77 



investigators designed the satisfactory combination Parshall flume 

 previously described in detail (p. 33) and shown in Figure 3- 

 This permitted a range of discharge from a minimum flow through a 

 3-inch Parshall flume up to a maximum of 23 second-feet through 

 the 2-foot throat. 



Flow recorders were used to eliminate a large part of the 

 routine work of calculation. They consisted essentially of a 

 spiral cam that mechanically computed the stream flow at the point 

 of control. The cam was geared to a float pulley wheel and a pen- 

 cil cord attached to the cam. For greater accuracy a 30-inch 

 diameter float was used, as it was found that smaller floats per- 

 mitted lag in the record. Movement of the float with rising or 

 falling water was transmitted through the pencil cord to a record- 

 ing chart which gave direct measurements in units of discharge. 

 Superposing two charts, one from each control, and measuring with 

 a planimeter the area between the two gave the daily loss of water 

 in the stream between points of measurement. Flow recorder in- 

 stallation is shown in Plate III-A. 



Equipment was likewise developed to measure the transpira- 

 tion opportunity. Briggs and Shantz (7) showed that evaporation 

 from a shallow black pan was in closer correlation with transpira- 

 tion than that obtainable with other devices. With this in mind 

 an evaporimeter was designed with such an evaporation pan attached 

 to the weighing mechanism of a recording rain gage, so that con- 

 tinuous records of loss of water were obtained. The practical 

 depth of the pan, sufficient for one day's maximum evaporation 

 loss, was limited to 0.6 inch. The diameter was 2 feet. It was 

 necessary to refill the pan with water each day on account of its 

 shallow depth. From the charts, hourly rates of evaporation were 

 obtained. 



