36 Quantity of Water Consumed in Irrigation. 



to a depth of 9 J inches over the filtration area. So that, 

 after allowing for the 35 days in each year when the sewage 

 was applied to other land, the enormous depth of 261 feet 

 per annum was purified by the filtration areas, and had been 

 so purified for the last three years."* Such an instance 

 is perhaps exceptional for sewage filtration, but it shows 

 what certain soil can do when the subsoil is thoroughly 

 drained. In the filtration of water for the supply of towns, 

 a very large quantity of water is passed through layers of 

 sand and other material. In London, a depth of from 4 \ 

 feet to 18 feet is filtered in 24 hours. The filters are 

 usually formed of 2 feet or 3 feet of sand over 3 feet to 5 feet 

 of gravel and other porous material. This shows what sand, 

 over a porous substratum, can carry off. 



21. The canal officer before mentioned, Mr. Beresford, 

 stated that he had seen water just reach an outlet or a 

 field, and no more, and " there are places where a fairly large 

 kuldba" (outlet) "in a whole week only irrigates two or three 

 fields."-)* The author recollects that some years ago, when 

 one distributary in the Anupshahr (then called the Fateh- 

 garh branch) division of the Ganges canal was first opened, 

 and for months afterwards, a discharge of something like 80 

 cubic feet per second in the upper part, was with difficulty 

 able to supply a fourth of this quantity 20 or 25 miles further 

 down ; all outlets between had to be closed in order to ob- 

 tain enough water in the lower part of the distributary to 

 irrigate from it. The author has known several small 

 tanks, where the subsoil was non- retentive, to fill after a 

 heavy fall of rain, and in a couple of days or so, a depth of 

 6 or 7 feet to soak away. The great facility for the percola- 

 tion of water offered by very sandy soil, with a porous 

 substratum, is shown by certain rivers in many parts of the 

 world, which are lost in sandy plains — in some cases during 

 the drier portions of the year only, in other cases all the year 

 round. Instances are to be found in parts of Australia. 



22. In nearly all these cases a portion of the loss was unques- 

 tionably due to evaporation. In canals, evaporation would 

 take place not only from the surface of the water, but also 

 from the moist part of the banks, where water is absorbed 

 and rises above the water-line in the canal. This addition to 



* Proceedings of the Institution of Civil Engineers, Vol. XL VIII., page 207. 

 +Professional Papers on Indian Engineering, Vol. V., new series (Roorkee, 

 1876), page 416. 



