A comparison of automatically and manually collected pan evaporation data

Pan evaporation is an important weather variable that has numerous applicat ions related to decision making in agriculture, forestry, ecology, hydrolog y, and other fields. The automation of pan evaporation measurements through the use of electronic sensors has the potential to increase the availabili ty and resolution of measurements, while reducing the overall cost of data collection. Information is needed with respect to the field performance of these devices. The objective of this study was to compare automated and man ual pan evaporation measurements. Daily pan evaporation measurements calcul ated from 15-min averages of water height in Class A evaporation pans of th e Georgia Automated Environmental Monitoring Network (AEMN) were compared t o daily pan evaporation data collected at National Weather Service (NWS) co operative stations. Collocated weather stations in Griffin and Watkinsville , Georgia were selected for the comparison. Data from 1991 to 1996 at the G riffin location and data from 1993 to 1997 data at the Watkinsville locatio n were used. Data sets consisted of 733 and 808 daily evaporation totals fr om Griffin and Watkinsville, respectively An estimate of potential evapotra nspiration was also calculated for each daily record using the Priestley-Ta ylor equation. Daily part evaporation amounts from the automated observatio ns were generally less than the evaporation measurements from the manual ob servations. Average total annual part evaporation from the manual observati ons was 537 mm for Griffin and 1051 mm for Watkinsville. The average total annual pan evaporation from the automated observations was, respectively, 4 14 mm and 676 mm for the same locations. The Priestley-Taylor approximation of pan evaporation was generally closer to the manual observations than th e automated observations. Average total annual evapotranspiration estimated by the Priestley-Taylor equation was 491 mm at Griffin and 842 mm at Watki nsville. The daily automated pan evaporation data included many low values for days in which considerable pan evaporation should normally occur: Recor ds of water height from the automated observations showed that mechanical p roblems with the sensor used in the automated pan evaporation system were r esponsible for much of the difference seen between the automated and manual observations. Improved maintenance of the automated observations is recomm ended to justify replacement of the manual observations. A change in the de sign of the float mechanism might also be considered by the manufacturer.