Revisiting Pan Evaporation Trends in Australia a Decade on

Clare M. Stephens, Tim R. McVicar, Fiona M. Johnson, Lucy A. Marshall
2018 Geophysical Research Letters  
Decreases in pan evaporation (E pan ) have been reported around the world despite increasing air temperatures; this was attributed to reductions in wind speed and solar radiation. Using 42 years of Australian E pan data, we reexamined E pan trends, adding over a decade of observations to previous analyses. Flexible local linear regression models showed that many previously reported decreasing E pan trends have plateaued or reversed. Attribution analysis confirmed that 1975-1994 E pan decreases
more » ... n southern/western Australia were chiefly driven by decreasing wind speeds. Increasing vapor pressure deficit subsequently became dominant, resulting in 1994-2016 E pan increases. Climate trend analyses should consider applying flexible statistical models to qualitatively understand temporal dynamics, complementing linear models that are able to provide quantitative assessments, especially when multiple drivers are involved. Plain Language Summary Evaporation pans measure atmospheric evaporative demand and are used to estimate water loss from storages (e.g., dams) and to provide inputs to hydrologic models and drought indices. In the late twentieth century, a surprising trend in annual pan evaporation was found: although temperatures were increasing, pan evaporation was decreasing in many parts of the world (including Australia). Pan evaporation responds to multiple drivers: net radiation, air temperature, wind speed, and vapor pressure deficit. In Australia, earlier studies showed that declining wind speeds (stilling) were chiefly responsible. We revisited the conclusions of these studies using an additional 12 years of pan evaporation data. Interestingly, we found that many previously decreasing pan evaporation trends are now increasing. Using a flexible regression technique in combination with linear regression, we showed that this change is due to increasing air temperature driving greater vapor pressure deficits. Possible reasons for increasing air temperatures include anthropogenic climate change and/or a period of drought (2000s) in Australia. Both of these factors likely contributed to increasing pan evaporation trends. Increased atmospheric evaporative demand may reduce water security due to greater evaporative losses from storages. Key Points: • Pan evaporation decreases between the 1970s and mid-2000s were previously attributed to decreasing wind speeds • We show that these trends actually reversed in the early 1990s, driven mainly by increasing vapor pressure deficit • The vapor pressure deficit increases were caused by increasing temperature (not reduced moisture) and may continue with global warming Supporting Information: • Supporting Information S1
doi:10.1029/2018gl079332 fatcat:dk5npzhyxzfwhgmo6lm6jjkqre