DRAINAGE AND IRRIGATION 31 



est. A narrow strip lying east of these states is called the 

 semi-arid region. It takes in parts of Kansas, Nebraska, 

 North Dakota, Oklahoma, South Dakota, and Texas. All 

 the remainder of the United States in ordinary seasons is 

 humid. In wet seasons many parts of the above-named states 

 do not need irrigation. Besides the states mentioned, irriga- 

 tion for rice growing is practised in Georgia, Louisiana, North 

 Carolina, South Carolina, and Texas. 



Irrigation has been practised in the Old World for hun- 

 dreds of years. It seems that the Egyptians were the first to 

 make use of this method of growing their crops, but all over 

 South-western Asia also are the remains of great irrigation 

 systems. Even to-day millions of acres are irrigated in India, 

 Egypt, and Italy, and it is extensively practised in many other 

 countries of the world. 



The principal reason for irrigating is to supply the plant 

 with the proper amount of moisture at the time when it 

 needs it. In the arid region the plant needs moisture applied 

 regularly. In the semi-arid region it is sometimes not neces- 

 sary to apply any extra water. In the humid regions irriga- 

 tion is needed only in periods of drouth. It must be under- 

 stood that there is no definite division between these three 

 large regi®ns. They overlap each other according to the 

 season. 



The amount of water necessary to produce a crop is very 

 variable. It depends upon the soil and the time of year when 

 the rain falls. Even in those sections where irrigation is car- 

 ried on most extensively, if the rain would fall at the proper 

 season it would be sufficient, but frequently it falls during 

 the cold time of the year when plants cannot use it. In parts 

 of California it has been found possible to produce good crops 



