70S 



THE IKRIGATION AGE. 



Irrigation of Orchards 



BY SAMUEL FORTEER 



Chief of Irrigation Investigations, Office of Experiment 

 Stations, U. S. Department of Agriculture 



Evaporation Losses from Orchard Soils. 



A light shower followed by warm sunshine may re- 

 fresh the foliage of fruit trees, but its effect on the soil 

 is more likely to be injurious than otherwise. A brief, 

 pelting rain followed by sunshine forms a crust on the 

 surface of most soils, and if this is not soon broken up 

 by cultivation it checks the free circulation of air in the 

 soil and also tends to increase the amount of water evap- 

 orated. 



It has been found (a) that the amount of moisture 



dry, the bulk of the water soon passes beyond the first 

 foot, and the surface can be cultivated soon after the 

 water is turned off. 



The well known effect of temperature on evaporation 

 is shown in figure 26. The dotted line shows the mean 

 monthly temperatures at Tulare, Cal., from January 1, 

 1904, to December 31, 1905, and the solid line the monthly 

 evaporation from a water surface for the same time. 



Effect of Soil Mulches in Checking Evaporation. 



The effect on evaporation of a layer of dry granular 

 soil when placed above moist soil has been shown by a 

 series of experiments conducted in tanks by irrigation in- 

 vestigations of this office. These tanks are water-jacketed 

 and placed in the open under normal conditions as regards 

 sunshine, wind and temperature. Each tank holds about 

 three-fourths of a ton of soil and is weighed at stated 

 intervals in a manner shown in figure 27. The results of 

 experiments made at Davis, Cal., in 1908 are given in the 

 following table: 



Evaporation from soils protected by different depths of soil mulch at Davis, Cal., September 1 to October 3, 1908.* 



Average weight of tanks, September 1, pounds. 



No mulch, 



tanks 1 and 2. 



1,104.7 



3-inch mulch, 

 tanks 3 and 4. 

 1,090.0 



Per ct. Per ct. Per ct. Per ct. 



Ave 

 Ave 



3 days September 1 to 4 



4 days September 4 to 8 



3 days September 8 to 11 



4 days September 11 to 15 



18 days September 15 to October 3. 



age loss 



age loss 



age loss 



age loss 



Ave age less 



Total loss, 32 days, September 1 to October 3 33.25 



*U. S. Dept. Agr., Yearbook, 1908, p. 468. 



16.75 

 4.5 

 3.0 

 1.5 

 8.0 



17.83 

 4.79 

 3.19 

 1.60 

 8.52 



1.75 

 .75 

 8.25 

 2.5 



7.0 



1.86 

 .80 

 2.4 

 2.66 

 7.45 



35.93 14.25 15.17 



held by the soil, the temperature of both soil and air, and 

 the rate of wind motion are the chief factors in the evap- 

 oration of water from soils. The influence of moisture 



The soil first received an irrigation of 6 inches in 

 depth over the surface and in the tanks which had no 

 mulch; over one-third of this amount was evaporated in 



Jan Feb Mar Apr May June July Aug SeptOct/ Nov Dec Jan Feb Mar Apr May June July Aug SeptOcl Nov Dec 

 1904 I90, 



Fig. 26. Relation Between Temperature and Evaporation From a Water Surface at Tu'lare, Cal. 



is shown in the following figures, obtained from tank 

 experiments made at Tulare, Cal., in. 1904: 



Evaporation from Tulare soils which received different 

 amounts of water, June 15 to September 15, 1904. 



1 and 2 



3 and 4 



6 and 6 



7 and 8 

 9 and 10 



11 and 12 



The 



Number of tanks. 



results of other 

 when the water is applied to the surface of orchard soils 

 the loss by evaporation is very great so long as the top 

 layer remains moist. Even in light irrigations this loss in 

 forty-eight hours after the water is put on may amount to 

 from 10 to 20 per cent of the volume applied. In order to 

 reduce this loss and moisten the soil around the roots of 

 trees, the practice of running small streams of water in 

 deep furrows has become quite common. In applying 

 water in this way the top soil remains at least partially 



thirty-two days, while less than 1 per cent was evap- 

 orated in the tanks which were protected by a 9-inch 

 mulch. 



Similar experiments carried on at Wenatchee, Wash., 

 in June, 1908, showed the following losses in twenty-one 

 days: No mulch, 14J/S per cent of water applied; 3-inch 

 mulch, 4 per cent; 6-inch mulch, 2 per cent; and 9-inch 

 mulch, 1 per cent. 



From the foregoing it is evident that western orchard- 

 ists can prevent the greater part of the evaporation losses 

 by cultivating orchards to a depth of at least 6 inches as 

 soon as practicable after each irrigation. 



Loss of Water Due to Percolation. 



In the preceding paragraphs attention has been called 

 lo the large amount of water which is vaporized from 

 warm, moist soils. The above heading calls attention to 

 another loss of a different character. In all modes of 

 wetting the soil, but more particularly when deep furrows 

 are used to distribute the water, a part is liable to sink 

 beyond the deepest roots. As a rule, the longer the fur- 

 row the greater is the loss from this cause. In furrows 



