Atmospheric conditions controlling extreme summertime evapotranspiration in Poland (central Europe)

Abstract

Extremely high evapotranspiration may constitute a threat to agriculture and to water storage, supply, management and quality. To better understand the occurrence of high evapotranspiration, our goals were to (1) document the synoptic conditions favorable for extreme evapotranspiration and (2) determine the anomalies in the factors that directly affect extremely high evapotranspiration on a daily scale. We calculated the daily reference evapotranspiration (ET0; June–August, 1971–2010) for 31 stations in Poland using the FAO-56 Penman–Monteith method and identified the days with the highest ET0 values (>90th percentile). For these days, we calculated the sea level pressure (SLP), the 500 hPa geopotential heights (z500), the air temperature at 850 hPa (t850) and the amount of precipitable water. We detected a correlation between the air pressure field and high ET0 values (r > 0.4, p < 0.001) in northeast Poland; positive anomalies in SLP, z500 and t850 were documented on days with high ET0. Substantial anomalies were also observed in the factors that directly affect evapotranspiration, i.e., sunshine duration and air temperature (positive) and relative humidity (negative). The conditions favorable for high ET0 events are characteristic of anticyclonic weather, so when an anticyclone center is near northeast Poland, high evapotranspiration can be expected.