TREES AND SHRUBS 319 



of following the rows. While this system does moisten the 

 land uniformly, it is complicated and involves consider- 

 able expense. Straight furrows running between the rows, 

 with occasional cross furrows, is the more- 1 satisfactory 

 system. 



After each irrigation, the furrow is covered to diminish 

 evaporation. The furrows, therefore, are temporary and 

 must be made before each irrigation. It is difficult to 

 control the water thoroughly even under the furrow 

 method of irrigation. Some water, of course, always 

 reaches the end of the furrows and is allowed to flow into 

 a cross ditch at the end of the furrows which acts as head 

 ditch to the furrows below, or this water may be taken on 

 to fields of lucern or other crops. (Fig. 87; also Figs. 41- 

 56.) 



190. Time of orchard irrigation. Fall and winter 

 irrigation is very advantageous in the maintenance of 

 orchards. In the colder parts of the arid regions, where 

 the ground, during winter, is frozen and well covered with 

 snow, fall irrigation alone is practised. The wood of the 

 trees is allowed to ripen thoroughly before fall irrigation. 

 If water is applied too early, so that new growth starts, 

 the trees are hi danger of winter-killing. In the warmer 

 parts of the arid region, with mild, open winters, as in 

 Arizona, winter irrigation is of greatest benefit. Lands 

 that receive little precipitation in the whiter are especially 

 benefited by winter irrigation. Districts in which the 

 precipitation comes largely hi the fall, whiter or early 

 spring, are not so greatly benefited by fall or winter 

 irrigation. In such places the added water may simply 

 cause seepage, which is not desirable. 



Unless the soil is dry in the spring, there is no need of 

 spring irrigation. As a general rule, trees must not be 



