RESULTS OF MEASUREMENTS. 55 



same relation exists over the entire area to the leeward, we have out 

 to 10-ht., in this case, a mean reduction in wind of 35.6 per cent 

 (Table 9) or 9.7 miles per hour as against 15 miles. 

 Applying these figures, with the aid of Fitzgerald's formula for 



y 

 evaporation (0.0166 (e s e d ) (1 TT) ) to a region in which the annual 



loss of water amounts to 60 inches, we have the evaporation from the 

 reservoir reduced to 41 inches per annum, or a saving of 19 inches, 

 for an area of 60 acres, which would be so protected by a windbreak 

 1 mile long and 50 feet high. This would be more than enough to 

 irrigate an equal area of cultivated land for an entire season. 



Immediately after water has been turned upon fields, and for a 

 few days while the surface soil is saturated, there is chance for great 

 loss by evaporation. The loss during the first few days after flooding 

 may be a very important item in the duty of water reaching the 

 crops. 



During parts of June, July, and August, 1908, an experiment was 

 conducted to determine the relation between the loss from a Pische 

 evaporometer, exposing moist filter paper, and another, similarly 

 exposed, supplying water to a shallow dish of sand with the same 

 area as the filter paper. The sand was kept continually saturated. 

 The evaporation from a saturated sand surface was found to be 

 about 60 per cent greater than that from the filter paper, and was 

 equally influenced by an increase in wind velocity. The greatest 

 evaporation for a period of 24 hours, during the observations cover- 

 ing 72 days, was 105.5 cubic centimeters, or a depth over the 

 area exposed of 1.6 centimeters, or 0.63 inch. The average wind 

 during the day was 12.8 miles per hour, and at night 9.1 miles. The 

 mean relative humidity was 74 per cent. These conditions are not 

 at all abnormal for the region of eastern Nebraska where the measure- 

 ments were taken, and the conditions favorable to evaporation are 

 exceeded very frequently in the dry regions where irrigation is 

 depended on. It may easily be seen, then, that during three days 

 when water was being turned onto a field, and while the surface soil 

 was yet moist, the loss of moisture in unprotected areas might amount 

 to 2 inches or even more. An efficient windbreak, 50 feet high, would 

 reduce this evaporation in a field 30 rods wide to the leeward by at 

 least 30 per cent. 



It must not be forgotten that the same conditions exist for a longer 

 or shorter period after each rain in every section of the country. 



VI. HEAT. 



The effect of a windbreak upon temperatures in the zone of its 

 influence is much greater than is commonly supposed. Just how such 

 effects are brought about and how the measurements of temperatures 



