162 EGYPTIAN AGRICULTURE. 



check the flow, and they prevent the growth of weeds 

 which soon choke a canal. 



Drains should be nearly the same size as canals for 

 corresponding areas, although only one third of the irri- 

 gation water used finally reaches the drains. The rest of 

 the water is lost by evaporation, absorption in the soil, 

 and exhalation by vegetation. 



Whilst a canal runs full, a drain should carry only a 

 little water in the bottom so that the water surface should 

 be nearly a metre below the ground surface. It is only 

 when the water in drains is at a low level that the water 

 table in the soil is kept down and capillarity prevented 

 from causing salt incrustation on the surface of the ground. 

 It is for these reasons that drains are made large. A drain 

 running full whilst removing temporary excess of water 

 is not fulfilling its true function of keeping down the 

 water table. 



Drains require only small banks, unless the banks are 

 needed for roads, and excess soil may be spread over 

 the adjoining land. They should, equally with canals, 

 receive a thorough annual clearance, but the silt deposit 

 should be very little. No red water should ever be seen 

 in drains as all silt should have been deposited on 

 the fields and only the clear effluent run off. Drains 

 should have a clear course throughout their entire length, 

 unobstructed by any pipes or syphons. Where pipes are 

 absolutely necessary, they should be of large section, at 

 least one half the sectional area of the drain, to prevent 

 heading up of water in the drain. It is better to syphon 

 a canal over a drain, rather than to carry a drain by a pipe 

 below a canal or road. 



