54 WINDBREAKS. 



by hand, and weighed carefully. The average yield for the whole 

 field (separator measure) was 15J bushels per acre. The greatest 

 yield was found at a distance from the trees equal to two and one- 

 half times their height and was equal to 25.3 bushels per acre, or a 

 gain of 10.1 bushels over the average for the field. At 5-ht. the yield 

 was 16.7 bushels per acre, or 1.5 bushels more than the average. The 

 total gain out to 7-ht. was almost sufficient to offset the damage 

 from shading and sapping, and left a net loss equal to the yield of a 

 strip only 3.3 feet wide. In a more extreme case of drought the 

 benefit would have more than offset the loss from sapping, which 

 had practically reached its limit here. 



The second case was that of a field of barley south of a grove of 

 scattered ash and honey locust about 20 feet high. It is quite prob- 

 able that the ground near the windbreak was well soaked from the 

 effects of snow which had drifted in the lee of the trees during the 

 winter. In this field the normal yield was obtained from sample 

 plots about 10-ht. distant from the grove. The average calculated 

 yield of the field was about 16 bushels per acre. The greatest yield 

 was found at about 3-ht. distant from the trees, where it was 30.6 

 bushels per acre. The great distance of this " crest" from the trees 

 is explained by the open character of the grove. The effects of sap- 

 ping were not apparent at the edge of the field, where a strip 5 feet 

 wide had not been cultivated. The total gain in the field was suf- 

 ficient to offset the loss and leave a net increase equal to the yield 

 of a strip 59.5 feet wide, or 2.98 times the height of the trees. 



Relation to irrigation. At a time when irrigation is occupying so 

 much of the attention of agriculturists in the West the effect of 

 windbreaks upon the water stored in reservoirs, that moving through 

 ditches, and that placed upon the fields is exceedingly important. 

 This effect results from the ability of the windbreak to check evapora- 

 tion of the water from the time that it reaches the reservoirs to the 

 time when it has soaked into the ground and is available to crops. 



Many of the large reservoirs established by the Reclamation 

 Service are in mountainous country, hemmed in by steep walls. 

 Windbreaks are obviously not needed here. A few, however, are 

 in open country and have immense surfaces exposed to the air. A 

 large area of the reservoirs might, in such cases, be protected by wind- 

 breaks planted along the borders. The amount of water saved may 

 be roughly calculated per mile of windbreak for a given region. 



It has been observed that in the case of the Pische instrument 

 the reduction in the percentage of evaporation is not so great as the 

 reduction of wind velocity. Take, for example, the case of a cotton- 

 wood belt (diagrams 15 and 19) which, with a wind velocity of 15 

 miles, reduces the evaporation at a given point only 20 percent, 

 while the wind velocity was reduced 46 per cent. Supposing that the 



