74 



"The heat supply for vaporizing this 0.778 inch of water 

 on August 22 must have come almost entirely from the moving air 

 currents passing through the growth in the tank. 



"However, when a large swamp area is considered, there 

 must be a rapid drop in temperature of the air as it passes 

 through the swamp growth if it is to give up its heat supply at 

 the rate indicated by the above experiment. As soon as the air 

 is cooled to the same temperature as the plants there can be no 

 further transfer of heat from the air to the plants. When this 

 condition is reached, the energy for vaporization must come 

 solely from insolation. 



"It may be expected, then, that small isolated patches of 

 swamp growth will show rates of loss per unit area higher than 

 that accounted for by insolation alone, but it also is probable 

 that the loss from an extensive swamp area is limited to a value 

 not widely variant from that indicated by insolation. 



"The inference is that in conducting tank work to gain 

 data for use in estimating losses from field areas, that the 

 tank should be set in a field of growth similar to that in the 

 tank and the outside growth must completely surround the tank 

 so the exposure of the growth in the tank is normal." 



Coldwater Canyon, California —' 



Canyon-bottom vegetation . --The mountain slopes of southern 

 California support a growth of dry-land chaparral which must de- 

 pend upon the immediate precipitation for moisture, but vegetation 

 adjacent to small canyon streams is of a more water-loving nature. 

 This includes such broadleaf trees and shrubs as alders, willows, 

 sycamores, and California laurels, changing at times to coniferous 

 types at higher altitudes. As evidence of their water-loving 

 character these species are seldom found away from a dependable 



17 The field investigation was made by Colin A. Taylor, Associate 

 Irrigation Engineer and Harry G. Nickle, Assistant Irrigation 

 Engineer, Division of Irrigation, in cooperation with the Division 

 of Water Resources, Department of Public Works, State of California, 



