102 



DIVISION OF WATER RESOURCES 



FLOW AT MIDDLE COLDWATER CANYON CONTROL, AUGUST 9-15, 1931. 



almost completely shaded and the indicated evaporation rate can not 

 be applied to open areas where the water surface is exposed to the sun. 



If transpiration is the main factor in causing the daily drop in a 

 stream flow, the demand on the stream for water to take care of the 

 transpiration needs of the vegetation growing in the canyon might be 

 expected to follow a typical rate of transpiration curve. That it does 

 tend to do this is shown by a comparison of a daily cycle of loss in 

 discharge from Coldwater Canj'on to the daily cycle of a typical rate 

 of transpiration curve. 



A graph of the drop in stream flow occurring at the upper and 

 lower controls is shown in Plate XXVIII-A. Plate XXVIII-B shows 

 a record of the loss of water by evaporation and transpiration on 

 September 11-12, 1930, from a tank of mixed swamp vegetation, chiefly 

 wire rush, tule, and willow at Ontario. 



The Ontario tank record shows 94 per cent of the loss occurring 

 betAveen 8 a.m and 8 p.m. or between 2| hours after sunrise and 2 

 hours after sunset, and the peak rate of loss occurring between 1 and 

 2 p.m. Only 6 per cent of the loss occurs between 8 p.m. and 8 a.m., 

 and the loss between midnight and sunrise is very low. The supply of 

 heat energy available for vaporizing this water comes from the sun, 

 and the insolation reaches a peak at noon and drops to zero at sunset; 

 but there is some storage of heat in the ground and the overlying air, 

 and therefore the cycle of transpiration lags behind the radiation cj^cle. 

 Air temperature is an index of the heat energy available and, in Plate 

 XXVIII-B, it is shown to have a good correlation with the rate of 

 transpiration during the daylight hours. Both of the curves for the 

 drop in stream flow have the same general shape as the rate of transpi- 

 ration curve. The cross-hatched area between the two curves repre- 

 sents the loss of Avater suffered by the stream as it passed from the 

 middle to the lower control. 



The demand of transpiration first affects the water table under- 

 lying the soil in which the trees are rooted and, as the water table 

 drops, water moves from the stream to replenish the draft. The maxi- 

 mum rate of drop in flow would be expected shortly after the time of 

 maximum transpiration opportunity. The discharge curves shown in 



