I want to discuss some ideas about how we manage underground【B1】______. If we only take as much out as comes in, we’re not going

Good morning, ladies and gentlemen. Last week we talked about some important terms in environmental science. Sure you still remember them, right? So in today’s environmental science class, I want to discuss a few other terms here, actually some ideas about how we manage our resources. Let’s talk about what that means. If we take resources like water, now maybe we should get a little bit more specific here, back from a more general case and talk about underground water in particular. So hydro geologists have tried to figure out how much water can we take out from underground sources. That has been an important question. Let me ask you guys, how much water, based on what you know so far, could you take out of, say, an aquifer under the city, as much as what gets recharged?
    Ok. So we wouldn’t like to take out more than naturally comes into it. The implication is that, well, if you only take as much out as comes in. You’re not going to deplete the amount of water that stores 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 message says is that you can pump as much water out of the system as naturally recharges, as naturally flows back in. So this principle of safe yield is 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, and a natural system of certain matter of recharge coming in and certain matter of water naturally flowing out through springs, streams and lakes, and over long terms 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? It’s not balanced any more?
    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 stores 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 level will drop. And when that happens, they can’t fix service water. How? Well, underground systems there are natural discharge points, places where the water flows out from the underground systems, and into lakes and streams. Well, a drop of water level can mean those discharge points will eventually dry up, and that means water’ s not getting to lakes and streams that depend on it. So we end up reducing the surface water supply, too. You know, in the state of Arizona, we’re managing some major water supplies with the principle of safe yield and under this method they will eventually dry up the natural discharge points of those aquifer systems. Now, why is this issue? Well, aren’t some of you going to want to live in the state for a while? Won’t your kids grow up here, and your kids’ kids? You may be concerned with "does Arizona have water supplies which is sustainable?" A key word here. What that means? The general definition of sustainable is whether it is enough to meet the needs of the present without compromising the ability of the future to have the availability to have the same resources.
    Now, I hope you see these two ideas are incompatible: sustainability and safe yield. Because what sustainability means is that it’s sustainable for all systems that depend on the water, for the people who use it, and for supplying water to the dependent water bodies like some streams. So I’m going to repeat this. So, if we are using a safe yield method, we’re only balancing what we take out with what gets recharged, but don’t forget, water also flows out naturally. Then the amount that has stored underground gradually gets reduced, and that is going to lead to another problem; these discharge points with water flowing out to the lakes and streams, they’re going to dry up. Ok?