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Decreases in pan evaporation (Epan) have been reported around the world despite increasing air temperatures; this was attributed to reductions in wind speed and solar radiation. Using 42 years (1975-2016) of Australian Epan data, we reexamined Epan trends, adding over a decade of observations to previous analyses. Flexible local linear regression models showed that many previously reported decreasing Epan trends have plateaued or reversed. Attribution analysis confirmed that 1975-1994 Epan decreases in southern/western Australia were chiefly driven by decreasing wind speeds. Increasing vapor pressure deficit subsequently became dominant, resulting in 1994-2016 Epan 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.