to use something in a careful or thoughtful way so that it won't run out

made polluted by adding something harmful or unhealthy

a shortage of food that may cause people to starve

to make something new out of something old

a large area of a country or the world

one thing that causes a certain outcome

to cause something to change

the effect of one thing on another thing

referring to a specific item or thing

very harsh

to happen

enough for a specific need

happening over a large area or to many people

to use up so that there is little or nothing left

water found underground in soil or rock

a way to supply water to land or crops

things that exist in a region that can be used to provide for people

a rule or law that limits something

to provide what is needed to continue doing something

to make something smaller in amount

describing a cause that is not the most important one

to become smaller or decrease

in a way that is important or enough to make a difference

something that is necessary

a need or requirement