CHAPTER VII. 

 RESERVOIRS AND PONDS. 



^TT^ HE fortunate irrigator who has a reservoir of his 

 * own has his water-supply constantly on tap — 



^^ the reservoir may also appropriately be called 

 the farmer ' s savings bank . An irrigation sys- 

 tem depending upon storage, when the storage works 

 are judiciously construdled, is the most reliable of all. 

 The reservoirs can hold the waters of a wet year for 

 use in a dry one, and in the possible sequence of sev- 

 eral dry years the smaller stored supply gives several 

 months' warning to irrigators, so that water can be 

 husbanded and made to perform a larger duty than 

 usual in order to tide over a period of scarcity. 



The problem of water storage for irrigation is a 

 very different one from that for the domestic supply of 

 a city. In the first place, it is important that water for 

 domestic use be as nearly as possible free from mud and 

 organic impurities, while for irrigation such impurities 

 are not only no objection to the water but often mate- 

 rially add to its value by enriching the soil to which it 

 is applied. Waters held in reservoirs and intended for 

 irrigation purposes are often rendered much warmer 

 than the flowing waters of streams, and are therefore 

 more beneficial to plant growth when drawn off and 

 applied. The reservoirs must also be credited with 

 having a salutary effe<5l on the atmosphere of the arid 

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