MONITORING FREE-WATER EVAPORATION AT AUTOMATED WEATHER STATIONS

The automation of weather stations necessitates an alternative approac h to the traditional manual measure of free-water evaporation made usi ng a class A pan. This study compared commercially available water-lev el sensing transducers mounted on class A pans to manual measurements using a class A pan. Measurements of free-water evaporation with two a utomated transducers over a 24-h period resulted in mean differences o f 0.23 and 0.98 mm. Hourly measurements for free-water evaporation all owed examination of the correlation between principal weather elements and evaporation. Evaporation from the pan was highly correlated with wind speed at night (r = 0.86) and with air temperature during the day (r = 0.75). In addition, it was found that during the summer some 33% of the daily free-water evaporation occurred at night. For a 24-h per iod, accumulated free-water evaporation was highly correlated with air temperature (r = 0.85), net radiation (r = 0.81), incoming solar radi ation (r = 0.80), and wind speed (r = 0.69).