SECTION 4 



APPLICATION OF EVAPORATION RATE MODELS 



Lntroduction 



Evaporation ponds are one means of disposing saline water from tile drains. Physical, 

 chemical, and biological factors affect evaporation parameters and can either increase or 

 decrease the efficiency of the ponds. These parameters include air and water temperatiire, solar 

 radiation, humidity, wind speed and direction, wave action, water color, turbidity, salinity 

 chemical composition, organic content and water depth. Wind speed, air and water temperature 

 and water saHnity are the most obvious factors that affect the evaporation rate, where the others 

 are less clear, yet just as important in the evaporation process. 



The water flux to the atmosphere is a physical process and is proportional to the vapor 

 pressure gradient between the water surface and the air above. A kiiowledge of the effect of 

 climatic factors on the vapor pressure leads to an estimate of the evaporation rate from the water 

 surface. Many models have been suggested and tested to estimate the evaporation rate from 

 water surfaces. 



Estimation of Evaporation Rates 



Dalton's model (1834) is used widely, and the equation is of the form: 



E = %)(es-ea) 



where: 



E = evaporation rate [L/T] 



es = vapor pressure in the film of air next to the water surface [M/T'L^] 



ea = vapor pressure in the air above water surface [MJT'U] 



K\i) = an empirical coefficient that depend on barometric pressure, wind velocity, 

 and other factors [T»L* M] 



This equation has been used tc calculate the evaporation rate from pure water surfaces 

 as well as from the floating pan containing saline water (EC = 14 dS/m) at Peck pond. The terms 

 of the equation are estimated as follows: 



• ea is used from CIMIS weather data (Table 4.1) 



• es is determined by the Janson (1959) equation as 



where: 



es = ew (1 - 0.0005373 S) 



ew = vapor pressure of pure water obtained from List (1951) 

 S = the salinity concentration (g/kg) 



The values of ew and es are presented in table 4.1. To estimate the wind coefficients, 

 ftp.), the wind speed and evaporation rate slopes found by Moore and Runkles (1968) are used 

 to construct new relations between the wind speed and wind coefficient [T'L^/M] (Figure 4.1). 

 Calculated and measured data are presented in Table 4.1. 



The comparison between calculated sind measured evaporation rates at the same elapsed 

 hour shows some discrepancy (Figure 4.2), while the comparison between the calculated ones at 

 certain elapsed hours with measured data after 10 hours shows slightly improved agreement 

 (Figure 4.3). Smoothing the measured data by excluding some of the above-range values led to 

 good agreement between the calculated and measured data (Figure 4.4). This agreement is due 

 to the heating time (6-10 hours) needed for water molecules to break the water tension and 

 escape from the water surface. More data is needed for verification. 



page 4.1 



