234 Journal of Agricultural Research voi. x, no.s 



EXTENSION OF THE EVAPORATION DEPTHS FROM I,AND PANS TO LARGER 

 OPEN-WATER SURFACES UNDER THE SAME CONDITIONS BY USE OF A 

 FLOATING PAN 



A part of Washington Park, South Denver, is shown in figure i. 

 The lake is artificial, material from the excavation having been taken 

 to form an embankment for the west and southwest sides. The other 

 shores are natural. The water level is maintained by the supply 

 from the city ditch. There is no outlet, but seepage is great. It has 

 been the custom to cut off the water supply late in the fall, and by 

 spring, when water was again turned in, but a small pond would be 

 left. The area of the lake is about 17 acres; the depth at the south 

 and southeast is 3 to 4 feet and 5 to 8 feet north of the island; a 

 maximum of 7.6 feet was measured at the point indicated by the star. 



The contours give an idea of the exposure; some groups of trees 

 are located about the lake, but these are still small. Plate 37, B, 

 looking southeast, shows the character of these tree obstructions. 

 The mean elevation of the water surface was for the season, 5,310.3 

 feet, or 36 feet lower than the highest point of the laboratory. 



Evaporation tank 9, United States Geological Survey floating 

 standard, was installed as soon as water was let into the pond in the 

 spring, and the records show the dates of its use. The pan itself 

 floated free in a 6-foot inclosure of the protecting raft. This raft 

 was 16 feet square and was built of 8-inch material standing on edge. 

 It proved an effective baffle for the wave action on this small reservoir. 

 Four casks, one at each corner, were necessary, as the lumber became 

 water soaked toward the end of the season. The raft, with floating 

 tank, anemometer, and rain gage, was anchored at the point indicated 

 by the star, 3,400 feet from the laboratory. The tank was reached 

 by canoe. Observations were made on water temperature, wind, 

 precipitation, and evaporation. No boats are allowed on the lake, 

 and swimmers are not permitted to use it; thus, the evaporation pan 

 was free from interference. Plate 37, B, shows a still well; this was 

 later removed (after two weeks' use), and observations were taken 

 when there was little wind movement. 



Tabulated results of the season's observations are given in Table 

 XII. 



Evaporation as measured from the floating pan was for the season, 

 108.9 per cent of that from the 12-foot land pan at the laboratory and 

 86. 1 per cent of that from a 3-foot square pan (equal in area and of similar 

 shape to the floating) at the laboratory. 



