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， in underground systems there are natural discharge points， places where the water flows out from the underground systems， out of 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 supply which is sustainable?" Key word here.
What does 6'sustainable" mean? 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， 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 stored underground gradually gets reduced， and that is going to lead to another problem; the lakes and streams are going to dry up. OK?
17.
In this first part of our lecture today we shall consider some basic facts about the workings of the brain. It may surprise you to know that the human brain is only about three pounds in weight. Men's brains tend to be slightly bigger than women's. The brain is grayish white in colour and is wrinkled rather like a shelled walnut. The surface area of the brain is thus much larger if the brain were smooth. Each working part of the brain is a nerve cell known as a neurone and there are at least a million million neurons in your brain. The neurones are， of course， tiny. The neurones both conduct electrical impulses and generate the power to send messages through the nervous system. The electrical impulses cause chemical reactions in cells to take place and these chemical reactions could be said to be responsible for the passing of messages. It has been estimated that from 100，000 to 1，000，000 different chemical reactions of this type take place in your body every minute.
In this second part of our lecture， let us now look in more detail at the structure of the brain. The brain can be divided into three main parts - the hindbrain， the midbrain and the forebrain. And each of these parts can be subdivided. Each area of the brain has its own particular function. The hindbrain consists of the brain stem and the cerebellum. The brain stem connects the brain with the spinal cord along which all messages to and from the brain must pass. The brain stem also controls the main automatic functions of the brain， such as heartbeat， breathing and consciousness. The cerebellum controls balance and coordination.
……
展开
