Water cycle:
The recycling of water between the earth and the atmosphere 
An ecosystem reuses water through the water cycle. Water enters the atmosphere as vapour from bodies of water and from the soil. Plants lose water through the pores in their leaves. Water vapour in the atmosphere condenses and forms clouds. The water droplets return to the earth as rain, snow, sleet, or hail and are used again by plants and animals and the cycle continues.

Water energy:
Energy produced by using the power of moving water

Water vapor:
The gas (gaseous) state of water

Well:
a dug or drilled hole used to get water from the earth.

Wetland:
land (marshes or swamps) saturated with water constantly. These lands mostly are near our coastlines or other areas of large amounts of water.

Wilderness:

land remaining in basically wild condition, with few if any traces of human activities.

Wildlife:

animals living in the wilderness without human intervention.

Wildlife refuges:

land set aside to protect certain species of fish or wildlife 

Windpower:

power or energy derived from the wind (via windmills, sails, etc.). We now have Windpower electric farms, that produce electricity for us to use.

Wild flower 
A flowering plant that grows in woods, deserts, or other natural areas and reproduces / seeds itself without human intervention

Wind breaks:
The practice of planting trees and shrubs to protect fields from soil erosion by wind 

Wind energy :
Energy from moving air