CH. V 



PRELIMINARY OPERATIONS 159 



branch out various secondary canals which lead the 

 waters to different sections of the ground, while the 

 distributing channels take the water to the particular 

 plot of ground which they are destined to irrigate. 



The above system is that used for perennial irriga- 

 tion, i.e. for that which has to be carried on practic- 

 ally throughout the year. It may also be used for 

 shorter periods, and if required only when the source of 

 supply is at the flood, the main canals are made more 

 shallow, being so constructed that the water flows into 

 them only when the river, lake, etc., has reached a 

 certain level. This method of irrigation, called Jioocl- 

 irrigation, may become necessary when the demand on 

 the water-supply is greater than can be supplied to all 

 applicants all the year round. In this case the most 

 common method of supplying water is by 6cmn-irrigation. 

 The area, having been carefully surveyed and levels 

 taken, is divided into a series of basins which are 

 bounded by dykes made sufficiently strong to hold 

 up a certain depth of water. These basins are flooded 

 each in turn, the water being allowed to stand and 

 gradually to soak into the soil, after which the crops 

 are sown or planted. 



On land with a very gentle gradient basin-irrigation 

 may be obtained from rainfall by constructing small 

 dams across the lower side of the ground and along the 

 sides to retain the surface drainage, which is allowed to 

 soak in, after which the crops are put in. For afforesta- 

 tion works basin - irrigation would probably be more 

 suitable to sowing than to planting, excepting on sandy 

 soils. 



AVhere irrigation is obtainable it is useful for the 

 reclamation of salt soils. The frequent washing out 

 of the soil may make it sufficiently sweet to be able 

 to grow the trees required. This process of sweeten- 

 ing may be assisted by growing crops which can 

 thrive in a soil which contains some salt, such as e.g. 

 the common So?yhum or Dura, the " Fox-tail " sedge 

 (Juncellus alopecuroides), etc. 



