Climatic Conditions 89 



on a side. But to cover such an area as this with 2 inches of 

 water once in 10 days would require more than three Nile rivers 

 flowing 1 at maximum flood a river 50 feet deep, 1.156 miles wide, 

 running three miles an hour. 



THE CLIMATIC CONDITIONS UNDER WHICH IRRIGATION 

 IS PRACTICED 



If we study the conditions of rainfall under which 

 irrigation has been practiced, we shall find rather wide 

 variations in the mean amounts which fall upon the dif- 

 ferent countries, especially when the mean annual rain- 

 falls are compared. In all of India except the extreme 

 northwest part; throughout China, Japan and Siam, 

 in Italy, and France, and Mexico, as much rain falls 

 during the year as falls in the United States east of 

 the 97th meridian, if we except Louisiana, Mississippi, 

 Georgia and Florida, an amount ranging from 23.6 

 inches to 51.2 inches, or between 60 and 130 centime- 

 ters. But in Asiatic Turkey, Persia, Afghanistan and 

 the extreme northwest of India ; in the irrigated parts 

 of Queensland, Victoria and South Australia ; in Cape 

 Colony, Algiers and Spain ; and in Argentina and the 

 western United States, south of Washington state, the 

 rainfall for the year drops from 23 inches to less than 

 8 inches. On the lower Ganges, from the Soane region 

 to Calcutta, and south along the east coast as far as the 

 Orissa- canals, the yearly rainfall is equal to that of the 

 southern states, or from 51 inches to 78 inches (130 to 

 200 centimeters). It is not, therefore, in regions of 

 small rainfall alone that irrigation systems have been 

 developed. Indeed, there must always be contiguous 



