40 BULLETIN 339, U. S. DEPARTMENT OF AGRICULTURE. 
irrigated with not over 6 acre-inches at one application if economy of 
water is desired. It is hardly considered that it will ever be practical 
for the farmer to predetermine just how much should be applied at 
each irrigation and then apply this quantity and no more, but it is 
believed that intelligent and economical practice demands at least an 
-approximate knowledge of the volume that is being applied. The 
fact that a head of 1 cubic foot per second delivers almost exactly 1 
acre-inch per hour should make it comparatively easy for an irrigator 
to determine at least approximately how much water he applies to 
his land. 
LENGTH OF IRRIGATION SEASON. 
The length of the season during which crops require water depends 
upon both the climate and the crops grown. Grains require water 
for only a comparatively short season, winter grains frequently re- 
quiring but one irrigation, while alfalfa and pasture grasses require 
irrigation from early spring until late fall. The climatic conditions, 
with particular reference to the temperature, which determines the 
length of the growing season, have a very direct influence upon the 
length of the irrigation season, since the grass crops, provided an 
abnormal amount of natural precipitation does not occur, will require 
water for as long as the climate will permit them to grow. . The 
length of the irrigation season in Idaho has been carefully determined 
by the investigation and is shown for the different crops by Table 
XX. 
TABLE XX.—Average length of irrigation season of plots included in the 4 years’ 
investigation. 
Total | | Maxi Average dates of 
Je AVOIAES Numer iawereee ser irrigation. 
= number | having 1 | length o 
Crop. Year. | of plots | of irriga- irrigation [irrigation |/¢nsth of 
| ae tions. | only. | season.t |"{820 | First ir- | Last ir- 
| : * | rigation. | rigation. 
| Days. Days 
Grainses ass seen ae 1910 76 3.6 3 46. 7 | May 27 | July 12 
Alfaha and clover....... 1910 27 4.7 0 95. 4 144 | May 12 | Aug. 12 
Gralns Sos ey eee ee 1911 96 74, I 17 35. 5 64 | June 13 | July 19 
Altalfa and clover...-.--- 1911 34 6.1 if 111.4 142 | May 14 | Sept. 2 
Grains) ese eee 1912 60 3.7 10 39. 7 61 |June 9] July 18 
Alfalfa and clover.......- NOTE 25 5.6 0 87.3 123 | May 23 | Aug. 18 
GLAING Rese es eee a 1913 66 Sl: 8 48.9 110 | June 5 | Aug. 20 
Alfalfa and clover......- 1913 15 ball 0 96.5 119 | May 16 Do 
1 Exclusive of plots having one irrigation only. 
Average length of irrigation season for alfalia for 4 years was 97.6 days. 
Average length of irrigation season for grain for 4 years was 42.5 days. 
SIZE OF IRRIGATION HEAD. 
The size of the irrigation head used, particularly with the porous 
soils, has a great effect upon the irrigation requirements. It is a fact 
well known to irrigators who are accustomed to handling porous soils 
that very little can be accomplished with small heads of water for 
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