THE IBKIGATION AGE. 



171 



one irrigation in order to bring soil in which most crops 

 would suffer to a good state of moisture. Of course 

 the exact amount will vary with the kind of crop and 

 with the stage of its development. For instance, crops 

 planted in wide rows need considerably less water dur- 

 ing early growth when plants are small, than later. 



SEASON OF GROWTH. 



Although there is no season of the year when some 

 crop or crops are not growing on the station farm and 

 on the other farms of the region, yet there are two 

 seasons during which most are produced : ( 1 ) November 

 to March inclusive, when grains, most hardy vegetables 

 and small fruits are planted and make the main part of 

 their growth; and (2) April to October inclusive, 

 when melons and other vegetables more sensitive to 

 frost than to heat, corn, cowpeas and sorghum make 

 the most of their growth. 



But there is no time of the year when some crops 

 are not maturing and others in the early stages of their 

 growth. The time when fewest crops overlap in this 

 way is about the time of the fall frosts that occur about 



February inclusive, the rainfall Vicing usually the 

 heaviest during the last month of the year. The rain 

 and snow falling in the mountains during the winter 

 rainy period usually furnish an increased supply of irri- 

 gating water until the end of March. From the latter 

 month until July the rainfall is light and the supply of 

 water usually gradually diminishes, becoming very low 

 during June. The summer rains swell the streams and 

 increase the supply of irrigating water temporarily. 

 Then follow about three months during which the rain- 

 fall is so light that the supply is again usually less than 

 the demand. 



The following table of the average rainfall in 

 inches for each month of the year, as recorded during 

 the past twenty-six years, will indicate how the rainfall 

 is distributed through the year at Phoenix : 



Mo e ntlf]y _Jan-eb-Mar : Apr : _MayJune July Aug. Sep. Oct. Nov. Dec. Tot. 



' 80 15 



' 95 



- 45 



7.18 



It will be seen that the average monthly precipi- 

 tation gradually decreases from the winter rainy season 



WATERMELONS AFTER TEN IRRIGATIONS. 



the middle of November. By this time the summer 

 crops sensitive to frost are either mature or are killed, 

 and few of the winter crops are planted. Hence this, 

 is the most appropriate time to make a beginning of 

 the crop year. Each irrigation year reported upon be- 

 low is considered as extending from the end of Novem- 

 ber, 1900. to the end of November, 1901. 



RAINFALL DURING YEARS COVERED. 



While precipitation may occur at any time of the 

 year in the vicinity of Phoenix, there are two seasons 

 during which the fall is heavier than daring the re- 

 mainder of the year, and the consequent supply of irri- 

 gating water much greater. The two seasons are, re- 

 spectively, July and August, and from December to 



until July, during which month and the following one 

 the average amount is somewhat above the average rain- 

 fall for the respective months of the winter rainy 

 season. 



The average monthly amount of precipitation be- 

 ing so small, the amount that falls during any one day 

 is seldom great too small usually to be of much direct 

 benefit to growing crops. The principal local benefit 

 is due to the temporary raising of the relative humidity 

 of the atmosphere near the surface of the soil, thus 

 checking evaporation and causing more of the irri- 

 gating water to be available to the growing plant. If a 

 rain can be followed with an irrigation, that the mois- 

 tening of the soil may be continued downward to the 



