42 IOWA ACADEMY OF SCIENCE Vol. XXVI, 1919 



snow. The Mississippi Valley is in the region of the prevailing 

 Westerlies, and practically all of the water received as rain and 

 snow comes directly or indirectly from the Gulf of Mexico. Small 

 amounts may come from the Atlantic Ocean. 



The moisture laden winds from the Gulf travel inland, become 

 a part of the great cyclonic movements which travel across the 

 continent from west to east and supply the Mississippi Valley 

 with an abundance of water making it the granary of the world. 

 The moisture received as rain or snow is distributed as follows : 

 runs directly off — the run-off, collects into ponds and lakelets, or 

 escapes through drainage lines, and eventually returns to the Gulf 

 from which it came. A second and much larger portion soaks into 

 the ground. The larger part of the water which enters the ground 

 is pumped to the surface through the force of capillarity and by 

 plants and returned to the atmosphere as vapor. A lesser part 

 descends through the force of gravity and becomes a part of the 

 "permanent" ground water supply. The water supply system for 

 the Mississippi Valley region comprises a central pumping sta- 

 tion, the Gulf of Mexico, the great cyclonic movements constitute 

 the distributing system and the lakes and lakelets serve as a re- 

 serve supply. 



The productivity of the valley region is dependent primarily 

 upon the adequacy of the water supply, that is a sufficient amount 

 of water properly distributed in time and space. No evidence is 

 available to show that man can vary the first factor, the total rain- 

 fall. The distribution can be varied both in time and space 

 through drainage and irrigation and methods of cultivation, as far 

 as farm crops are concerned. The natural distribution of water is 

 variable. The porous soils and subsoils tend to equalize such 

 variability. Water so held is available when within the reach of 

 plant roots with the aid of .capillarity. The zone of perpetual 

 groundwater is the reservoir. The height to which capillarity 

 can lift varies inversely as the diameter of the capillary tubes. In 

 arid and semi-arid regions the ground water level may be so low 

 that capillarity fails to bring the water within the reach of plants, 

 while in humid regions the level may be so high as to drown many 

 plants. 



The ground water level is not a constant but varies with the 

 seasons or through a term of years. The ground water level is 

 important from an agricultural standpoint. Where it is too high 

 drainage may be required and where it is too low irrigation may 

 be necessary. . 



