THE IRRIGATION AGE. 



VOL. IX. 



CHICAGO, MARCH, 1896. 



NO. 3. 



WATER SUPPLIES FOR IRRIGATION. 



III. MEASUREMENT OF STREAMS, GAUGING THE UNDER- 

 FLOW. 



BY F. C. FINKLE. 



RAINFALL is variable in all countries. 

 At times a storm occurs when as 

 much water falls in the space of a few 

 hours as during all the balance of the year. 

 Again, certain seasons are subject to fre- 

 quent and heavy storms, while others are 

 wholly, or almost so, devoid of rainfall. 

 This marked division of the year into a dry 

 and a wet or rainy season is particularly 

 the case in countries where irrigation is 

 necessary, and as has been already shown, 

 is one of the principal causes which make 

 irrigation a necessity in such districts. The 

 natural streams, being fed by the fall of 

 rain on the watersheds tributary thereto, 

 will vary in their discharge as the season 

 is dry or wet. During the wet season, 

 when rains are heavy and frequent, the 

 streams will rise and flow large quantities 

 of water, which cannot be diverted and 

 used for irrigation, as the water is not 

 needed at that time of the year, the ground 

 being already saturated by the natural rain- 

 fall. The only manner in which this storm 

 flow can be utilized is by storing it in res- 

 ervoirs, and thus conserving it until the 

 dry season; this subject will be subse- 

 quently treated under the head of storage 

 reservoirs. 



At present our discussion will be on the 

 value of natural streams for irrigation pur- 

 poses during the irrigation season, when 

 the flows of streams are at their minimum. 



Before an enterprise is founded with the 

 natural flow of a stream as a water supply, 

 it is necessary to make a careful determin- 



ation of the minimum discharge of the 

 stream during the season of irrigation. 

 As soon as the extent and character of the 

 watershed which feeds the stream is known, 

 we are able to judge approximately of its 

 value as a source of water supply. A flow- 

 ing stream derives its supply of water 

 during the season of no rains either from 

 melting snows or from springs. Both of 

 these are the accumulations from the sea- 

 son of rains, and to what extent depends 

 on the area, elevation and nature of the 

 watershed. 



If the watershed is large and high it will 

 contain a plentiful quantity of snow at the 

 end of the rainy season, which by reason 

 of the coolness of the atmosphere at a high 

 elevation, will have fallen, and will grad- 

 ually melt during the dry and warm season 

 following, thus feeding the stream flowing 

 from it. 



Again, if the watershed is composed of 

 loose and impervious layers, intermingled 

 in such a way as to create underground 

 basins and channels, which accumulate the 

 water and again conduct it to the surface 

 later in the season, the stream to which it 

 is tributary will be fed by springs. In 

 these two ways, viz., by melting snow and 

 springs, the streams are supplied with 

 water during the dry season, and under- 

 standing these principles we are able to 

 judge somewhat of the capacity of a stream 

 for supplying water for irrigation, as soon 

 as we have become acquainted with the 

 characteristics of its watershed. But it ia 



