184 



THE IRRIGATION AGE. 



DUTY OF WATER AND HOW TO DETERMINE IT 



By E. C. MeClellan, Irrigation Engineer, Los Angeles, California 



The term of "Duty of Water," as used today in 

 consideration of use of water in irrigation in the 

 semi-arid sections of the country, is a phrase hav- 

 ing practically no significance to the great majority 

 of irrigators, because as used it bears no relation 

 to the growth of plants. It is used to determine 

 the quantity of water that can be placed into the 

 land without totally destroying the plants grown 

 there, and without regard for the ruining of the land 

 being used for agricultural purposes. As a fact, 

 methods used today ruin both land and plants, as 

 is shown wherever crops have been grown in a sec- 

 tion for any length of time. 



To understand what irrigation should mean, 

 there are a few fundamental matters that must be 

 considered and they are : 



First Plants have evolved to their present 

 state of perfection through millions of years growth 

 and development, during which time one of the 

 greatest changes they have had to meet and adapt 

 themselves to is a gradual decrease in the supply of 

 water in the ground where they are growing. 



That change has always been tending from a 

 moist to a dryer state, and is so gradual that it is 

 hardly perceptible during the few thousands years 

 we have any record of. There is, however, a rea- 

 sonably certain and fixed proportion or ratio be- 

 tween that moisture supply and the other elements 

 stored in the ground and used by the plants for their 

 structural growth and collection, manufacture and 

 storage of those certain substances needed by man 

 and animals for their growth. 



Second the plants have adapted themselves to 

 that certain proportion or ratio, and are capable 

 of securing the highest efficiency in growth and 

 storage of food substances when under the influence 

 or surroundings where such proportion exists. 



Third The elements needed by the plants are 

 found, in the air and in the ground. Those in the air 

 are so regularly disposed or mixed together that the 

 plant having evolved a method of securing them is 

 not obliged to change in any particular at any part 

 of the world. Those in the ground are very seldom 

 exactly the same at all places ; but after being 

 placed in the ground whether by nature or man, in 

 fertilization remain there, with the exception of 

 those elements entering into the composition of 

 water. Water is always moving, flowing down or 

 away in the form of a liquid, or spreading through 

 the ground, or rising up and escaping into the air 

 as a vapor. It is necessary to constantly renew the 

 water supply in the ground, to keep the proportion 

 between it and the other elements required by the 

 plants. 



In a large section of the world this renewal 

 is amply provided for by rain or snow fall. 



Where the rain or snow fall is not sufficient to 

 provide for the proper proportion or ratio between 

 the moisture supply and the other elements needed, 

 irrigation is needed, to provide the necessary 



quantity of water, exactly as fertilization, so-called, 

 is used to supply the deficiencies in other elements 

 lacking. Irrigation is in fact just as much fertiliza- 

 tion as the supplying of any other elements needed 

 is fertilization. 



In fertilizating, the constant aim is to supply 

 the ground with just the necessary quantity of ele- 

 ments lacking, so the plants will return the greatest 

 amount of food values for the quantity of fertiliza- 

 tion used. 



In irrigation, the aim should be to supply the 

 ground with the quantity of water lacking, so the 

 plants will return the greatest amount of food values 

 for the quantity of water used. 



Nature, having established a standard or ratio 

 between all the elements entering into plant growth, 

 the first aim should be to keep to that standard in 

 water supply as well as other elements. Irrigation 

 should mean bringing the water content in the 

 ground up to that standard. 



Plants having evolved from a wetter to a dryer 

 moisture supply in the ground, have their in- 

 telligence turned in that direction, and their 

 tendency is towards securing, manufacturing and 

 storing a finer line of food substances under a less 

 than standard moisture supply. 



When the standard of moisture supply is raised 

 by application of a greater quantity of water, the 

 plant is driven back in its evolution. The result 

 is shown by a greater structural growth, and the 

 collection, manufacturing and storage of a less 

 quantity of depreciated food substances. 



This is due to the fact that, as the structural 

 growth is mainly composed of the elements entering 

 into the composition of water, the plant uses as much 

 of it as it can, and provides a greater growth ; but 

 the presence of the excess water in the ground stops 

 the combinations being formed there, that are taken 

 up and stored by the plant, and known as proteids, 

 or nitrogeneous substances, and the excess water in 

 the plant stops the manufacture of the sugars and 

 starches, called carbo-hydrates ; and these are the 

 main food substances in the plants. In talk, water is 

 called the life blood of the semi-arid sections ; but 

 in practice, its value is never considered. This is 

 shown in all the reports of the United States, and 

 the different states upon the effects of the use of 

 water in irrigation. As illustration of this, I desire 

 to comment upon a late bulletin issued by the Uni- 

 versity of California, No. 280, entitled "The 

 Economical Irrigation of Alfalfa in Sacramento Val- 

 ley." On the second page is given a summary of six 

 years experiment at Davis, 1910-1915, inclusive, and 

 also the precipitation for that period, as follows : 

 (Bulletin No. 280, California University) 



Summary of alfalfa dutv-of-water investigations 

 at Davis, 1910-1915. 



(First column gives number of irrigations; sec- 

 ond column gives depth of each irrigation in inches : 

 third column gives total depth of water applied). 



