THE IRRIGAHON A GE. 185 



selves. On slopes the water may be carried in temporary laterals 

 plowed out approximately on the level, or such laterals may be per- 

 manently made and retained, as their low sloping banks need not in 

 terfere with the crossing of field machinery. The water has free flow 

 down the slope until the overflow is caught by the next ditch below 

 and flows from it to the lower slopes. 



The evenness of the distribution depends in part upon the uni- 

 form grade of the land and its freedom from knolls or hummocks and 

 in part upon the ditches being level; for then as they fill there is an 

 overflow all along the lower banks and all points are reached in the 

 downward movement of the water. On nearly level lands the tempo- 

 rary ditches are made by plowing two furrows thrown away from each 

 -other, or by making a furrow with a double moldboard plow. A ditch 

 thus roughed out may need but little cleaning, as very free flow of 

 water is not generally desired. 



The water is made to rise in the ditch by damming with earth, a 

 cloth dam, or a metal tappoon, as already described, and is released 

 by cuts in the bank as shown in Fig. 12. The irrigator aids the water 

 to reach slighted parts by a little work with a spade here and there. 

 When one section of the field is thoroughly wet the dam is moved to a 

 lower point, and so on. This method of irrigation is best suited for 

 small grain and forage plants, but the difficulty of securing an even 

 spread of water and the amount of work required in* handling the 

 water have caused its abandonment in many places for one of the 

 check systems, which require some outlay in first cost, but enable the 

 irrigator to do better work afterwards with a minimum of labor. 

 Where irrigation is not regularly needed a recourse to free flooding 

 may save a crop when threatened by temporary drought. It is also 

 used, to some extent in the drier parts of California for winter irriga- 

 tion of land to be plowed and sown to grain as soon as the soil dries 

 sufficiently for it. Land deeply soaked in winter will mature a grain 

 crop without subsequent irrigation. In this case it is merely a sub- 

 stitute for winter rainfall. 



(TO BE CONTINUED.) 



