THE IRRIGATION AGE. 



ira 



ing upon vegetation. From August to December the 

 weather commonly grows gradually cooler, and the rela- 

 tive humidity usually increases. 



The accompanying table of the average monthly 

 temperature and relative humidity during the four past 

 years will indicate the changes in the weather from 

 season to season of the year, and what the weather of 

 the period has been. The temperatures are those at the 

 farm, and the relative humidity figures, those obtained 

 at the Weather Bureau at Phoenix : 



TABLE II. TEMPERATURE AND RELATIVE HUMIDITY. 



Mean Temperature. 

 1898 1899 1900 1901 



53 



53 

 63 

 62 

 76 

 84 

 90 

 85 

 77 

 69 

 60 

 52 



52 

 55 

 59 

 64 



73 

 81 

 91 

 88 

 SI 

 71 

 61 

 50 



Mean Relative Humidity. 

 1898 1899 1900 1901 

 56 49 41 47 



cates approximately the rate of loss of water from crops 

 and from soil. Consequently evaporation data are help- 

 ful in considering irrigation questions. 



The record of evaporation from a water surface 

 has been kept at the farm for but one year. The begin- 

 ning was made last April and a record is now being; 

 made regularly. The record is being kept in the usual 

 way from the surface of a tank of water sitting with 

 the surface even with the surface of the soil. The re- 

 sults for May to November (end of the last irrigation 

 year), inclusive, are given in the table that follows: 



TARLE III. EVAPORATION FROM WATER SURFACE. 



68 69 69 



36 36 33 37 



Annual Mean.68 



It will be seen that the mean annual temperature 

 and the mean relative humidity differ from year to year 

 far less than the rainfall. The changes in the tempera- 

 ture from month to month are, on the whole, quite regu- 

 lar. Only a few marked irregularities occurred during 



Evaporation 

 Week ending in inches. 



May 7th 1.68 



May 14th ' 1.92 



May 21st 1.92 



May 28th 2.04 



June 4th 2.04 



June llth 2.16 



June 18th 2.04 



June 25th 2.28 



July 2nd 2.28 



July 9th 1.92 



July 16th 2.52 



July 23rd 2.64 



July 30th 2.52 



August 6th 2.88 



August 13th 72 



August 20th 1.44 



Evaporation 

 Week ending in inches. 



August 27th 1.68 



September 3rd .... 1.56 



September 10th 1.68 



September 17th 1.80 



September 24th 1.6S 



October 1st 1.56 



October 8th 1.32 



October 15th 1.08 



October 22nd 1.08 



October 29th 84 



November 5th 48 



November 19th 36 



November 26th 48 



December 3rd 48 



Total . . .49.48 



The total of the seven months approximately 

 49i/i inches was divided among the months as follows: 

 May 8.28 inches, June 9.42 inches, July 11.29 inches, 



SMALL GASOLINE ENGINE RAISING SANTA CRUZ UNDERFLOW FOR IRRIGATION. PHOTOGRAPHED BY W. W. SKLNNER, 



UNIVERSITY OF ARIZONA AGRICULTURAL EXPERIMENT STATION. 



the four vears. One of these was the lower mean tem- 

 perature during March, 1898, than during February, 

 and another the lower mean temperature during April, 

 1900, than, during March. The coolest month during 

 the four years was January, 1898, and the warmest, 

 July, 1901. The dampest month during the period was 

 February, 1901, and the dryest, June, 1900, the latter 

 year being the dryest of which there is a record at the 

 Phoenix weather bureau. It will be noted that July 

 has been uniformly the warmest month of each of the 

 four years. 



EVAPORATION. 



The rate of evaporation from plants, from soil, or 

 from water, varies with the temperature, the relative 

 humidity, and the wind velocity, the rate from soil be- 

 ing affected less by the last factor than the rate from 

 water. The amount of evaporation from water indi- 



August 6.55 inches, September 7.08 inches, October 

 4.81 inches, and -November 2.05 inches. 



The comparatively low evaporation for the week 

 ending August 13th, due to the humid condition of the 

 atmosphere, is a striking illustration of how evapora- 

 tion is modified by such a condition. It will be noted 

 that though the temperature was higher during August 

 than during September, the evaporation was greater 

 during the latter month. This was due to the frequent 

 showers of August, and the absence of rain during Sep- 

 tember. From the latter month to the end of the year 

 the rate of evaporation decreased gradually and quite 

 regularly. 



The evaporation for December, 1901, to April, 

 1902, inclusive, amounts to 16.15 inches, making a 

 total of 65.63 inches for the year during which a record 

 has been kept. 



