WATER LOSSES FROM WET AREAS 



61 



The cooperative station at Prado was intended principally to deter- 

 mine the rate of use of water by tales in connection with a study of 

 the flow of the Santa Ana Kiver that was being made by the United 

 States Geological Survey. As the station was isolated and daily visits 

 were not practicable, recording devices were attached to both tule tank 

 and Weather Bureau evaporation pan. From the records so obtained 

 the hourly rates of consumptive use and of evaporation have been 

 computed. These are plotted in Plate IX, which shows air and 

 water temperatures, consumptive use of water by tules in an isolated 

 tank, and evaporation from a four-foot pan for each hour of the day 

 from August 21 to August 28, 1931, inclusive. There were periods 

 during the early morning hours when the loss of w^ater was too small 

 to be recorded and evaporation or transpiration, during those hours, has 

 been listed as zero. Characteristic of both evaporation and transpiration 

 is the daily increase or decrease with a rising or falling temperature. 

 The minimum rate occurs near sunrise and the maximum is in the after- 

 noon. Consumptive use is greater than evaporation and responds more 

 readily to sunlight and changes of temperature. The rate of evapora- 

 tion does not increase rapidly until water in the pan has been warmed 



PLATE IX 



HOURLY RATE OF USE OF WATER BY TULES, EVAPORATION FROM 



STANDARD WEATHER BUREAU PAN AND AIR AND WATER 



TEMPERATURES, PRADO STATION. 



by the sun and is relatively slow in comparison with the rate of con- 

 sumptive use, which increases rapidly, comes to a peak sooner, and 

 declines more quickly. 



On the morning of August 28 temperature was less than normal 

 and a light rain occurred shortly after noon. The effect of the rain in 

 deferring the morning increase in consumptive use and in evaporation 

 from water until about 2 p.m. is shown on the chart. The rain was 

 apparently the cause of a small decrease in the rate of evaporation 

 until its effect was overcome by a rise in temperature. In general, the 

 highest air temperature occurred at about 1 or 2 p.m., while the highest 

 water temperature occurred about two hours later. The same interval 

 also is noticeable in minimum temperatures. Observations in other 

 localities have shown that the highest consumptive use of water by tules 



