uh well if you only take as much out as comes in you re not gonna deplete the amount of water that s stored in there right Wrong but that s the principle. That s the idea behind how we manage our water supplies. It s called safe yield. Basically what this method says is that you can pump as much water out of a system as naturally recharges . as naturally flows back in. So this principle of safe yield it s based on balancing what we take out with what gets recharged. But what it does is it ignores how much water naturally comes out of the system. In a natural system a certain amount of recharge comes in and a certain amount of water naturally flows out through springs streams and lakes. And over the long term the amount that s stored in the aquifer doesn t really change much. It s balanced. Now humans come in . and start taking water out of the system. How have we changed the equation Female student It s not balanced anymore Professor Right. We take water out but water also naturally flows out. And the recharge rate doesn t change so the result is we ve reduced the amount of water that s stored in the underground system. If you keep doing that long enough if you pump as much water out as naturally comes in gradually the underground water levels drop. And when that happens that can affect surface water. How Well in underground systems there are natural discharge points places where the water flows out of the underground systems out to lakes and streams. Well a drop in the water level can mean those discharge points will eventually dry up. That means water s not getting to lakes and streams that depend on it. So we ve ended up reducing the surface water supply too. You know in the state of Arizona we re managing some major water supplies with this principle of safe yield under a method that will eventually dry up the natural discharge points of those aquifer .
