56 Water and Land 



It is commonly estimated that at least one-half of 

 this rainfall is evaporated, in part from soil and water 

 surfaces, but much more from growing vegetation; for 

 the transpiration of plants gives back immense quanti- 

 ties of water to the atmosphere. Hellriegel long ago 

 showed that a crop of com requires 300 tons of water 

 per acre : of potatoes or clover, 400 tons per acre. At 

 the Iowa Agricultural Experiment Station it was shown 

 that an acre of pasturage requires 3,223 tons of water, 

 or 28 inches in depth (23^ acre-feet). Before the days 

 of tile drainage it was a not uncommon practice to 

 plant willow trees by the edges of swales, in order that 

 they might carry off the water through their leaves, 

 leaving the groiond dry enough for summer cropping. 

 The rate of evaporation is accelerated also, by high 

 temperatures and strong winds. 



The rain tends to wet the face of the ground every- 

 where. How long it will stay wet in any given place 

 will depend on topography and on the character of the 

 soil as well as on temperattire and air currents. Show- 

 ers descending intermittently leave intervals for com- 

 plete run-off of water from the higher ground, with 

 opportunity for the gases of the atmosphere to enter 

 and do their work of corrosion. The dryer intervals, 

 therefore, are times of preparation of the materials 

 that will appear later in soil waters. Yet all soils in 

 humid regions retain sufficient moisture to support a 

 considerable algal flora. Periodical excesses of rainfall 

 are necessary also to maintain the reserve of groimd 

 water in the soil. Suppose, for example, that the 35 

 inches of annual rainfall at Ithaca were uniformly 

 distributed. There wotdd be less than one-tenth of an 

 inch of precipitation each day — an amount that would 

 be quickly and entirely evaporated, and the ground 

 would never be thoroughly wet and there wotdd be no 

 ground water to replenish the streams. Storm waters 



