WATER SUPPLIES FOR IRRIGATION. 



113 



sufficient to force it to the surface when 

 the ground is penetrated by borings. It 

 also differs from subsoil or surface water, 

 which sinks from the rainfall and fills the 

 voids in the subsoil, in the fact of its 

 possessing motion or velocity caused by the 

 slope of the water- bearing stratum and the 

 pressure of water on a higher plane gravi- 

 tating downward and tending to displace 

 the particles of water on a lower plane. 



EXISTENCE OF UNDERFLOWS. 



There are many streams which do not 

 possess any underflow of water, on account 

 of the absence of such conditions as are 

 necessary to produce one. The existence 

 of an underflow is by no means an invari- 

 able accompaniment of a river or creek, 

 but is possibly lacking in more cases than 

 it is present. An underflow is also fre- 

 quently -found where there is nothing 

 more than a dry wash to indicate the ex- 

 istence of a channel, which carries water 

 only in times of extraordinary floods or 

 cloudbursts. 



The conditions required to produce an 

 underflow in a stream are the following: 

 (1) A suitable watershed; (2) a favorable 

 surface topography; (3) sufficient and 

 properly distributed precipitation; (4) 

 proper character of channel and surface 

 formation; (5) a hard substratum. 



The above five heads cover all the con- 

 ditions necessary to create an underflow of 

 some importance in a stream, and by 

 means of a thorough understanding of 

 these it is possible to form an opinion on 

 the existence of an underflow in any 

 stream, after becoming acquainted with the 

 natural characteristics of the stream and 

 its drainage area. The discussion of the 

 requirements above enumerated is a mat- 

 ter of importance, and while the articles 

 on each topic will be brief, it will be the 

 aim to cover the subjects so as to include 

 all that is very essential. 



SUITABLE WATERSHED. 



A watershed must be of ample size to 

 create the existence of any considerable 

 underflow in the stream which drains it. 

 But size is not the only or most important 

 consideration, as a watershed of fifty square 

 miles or even less has been known to fur- 

 nish a goodly amount of underflow, where 

 other conditions were favorable. On the 

 other hand it often happens that a very 



large watershed creates no underflow at 

 all, on account of being lacking in other 

 respects. 



A high watershed is very desirable, as 

 the cool air at high elevations is a good 

 condenser of the vapor in the atmosphere, 

 and this serves to increase the rainfall. 

 Besides, if the elevation is sufficiently high 

 to cause a large portion of the precipita- 

 tion to fall in the form of snow, this is a 

 considerable advantage, as will be seen 

 later on. Another advantage of a high 

 watershed is that evaporation is at a mini- 

 mum, and the loss from this cause much 

 less than at lower elevations. It is safe to 

 say, however, that the most important con- 

 sideration in the character of a watershed, 

 when its ability to produce an underflow 

 is considered, is the nature of its soil for- 

 mation. The surface must be loose and 

 porous, allowing water to sink readily and 

 penetrate it, while there must be a hard 

 and impervious substratum at a reason-' 

 able depth, which is continuous, or practi- 

 cally so, from the summit of the watershed 

 to the channel of the stream. If the sur- 

 face soil is too hard and compact the rain- 

 fall will practically all flow off as surface 

 flow into the channel of the stream, and 

 very little, if any, will find its way into 

 underground channels and produce an 

 underflow. 



Even if the surface of the watershed is 

 as soft and porous as may be desired, if 

 the substratum is not hard and continu- 

 ous, but contains deep crevices and fissures 

 leading to underground layers of softer 

 material, or if the substratum is too deep 

 and conducts the water down to other 

 strata of loose material, the water which 

 sinks will be conducted to considerable 

 depths and distances, and instead of form- 

 ing an underflow may reappear as springs 

 or artesian water. 



A FAVORABLE SURFACE TOPOGRAPHY. 



The nature of the topography of the 

 watershed or country contiguous to a 

 stream has a remarkable effect upon the 

 creation of an underflow in the stream. 

 If the watershed pitches steeply downward 

 and is very abrupt to the banks of the 

 stream and its branches or tributaries, the 

 water will flow off and into the channel 

 very quickly without much of it entering 

 the ground. After reaching the channel 

 very little water sinks as a rule, except in 



