Quantity of Water Consumed in Irrigation. 37 



the loss may be unimportant in a large canal, but it may add 

 very materially to the loss from a small watercourse, where 

 it would in similar soil be as much as in the canal, and, con- 

 sequently, bear a larger proportion to the water surface. Till 

 more is known on the point, it will be convenient to take 

 the wetted perimeter for both evaporation and percolation. 

 The author is of opinion that a quarter-inch per diem may 

 be taken as the average loss from a canal and its distribu- 

 taries. This might be too much to allow from a large area of 

 deep water, except, perhaps, on very hot, dry, and windy days 

 but it is not too much to allow in hot and dry weather, when 

 irrigation is most needed, as an average for streams, some a 

 few feet deep, and others a few inches only. In a moist and 

 cool climate, and in damp weather also, evaporation would 

 generally be much less than a quarter-inch a day, but under 

 such conditions, irrigation would scarcely be a necessity, un- 

 less for what would in India be monsoon crops, which are not 

 considered in this paper, with the exception of rice. In 

 Appendix A, less loss by evaporation has been allowed for 

 rice cultivation, owing to the conditions attending it. 

 Whether evaporation be estimated at a quarter-inch or a 

 little more, since the loss from percolation has been shown 

 to be very great, that by evaporation from water flowing in 

 channels may generally be neglected, except in very clayey 

 soils, where percolation is comparatively little. 



23. The depth of water in the canal has not been taken 

 into consideration in the foregoing remarks, because the 

 author's observations on the loss of water from tanks has 

 led him to the conclusion that where the variations of water 

 level are regular, the loss is practically independent of the 

 depth of water in the tank. Doubtless the comparatively 

 great thickness of soil through which the water usually has 

 to pass, is one reason why the effect of a varying depth 

 of water in a tank on percolation is not perceptible. The 

 author sees no reasons for supposing that percolation from 

 any channel, in which water is constantly flowing, would be 

 materially different from percolation from a tank. The 

 author's contention is, that in any given case (tank or canal) 

 the loss in depth would, under ordinary conditions, be prac- 

 tically the same, whatever the depth of water. If the depth 

 of water is one factor, the thickness of soil through which 

 percolation takes place is another factor. 



24. The fact of water being often stored in open and 

 unlined reservoirs, formed on the surface of the ground or in 



