The Water Cycle (1946)

every Mist every cloud every rainstorm is part of a never ending cycle on which all life depends every bubbling spring every rushing torrent every smooth flowing river is a part of this life sustaining cycle in this cycle water from the broad expanse of ocean evaporates and forms clouds from clouds to Earth it falls as rain finding its way into streams and rivers this water flows Back to the Sea let us examine this cycle again more closely by evaporation tiny water particles escape from water surfaces in the air these particles may come together and form droplets that appear as clouds these clouds May grow in size droplets in the clouds May combine to form larger drops that fall to the Earth as rain eventually most of the rainwater joins in rivers and flows Back to the Sea this continuous circulation of water goes on and on this is the water cycle let us consider the evaporation phase of this cycle water like all matter is made up of extremely small particles called molecules these molecules are always in motion some of the faster moving molecules escape from the surface and enter the Air Above This is evaporation a few of them return as the sun warms the water the molecules move faster and so more of them Escape we say that the space above the water is saturated when as many water molecules return to the water as leave it if the temperature of the space above the water rises the number of molecules entering this space again exceeds the number returning until once more a balance occurs producing saturation if the temperature of the space Falls some of the molecules form droplets of water this is condensation this condensation may form clouds close to the surface of the Earth such clouds we call fog ordinarily however when we speak of clouds we mean those that form higher in the sky the formation of these clouds depends upon the movement of air warm air near the Earth's surface moves upward because it is less dense than cold air as a current of warm air rises the pressure of the atmosphere upon it decreases and the air expands and cools this cooling causes the moisture in the air to condense forming clouds and sometimes producing local showers when a large mass of cold air as represented here at the right meets a large mass of warmer air as represented at the left the warmer air rises expands and cools clouds begin to form and a storm may result here we see such a storm developing this kind of storm may extend over a broad geographical front most of the rain that falls to the Earth runs off the surface of the ground as soon as it falls tiny trickles of water merge into rivulets the rivulet flow along sometimes trickling over rocks and sometimes tumbling down hillsides as miniature waterfalls but always increasing in volume as the rivulets join Mountain Streams turn along their Rocky ways down the steep slopes finally they reach the lowlands where they flow more slowly the small streams flow together and form the larger Rivers the rivers flow onward to the Sea some of the water that falls to Earth sinks into the ground and seeps slowly downward through the porous layers until it reaches a depth where the soil is already saturated the downward moving water raises the level of this saturated Zone the upper level of this saturated Zone we call the water table thus we may find the water table below the surface of the ground but where the water table is above the ground we find lakes and swamps at all times there are some molecules of water that rise to the surface of the ground and Escape by evaporation some of the moisture enters the roots of plants and trees after entering through the roots the water passes upward into the branches water evaporates from a plant or tree through its leaves large amounts of water return to the air in this way we have seen that part of the rainfall sinks into the soil and forms groundwater we have also seen that great amounts of this water return to the Atmosphere by evaporation from the soil and from leaves of plants but again most of the water that falls as rain flows through rivers to the Sea during heavy prolonged rain large amounts of water May reach the rivers causing destructive floods man cannot control the amount of rain that falls to Earth and swells the rivers but to some extent he can prevent floods such as these by controlling the circulation of surface water where growing plants cover the ground they hard the waters flow most of the water then sinks into the ground instead of running off the surface but wherever erosion has left Barren ground and deep gullies dams made of Earth and brush may be used to control the flow of water large dams such as this one provide provide constant control of Mighty rivers in times of heavy rainfall this Dam holds back the flood waters after the threat of a flood has passed water can be released slowly to continue its flow to the Sea we have seen how flowing water can be by vegetation and by dams Great and Small for man's use as well as for his protection but evaporation of water from the ocean formation of clouds in the sky precipitation of rain from the clouds and circulation of rain water on the Earth go on and on without this eternal water cycle life on Earth could not exist