NURSERY PRACTICE FOR PRAIRIE-PLAINS PLANTING 61 



The portion of the casing below water is then perforated by means of 

 a special tool devised for this purpose. Details on installations and 

 types of casings are described by Ewing (10). 



Of equal importance is proper well development by controlled 

 pumping and surging, or the removal of the particles of sand that 

 bridge over the openings in the well screen or casing and may appre- 

 ciably reduce the flow of water. They are commonly removed by 

 working a plunger up and down with a well rig, especially in connec- 

 tion with the development of a new well. The plunger is slightly 

 smaller in diameter than the casing and its action draws the fine 

 particles into the casing where they may be removed with a sand 

 bucket. Another method of surging (3d) used generally with the 

 smaller-sized piston or horizontal centrifugal pumps, especially when 

 attached to sand points, consists of pumping water down into the 

 well pipe under pressure so that the water is forced through the 

 screen in the sand point. This process cleans the screen by carrying 

 the finer sand and soil particles some distance away from the screen, 

 thus increasing the pore space around it and permitting movement of 

 a larger volume of water through it when pumping i> resumed. 



When the well screen or point is attached directly to the suction 

 line, it is essential that it be some 10 to 20 feet below the surface of the 

 water to allow for drawdown, or lowering of the water table. Other- 

 wise when the water level drops the pump will not function properly. 

 The degree of drawdown depends upon the ability of the water to 

 flow in fast enough laterally, and this in turn depends largely on the 

 size of the particles in the water-bearing stratum, the capacity of the 

 pump, and the length of time of pumping. Very often in situations 

 where the water-bearing stratum is a medium to fine sand, it is neces- 

 sary to put down two or three weUs hooked up to a single intake or 

 suction pipe in order to afford sufficient flow. The spacing of the 

 points or wells will vary from 30 to 50 feet, with the pumps of 800 

 to 1.000 gaUons-per-rninute capacity. 



Two other types of pumps used occasionally for shallow wells where 

 the suction lift is not over 20 feet are piston and rotary pumps. 

 These do not. however, have the capacities demanded in a large 

 nursery. The rotary pump particularly is not well adapted to situa- 

 tions where the water contains any appreciable amount of sand or 

 grit, as this material cuts the rotors and in short order seriously 

 reduces the maximum suction lift of the pump as well as its capacity. 



i>fj:p-\vell pumps 



Where the vertical distance from the pump location to the lowest 

 water level is more than 20 to 25 feet, there are several types of pumps 

 available, namely, vertical turbine, vertical centrifugal, deep-well 

 cylinder, and air-lift pumps. 



The vertical turbine and the vertical centrifugal types are similar 

 in that each has a casing which extends below the water table, and 

 which houses an impeller-equipped shaft capable of rotating at speeds 

 of 800 to 3.500 r. p. m. The impellers are housed in bowls near the 

 bottom of the casing. In popular usage the term "vertical centrifugal 

 pump" is generally applied to the type which has only one impeller 

 blade of large diameter and is well adapted for pumping from depth- 



