246 SOILS 



The bottom of the reservoir should then be har- 

 rowed, and, if necessary, covered with clay and 

 puddled. 



The banks of small reservoirs should have a rise 

 of not more than one foot in two. The top of the 

 bank should be at least two and one-half feet wide. 

 For example, if a bank is five feet high it should 

 be about seventeen feet wide at the base. The 

 rim of it should be sodded or faced with stone to 

 prevent erosion by waves. A circular shape is 

 preferred because it requires the least number of 

 feet of wall to inclose a certain area, and seepage 

 is less. The outlet should be just above the 

 bottom and may be masonry, a plank sluiceway, 

 sewer pipe or wrought-iron pipe, according to the 

 size of the reservoir. It should be provided with 

 a gate. The loss of water by seepage from these 

 small reservoirs varies with the character of the 

 soil, but need not exceed eight inches a year and 

 is usually less. The loss by evaporation is usually 

 much greater, especially in very dry or windy 

 climates. It can be lessened only by planting 

 windbreaks. 



PUMPING WATER FOR IRRIGATION 



Most of the irrigation in this countrv is done by 



i * 



diverting water from streams or reservoirs by 

 gravity. In most cases this is the only kind of 

 irrigation that is at all practicable. But occasion- 

 ally it is necessary or expedient to raise water from 

 the supply to the main ditch or to the field. This 

 is done chiefly by windmills, engines, hydraulic 

 rams, water-wheels. Pumping water for irrigation 

 has become quite common in California, where 

 wells are sunk in or near the beds of underground 



