Mar. i, 1924 
Movement of Water in Irrigated Soils 
679 
the accumulation of harmful quantities of soluble substances in the soil. 
Where conditions are such that it is possible to measure or to estimate 
the quantity of irrigation water used on a certain area of land and to 
measure also the drainage discharge from the land, analyses of the two 
waters make it possible to know whether the land is gaining or losing in 
salt. 
Two examples may be cited as illustrations of this point. One is a 
valley containing about 100,000 acres of irrigated land so situated that 
the return flow of drainage water may be measured and sampled. Prior 
to the construction of a drainage system, the land on this valley was badly 
water-logged. The annual diversion for irrigation purposes aggregated 
300,000 acre-feet of water which carried an average of 350 parts per 
million of dissolved solids. Upon the completion of the drainage system 
it was found that the aggregate annual drainage discharge was about 
100,000 acre-feet of water, carrying an average of 1600 parts per million 
of dissolved solids. An acre-foot of water weighs 2,716,000 pounds, or 
1,358 tons. With 350 parts per million of dissolved solids, an acre-foot 
of water carries 950 pounds. The incoming irrigation water carried to 
these valley lands 142,500 tons of dissolved solids, or nearly 1.5 tons per 
acre. The outgoing drainage, on the other hand, carried away 2.17 tons 
of salt with each acre-foot, or 217,000 tons for the year, or nearly 75,000 
tons more than the irrigation water brought in. This situation indicates 
a marked decline in the salt content of the irrigated land. The general 
increase in crop yields in this valley also bears testimony to the improve¬ 
ment of the soil. 
In another valley a similar study of conditions shows a different situa¬ 
tion. The facts are substantially as follows: ‘ The valley includes about 
53,000 acres of irrigated land. The annual diversion of water is 200,000 
acre-feet, containing an average of 1,000 parts per million of dissolved 
solids. The annual drainage discharge is about 48,000 acre-feet, carrying 
an average of 1,410 parts per million of dissolved solids. Thus the irriga¬ 
tion water brings in 272,000 tons of salt annually and the drainage water 
takes out 92,000 tons, leaving a residue of 180,000 tons of salt in the 
valley. If this residue were distributed uniformly to the irrigated lands in 
the valley it would be equivalent to 3.4 tons per acre. 
These two examples serve to show how one set of comparisons may be 
made between the quality of the irrigation and of the underground 
water. It is not always possible to make such comparisons because the 
percolating waters from irrigated land are sometimes lost into the country 
drainage and can not be measured or sampled. 
In some situations where it is not practicable to measure the volume of 
the drainage discharge it is possible to obtain samples of the drainage 
water and by analysis compare its quality with that of the irrigation 
water. Such comparison makes it possible to judge of the reactions that 
are taking place in the soils. If no exchange reactions take place the drain¬ 
age water should differ from the irrigation water only in being more con¬ 
centrated as a result of the losses by evaporation or the use by plants. As 
a matter of fact, it is rather unusual to find a place where the underground 
water does not differ profoundly from the irrigation water both as to 
total solids and in percentage composition. 
Where the irrigation water carries only a small quantity of total 
solids and these consist in large part of calcium and bicarbonate it is 
to be expected that as a result of the aeration and evaporation that 
takes place when the water goes on to the land these constituents 
