WATER LOSSES FROM WET AREAS 97 



This first flow recorder attaeliment was installed to operate by a 

 12-incli float. After testing in the field, it was found that the pencil 

 lag- on the record chart was too great for tlie work being undertaken. 

 This lag amounted to as much as 0.020 second-foot and as the fluctua- 

 tions that were being measured amounted to 0.200 second-foot and 

 less, the error was 10 per cent or more. 



In order to secure greater sensitiveness, the gears were eliminated, 

 the cam was balanced, and a 30-inch float was used. Nonadjustable 

 cams were designed and made for use with 3-inch Parshall measuring 

 flumes. With the improved flow recorder attachment, the pencil lag 

 was reduced to 0.002 second-foot for flows above 0.50 second-foot, and 

 for flows of less than 0.20 second-foot no lag could be detected. 



By using these flow recorder charts, the loss per day may be 

 obtained directly by superimposing two charts, one from each control, 

 on top of a light-table, and planimetering the area between the two 

 curves. This area represents the daily loss between controls and, when 

 multiplied by the proper constant, can be converted into whatever units 

 are desired. 



Evaporimeter 



It became apparent, during the course of the investigation of the 

 loss of water along stream channels by evaporation and transpiration, 

 that the study would be materially aided if a continuous record could 

 be obtained of the transpiration opportunity. Briggs and Shaiitz* 

 showed that the evaporation from a shallow black pan correlated more 

 closely with actual transpiration than that from any of the other 

 devices which they tested. Loss of water from deep paas is affected 

 by heat storage within the water. During the morning much of the 

 heat received from the sun is used in raising the temperature of the 

 water, and if the tank is 10 inches or more deep there is a lag of several 

 hours in the curve of evaporation, behind the cycle of insolation. As 

 the depth is decreased, the lag becomes less. The practical lower limit 

 for the depth to be used appeared to be that depth which would be 

 sufficient for one complete day's record on the hotter days. The 4-foot 

 pan maintained at Ontario from 1928 to 1930 showed a peak rate of 

 slightly less than 0.50 inch per day; therefore 0.60 inch was chosen as 

 the most practical maximum depth for the water in the evaporimeter. 



A Fergusson recording rain gage was used as the recording device. 

 The evaporimeter pan was made 2 feet in diameter and 0.7 inch deep. 

 This pan was attached to a cylinder that would fit inside tlie rain gage 

 and take the place of the usual rain-gage bucket. The pan and recorder 

 were then placed in a box 30 inches square by 27 inches high, to 

 provide lateral heat insulation. The evaporimeter is shown in opera- 

 tion in Plate XXIV-A with a standard 8-inch rain gage to the right. 

 The recording mechanism is shown in Plate XXIV-B. The chart scale 

 is 9 to 1 ; that is, 9 inches on the chart is equivalent to 1 inch of evapo- 

 ration. Record charts are shown in Plate XXV for typical days in 

 August and October, 1931. 



* Reprint from Journal of Agricultural Research, Vol. IX, No. 9, May, 1917. 

 "Comparison of the Hourly Evaporation Rate of Atriiometers and Free Water Sur- 

 faces with the Transpiration Rate of Medicago Sativa," by Lyman J. Briggs and 

 H. L. Shantz. 



7—4503 



