59 



3rd. The time and frequency of cultivation. Thorough cultivation as 

 soon as possible after irrigation decreases evaporation losses. 



4th. Number of seasons irrigation is practiced. In all irrigated dis- 

 tricts it has been the experience that irrigation causes a rise of the water 

 table. If this rise is too great it may submerge the roots and waterlog the 

 soil, but if the water table rises only .to a deptfi where the soil' water can be 

 drawn by capillarity to the roots, this will lessen materially the necessary 

 amount of irrigation water. 



5th. Climate. Rainfall, temperature, humidity in the air, wind move- 

 ment, all have an effect. The rainfall and its distribution are important. 

 Abundant rainfall or snow in the winter will be partly stored in the soil 

 and is available to deep rooted plants during the growing season, thus 

 decreasing the necessary irrigation. On the other hand, light showers 

 during the summer may do more harm than good by destroying the soil 

 mulch and increasing soil evaporation. An increase in temperature and in 

 wind movement will increase the soil evaporation. 



6th. Character of soil and subsoil drainage. A sandy soil underlaid 

 with a porous subsoil which drains readily will take care of large volumes 

 of water without waterlogging and encourage waste. 



7th. The value of water, method of payment for water, judgment of 

 irrigator. A high cost of water leads to higher duty. If water is sold at so 

 much per acre of land independent of the > amount of water used, it is 

 human nature for the average irrigator to use all he can. On the other 

 hand, where the water charge is based on 'the quantity actually used, this 

 leads to careful use and high duty. This factor combined with the judg- 

 ment or lack of judgment may have more effect on value of the duty of 

 water than all the other factors together. 



Because of all the factors on which the duty of water depends, it is to 

 be expected that there is a great difference in the value of the duty of 

 water obtained in different localities or even in the same localities for 

 farms, or orchards under the same conditions but owned by different 

 parties. There are no measurements of the duty of water in British Colum- 

 bia, but values of the duty of water in the arid states of the United States 

 are very numerous. These measurements have been largely made by the 

 Irrigation Investigations Office of the U. S. Department of Agriculture, 

 usually in cooperation with the state governments. In a review of ten 

 years of irrigation investigations, carried on by the U. S. Department of 

 Agriculture since 1898, Mr. R. P. Teele gives a large number of duty of 

 water measurements which are' very instructive. ; Tnese ntSa*stil'e i me-nts; 

 show that the gross duty ranges from a maximum average of 13.18 acre 

 feet per acre for 7,000 acres of the Modesto system in California, to a 

 minimum of 2.0 acre feet per acre for 5,160 acres irrigated by several 

 small systems in the Santa Ana Valley, of California. The gross duty for 

 50,000 acres in the Yakima Valley irrigated by several canal systems is 

 5.70 acre feet per acre. For 20,000 acres of the Bitter Root Valley the 

 average is 4.69 acre feet per acre. 



The duty of water is generally lower for a new system, because seepage 

 losses are greater for new canals, new lands require more water, water is 

 more plentiful as only a part of the land is irrigated, and the irrigator is 

 less skillful. As the system gets older, less water is used and the gross 

 duty increases. This is well shown in the following table. 



