TREES AND SHRUBS 323 



water is available, most water is used. For instance, in 

 Wyoming, in well-watered sections, the duty of water is 

 70 acres per second-foot; in California, where water is 

 scarce, the duty is 400 acres per second-foot. Yet, in the 

 latter place citrus trees with long growing periods are 

 largely grown, and the climate is hotter than in Wyoming. 

 The whole question of the quantity of water needed for 

 orchards needs careful investigation. It is probably safe 

 to say that from 12 to 24 niches is an ample seasonal 

 depth of water for orchard crops. More than 6 acre- 

 inches is seldom needed in any one month, even under low 

 rainfall and high evaporation. That means, for an irri- 

 gation season of two months, 12 inches, and of three 

 months, the usual limit, 18 inches. The long-season 

 citrus fruits seldom need more than 3 acre-niches of water 

 per month, although according to Wickson, citrus trees 

 require 50 per cent more water for each crop than do 

 deciduous trees. Wickson declares, however, that 20 

 acre-inches are ordinarily sufficient, annually, for the 

 irrigation of citrus trees, and that 10 inches are frequently 

 sufficient. 



192. Other conditions of orchard irrigation. In young 

 orchards, and occasionally hi old orchards, inter-culture 

 is often practised. Corn, potatoes, beets, squash and 

 various vegetables or small fruits are planted between the 

 rows of trees. Moreover, to maintain the fertility of the 

 land, cover-crops are occasionally planted between the 

 rows of older trees. Inter-tillage in orchards invariably 

 means a higher water requirement than does clean cul- 

 ture. The increase corresponds to the degree of inter- 

 culture. 



The great danger in orchard irrigation is over-irri- 

 gation. Only by moderate irrigation can the root-system 



