ACOMPARISON OF AUTOMATICALLY AND MANUALLY COLLECTED PAN EVAPORATION DATA

摘要

Pan evaporation is an important weather variable that has numerous applications related to decision making in agriculture, forestry, ecology, hydrology, and other fields. The automation of pan evaporation measurements through the use of electronic sensors has the potential to increase the availability 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 manual pan evaporation measurements. Daily pan evaporation measurements calculated from 15-min averages of water height in Class A evaporation pans of the Georgia Automated Environmental Monitoring Network (AEMN) were compared to daily pan evaporation data collected at National Weather Service (NWS) cooperative stations. Collocated weather stations in Griffin and Watkinsville, Georgia were selected for the comparison. Data from 1991 to 1996 at the Griffin location and data from 1993 to 1997 data at the Watkinsville location were used. Data sets consisted of 733 and 808 daily evaporation totals from Griffin and Watkinsville, respectively. An estimate of potential evapotranspiration was also calculated for each daily record using the Priestley- Taylor equation. Daily pan evaporation amounts from the automated observations were generally less than the evaporation measurements from the manual observations. Average total annual pan evaporation from the manual observations was 537 mm for Griffin and 1051 mm for Watkinsville. The average total annual pan evaporation from the automated observations was, respectively, 414 mm and 676 mm for the same locations. The Priestley-Taylor approximation of pan evaporation was generally closer to the manual observations than the automated observations. Average total annual evapotranspiration estimated by the Priestley-Taylor equation was 491 mm at Griffin and 842 mm at Watkinsville. The daily automated pan evaporation data included many low values for days in which considerable pan evaporation should normally occur. Records of water height from the automated observations showed that mechanical problems with the sensor used in the automated pan evaporation system were responsible for much of the difference seen between the automated and manual observations. Improved maintenance of the automated observations is recommended to justify replacement of the manual observations. A change in the design of the float mechanism might also be considered by the manufacturer.