[00:00.00]Listen to part of a talk in an environmental science class.
[00:03.91]Professor: So I wanted to discuss a few other terms here ... actually, some, uh, some ideas about how we manage our resources.
[00:13.82]Let’s talk about what that …what that means. [00:16.81]If we take a resource like water. ..well, maybe we should get a little bit more specific here-back up from the more general case-and talk about underground water in particular.
[00:28.90]So hydrogeologists have tried to figure out... how much water can you take out from underground sources? [00:36.74]This has been an important question.[00:38.81]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?
[00:47.70]Male Student: As ... as much as would get recharged?
[00:51.01]Professor: OK. So we wouldn't want to take out any more than naturally comes into it. [00:57.24]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?
[01:07.28]Wrong, but that’s the principle. [01:09.00]That’s the idea behind how we manage our water supplies. [01:13.60]It’s called "safe yield.“[01:16.54]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.
[01:27.37]So this principle of safe yield-it's based on balancing what we take out with what gets recharged. [01:35.95]But what it does is, it ignores how much water naturally comes out of the system.
[01:41.82]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. [01:50.64]And over the long term the amount that’s stored in the aquifer doesn’t really change much. [01:55.43]It's balanced. [01:56.76]Now humans come in . . . and start taking water out of the system. [02:01.42]How have we changed the equation?
[02:04.12]Female Student: It’s not balanced anymore?
[02:06.32]Professor: Right. We take water out, but water also naturally flows out. [02:11.65]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.
[02:18.78]If you keep doing that long enough-if you pump as much water out as naturally comes in-gradually the underground water levels drop. [02:27.18]And when that happens, that can affect surface water. [02:30.95]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. [02:40.86]Well, a drop in the water level can mean those discharge points will eventually dry up. [02:45.92]That means water’s not getting to lakes and streams that depend on it. [02:49.97]So we’ve ended up reducing the surface water supply, too.
[02:53.71]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 systems.
[03:06.01]Now, why is this an issue? [03:09.03]Well, aren’t some of you going to want to live in this state for a while? [03:13.28]Want your kids to grow up here, and your kids' kids? [03:16.57]You might be concerned with . . . does Arizona have a water supply which is sustainable-key word here? [03:24.29]What that means . . . the general definition of sustainable is will there be enough to meet the needs of the present without compromising the ability of the future to have the availability ... to have the same resources? [03:37.91]Now, I hope you see that these two ideas are incompatible: sustainability and safe yield. [03:44.60]Because what sustainability means is that it's sustainable for all systems dependent on the water-for the people that use it and for... uh, for supplying water to the dependent lakes and streams.
[03:57.79]So I’m gonna repeat this: so if we're using a safe-yield method, if we're only balancing what we take out with what gets recharged, but-don’t forget, water's also flowing out naturally. [04:09.39]Then the amount stored underground is gonna gradually get reduced and that’s gonna lead to another problem. [04:15.60]These discharge points-where the water flows out to the lakes and streams-they’re gonna dry up. OK.
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