4 BULLETIN 339, TJ. S. DEPARTMENT OF AGRICULTURE. 



The spikes in the tops of the stakes were leveled with the weir crests 

 by means of long carpenter's levels, great care being exercised to 

 keep them level at all times. It was found that excess velocity of 

 approach, due to the filling up of the weir pools, was the most fre- 

 quent source of error in weir measurements, and special care was 

 taken throughout the entire investigation to maintain weir pools of 

 sufficient size and depth to insure a slow velocity of approach. The 

 formula used in computing the discharge over the weirs was Q = 3.367 

 L H^, where Q = discharge in cubic feet per second, L = length of crest 

 in feet, and H = head or depth of water on the crest in feet. 



Usually the head was measured every hour, but where the flow 

 remained rather constant measurements sometimes were taken at 

 two-hour intervals, and where much fluctuation occurred the head 

 was measured oftener than every hour. Plate I, figures 1 and 2, 

 should furnish a clear idea of the type of weirs used. The volume of 

 water diverted by the large projects was determined by daily gage 

 readings and current-meter determinations. 



DETERMINATION OF AREAS AND YIELDS. 



With the exception of the large projects, all areas included in this 

 investigation have been determined by transit and chain surveys. 

 The transit notes were plotted on a scale of 100 feet to the inch, and 

 the acreage was determined from the map with a polar planimeter. 

 It is believed that the areas of all of the tracts in this variation ex- 

 periment have been determined within 0.01 acre. 



The crops from the three plots into which each tract has been di- 

 vided -have invariably been cut, stacked, and thrashed separately, 

 and weighed wherever possible. In cases where grain or alfalfa fields 

 were located at long distances from a set of wagon scales and weigh- 

 ing has been impracticable, the yields from the different plots have 

 been determined by the automatic weighers attached to the thrash- 

 ing machines and by the measurement of the hay in the stack. 

 While the determination of the yields by the latter method has not 

 been absolutely accurate, the same method has been used with all 

 plots into which a tract was divided, thus insuring comparable 

 results. 



CLASSIFICATION OF SOIL. 



The soils on the different tracts have been classified with respect 

 to their mechanical composition only, chemical analyses having been 

 made in but few cases. Representative samples of the first, second, 

 third, and fourth foot of soil from each tract experimented upon 

 have been secured and prepared and are now filed away for further 

 reference. The soils varied from the finest of adobe clays to the 

 coarsest gravels. The extremes of soil which have been included 



