73 



"The discussion of the receipt of energy, other than the 

 vertical component from the sun, leads to a consideration of what 

 the effect might be on very small patches of swamp growth. The 

 extreme case may well be considered as an isolated tank of swamp 

 growth two feet in diameter set in otherwise barren ground. The 

 radiant energy intercepted by the plant growth in the tank must 

 necessarily be a greater amount than the same area of growth in 

 a swamp would receive because the isolated tank growth has a side 

 exposure that in a swamp would be protected by surrounding plants, 

 The analogy that might be drawn is that of a lens focusing the 

 sun's rays on the restricted area of the tank. 



"Take the case of the two-foot tank used at Ontario in 

 studying the correlation between air temperature and transpira- 

 tion. The loss for the month of August, 1930, from the Ontario 

 willow and reed tank was 30.6? inches depth. This is about two 

 and one-half times the depth of water that could be vaporized by 

 the insolation falling on the horizontal area of the tank. A 

 partial explanation is that the tall growth in the isolated tank 

 intercepts a much larger amount of insolation than the same area 

 of growth would receive in a swamp. But in the case of the small 

 tank, the heat energy brought to the growth in the tank by air 

 movement also is relatively large. An experiment investigating 

 this point was performed at Ontario on August 22, 1930. On this 

 date, the willow and reed tank was shielded from the direct rays 

 of the sun by a corrugated iron roof, eight by ten feet, placed 

 just high enough to clear the plants and allow free lateral wind 

 movement. The record of water loss is shown in Figure 8 (this 

 bulletin) . The full line on August 22 is the actual loss with 

 the tank shielded. The dotted line is the average of August 21 

 and 23. The values are: 



Loss August 21 1.296 Inches 



Loss August 23 1.274 inches 



Average loss for August 21 and 23 1.285 inches 

 Loss August 22 0.778 inch (with 



tank shielded) 



