CIMIS Weather Data 



The CIMIS hourly weather data from stations near to the evaporation ponds under 

 investigation (Peck, Pryse, and Barbizon) for 24-hour periods in March Eind August, 1989 are 

 included in ^pendix B. Fluctuation of conditions recorded in the CIMIS data include: tempera- 

 ture from 7 to 17°C; relative humidity from 40 to 75%; wind speed from 1 to 4.8 mile/hr (usage 

 of mile&^r rather than m/s is necessary for calculations. However, all records of data are in m/ 

 s); and vapor pressure from 3.2 mbar to 9 mbar. This suggests that the evaporation rate will also 

 vary similarly. 



Floating Evaporation Pan Data 



Table 3.2 shows the evaporation measurements from floating evaporation pans located 

 at Peck evaporation pond with different salinity levels. CJenerally, evaporation rate decreases 

 as salinity increases due to a salinity effect on reducing water surface vapor pressure. Figures 

 3.17, 3.18, 3.19 and 3.20 show that the evaporation rate from water having an EC=14dS/m was 

 higher than water with an EC of 30 dS/m. Additionally, water with an EC of 30 dS/m has a higher 

 daily evaporation rate than water of 47 dS/m. Figure 3.21 shows that the cumulative evaporation 

 from the floating pans was of the order EC= 14 dS/m > =30 dS/m > =47 dS/m >ET^. Table 3.3 

 contains the 2- day hourly evaporation loss (mm/hr) and cumulative loss (mm) from the floating 

 pans having an EC of 14 dS/m. The data shows that the evaporation rate during night time 

 contributed significantly to the total evaporation. Figures 3.22 to 3.26 show the effect of the 

 individual weather parameters on the potential evapotranspiration (ET^) and the average 

 evaporation rate from the floating pans containing saline water (EC= 14 dS/m) at Peck pond. 

 Average measured daily evaporation rates of agricultural drainage decreased from 8.0 mm/day 

 in August to 2.3 mm/day in November for the 14 dS/m water (Figure 3.27). 



Potential Use of CIMIS ET^ as a Predictor o f Evaporation Rate from Evaporation Ponds 



California's network of CIMIS weather stations could provide a useful tool for predicting 

 evaporation rates firom evaporation ponds. Cumulative evaporation measurements from Peck 

 pond were well correlated to CIMIS-calculated ET^ as reported from the nearby station at 

 Murrietta farms. An ET__ correction factor was calculated for cumulative evaporation rates at 

 different salinity levels from data collected during August through October, 1989. This 

 correction factor was then correlated to the EC of the water up to 61 dS/m (Figure 3.28). The 

 result is a simple linear model with an r-squared value of 87% which yields an ET^ correction 

 factor from input of the drainage water EC: 



Y = 1.3234 - 0.0066 EC (dS/m) 



where Y is the ET^ correction factor. 



The actual relation might be not linear, but this relation could be used for making 

 ballpark estimates within the range of salinity used, and it illustrates the potential for 

 developing such a model. 



Figures 3.29 a, b and c show the calculated (using the above model) and the measured 

 evaporation rates as well as the cumulative rates (Figures 3.30 a, b and c) from the floating pans 

 containing different salt concentrations (14, 30, and 47 dS/m) at Peck pond. The agreement 

 between the measured and the predicted rate is within acceptable limits. 



page 3.18 



