6 BULLETIN 1004, IT. S. DEPARTMENT OF AGRICULTURE. 



There is, of course, a considerable number of experimental errors 

 in the determinations. Even in the most uniform soils, duplicate 

 determinations of soil moisture are nearly certain to vary at least 

 one-half of 1 per cent, and in less uniform soils variations of much 

 greater magnitude occur. The probable error of the total of any 

 individual sampling or of the difference between the totals of any 

 successive samplings may be relatively large or small, depending 

 upon the dispersion of the errors of the units making up the totals 

 and the extent to which they compensate each other. An error in 

 the same direction of one-half of 1 per cent in each foot in a deter- 

 mination to the depth of 6 feet on a soil weighing 75 pounds per 

 cubic foot would be equivalent to 0.42 inch of water. In soils lacking 

 uniformity the experimental error may be much larger. 



In the present study of the use of water the following practices 

 have been followed for the purpose of reducing the experimental 

 error as much as possible: 



(1) The depth to which the samples have been included in each deter- 

 mination has been limited to the zone of change of water content. For 

 example, even if samplings have been made to the depth of 6 feet or more 

 but all changes in water content (either reduction by use or addition 

 by rainfall) have been confined to the first 3 feet of soil, the calculations 

 to determine the quantity of water used have been limited to 3 feet. 

 Inclusion of additional depths would only increase the experimental error 

 without contributing anything of value to the results. 



(2) Periods of time as long as practicable have been used to deter- 

 mine the rate of use in each instance. 



To obtain the rate of use per day, the quantity of water used in 

 any period is divided by the number of days in that period. Any 

 error that may exist in the total quantity of water used is conse- 

 quently reduced in proportion to the number of days in the period. 

 In a general way, experimental errors of successive samplings tend 

 to compensate each other, and the experimental error for the quan- 

 tity of water used in a season should be small and is limited to the 

 errors in the first and last samplings. 



DAILY RATE OF THE USE OF WATER WHILE THE CROP IS 

 GROWING RAPIDLY. 



Under this heading are included the results of attempts made to 

 determine the daily rate of the use of w ater by the wheat crop under 

 full and uninterrupted growth. This rate was calculated in all years 

 when two soil-moisture determinations at least two weeks apart 

 were made during the period of rapid growth of the crop and while 

 it was not suffering for lack of water. As long a period as possible 

 was selected for each year, in order to make the error in the rate 

 per day due to the experimental error of the determinations as small 

 as possible. In years when no shortage of water occurred the end 

 of the period is determined by harvest. No years were included {1) 

 when the crop had suffered from drought before the period of rapid 

 growth was reached ; (2) when the second sampling during the rapid- 

 growth period was delayed until after the crop commenced to suffer 

 from drought; (3) when some condition, such as hail, rust, or insects, 

 destroyed the crop. Years when some factor other than a lack oi 

 water was responsible for crop injury were excluded, to avoid the 



