SPRAY IRRIGATION. 



9 



A simple way to develop ground water is to dig a sump or pit in a 

 swamp, in a sidehill-spring bog, or in seeped lands bordering a stream 

 or canal. In many cases, where water-bearing sand or gravel can be 

 reached near the surface, the sump furnishes a very cheap water sup- 

 ply which can be enlarged to considerable storage by removal of ma- 

 terials. Pumping from a sump or pit requires the simplest kind of 

 machinery and is a possible source of water for spray irrigation in 

 many of the trucking districts of the East and the sur- 

 face irrigated districts of the West. 



Shallow open wells are a modification of sumps, but 

 generally are deeper and smaller in diameter. Their use 

 as sources for irrigation water usually is limited to spe- 

 cial cases where the materials passed through above the 

 water stratum will stand without expensive curbing, and 

 where the water-bearing materials are coarse, resembling 

 gravel or porous rock, and need little screening. 



A common way of developing ground water is by the 

 use of one or more well points. A well point (fig. 3) 

 consists of a section of galvanized-iron pipe which is 

 perforated, usually with elliptical holes, and has a sharp 

 iron plug swaged into place in one end and is threaded 

 on the opposite end. The point is screwed into galva- 

 nized wrought-iron pipe and driven into the earth to the 

 water-bearing stratum. Extra heavy " guaranteed " 

 wrought-iron pipe should be used if the pipe is to be 

 driven through hard, resistant materials. When used in 

 coarse materials the perforations may be left uncovered, 

 although it usually is necessary to cover the body of the 

 point with a perforated brass jacket to prevent sand 

 being drawn into the pump. Where very fine sand is 

 encountered a fine mesh gauze is placed between the pipe 

 and jacket. For irrigation wells it is advisable to use 

 as large perforations as possible, and many wells which 

 yield considerable sand in the beginning improve in this 

 respect as they are pumped. Points of this type are 

 made up to 4 inches in diameter. Two-inch points and smaller can 

 be driven by hand through soil and sandy materials to a depth of 50 

 to 75 feet, while those larger in diameter usually require a machine 

 for sinking. 



Piston or plunger pumps may be connected directly to the casing 

 of a driven well when the water comes within suction lift, which 

 is practically 25 feet at sea level and becomes less as the altitude 

 increases (see Table 5). Where the water draws down to a greater 

 distance from the pump it is necessary to sink the pump in a pit, 

 66687°— Bull. 495—17 2 



Fig. 3. — Well 

 point. 



