IRRIGATION AND DRAINAGE 149 



Number of irrigations necessary in a season. For nearly half 

 the entire year the fruit trees of Wyoming and Montana have 

 little active, visible growth, whereas in the citrus districts of Cali- 

 fornia and Arizona the growth is continuous. A tree when dor- 

 mant gives off moisture, but the amount evaporated from both 

 soil and tree in winter is relatively small, owing to the low tem- 

 perature, the lack of foliage, and feeble growth. A heavy rain 

 which saturates the soil below the usual covering of soil mulch 

 may take the place of one artificial watering, but the light shower 

 frequently does more harm than good. 



The number of irrigations needed depends on the capacity of 

 the soil to hold water. If it readily parts with its moisture, light 

 but frequent applications will produce the best results ; but if it 

 holds water well, heavy applications at longer intervals are best, 

 especially when loss by evaporation from the soil is prevented by 

 the use of a deep soil mulch. 



In the Yakima and Wenatchee fruit-growing districts of Wash- 

 ington the first irrigation is usually given in April or early in 

 May. Then follow three or four waterings at intervals of twenty or 

 thirty days. At Montrose, Colorado, water is used three, four, 

 or five times in a season. At Payette, Idaho, the same number 

 of irrigations is applied, beginning about June 1 in the ordinary 

 seasons and repeating the operation at thirty-day intervals. As a 

 rule the orchards at Lewiston, Idaho, are watered three times, 

 beginning about June 15. From two to four waterings suffice for 

 fruit trees in the vicinity of Boulder, Colorado, the last irrigation 

 being given on or before September 5, so that the new wood 

 may have a chance to mature before heavy freezes occur. In the 

 Bitter Root valley, Montana, young trees are irrigated earlier and 

 oftener than mature trees. Trees in bearing are, as a rule, irri- 

 gated about July 15, August 10, and August 20 of each year. In 

 San Diego County, California, citrus trees are watered from six 

 to eight times, and deciduous trees three or four times a season. 



Duty of water in irrigating apple orchards. The duty of water 

 per acre as fixed by water contracts varies from -^ to ? i^ cubic 

 foot per second. In general the most water is applied in districts 

 that require the least ; that is, wherever water is cheap and abun- 

 dant the tendency seems to be to use large quantities, regardless 



