The influence of increasing radiation (sunshine duration and global radiation) on the increase in potential evapotranspiration

Potential evapotranspiration (ETp) is a variable driven by many factors and one which is heavily affected by climate change. In many cases, the observed increase in ETp is attributed to rising air temperatures in the past and temperature is used as the main prediction variable for future developments of ETp in climate change projections.

The climate station at the lysimeter station Brandis (Saxony, Germany) has been recording a wide range of climate variables since 1980.  From the observations it is evident that, in addition to the increase in air temperature at the site, there has also been a significant increase of sunshine duration (average increase of 0,29h d⁻¹ decade⁻¹) and global radiation (average increase of 47,45 J cm⁻² d⁻¹ decade⁻¹). This combination of higher temperature levels and increased energy availability leads to significant increases in ETP (average increase of 0,11 mm d⁻¹ decade⁻¹), which is a mayor driver of the local water balance and an important variable in describing the atmospheric demand in modeling studies. Based on the observed trend in sunshine durations we provide an analysis of the individual contributions of increases in global radiation and air temperature, to assess:

• the individual contributions to the overall increase in potential evapotranspiration (according to Turc-Wendling)
• the influence of global radiation and air temperature on the intra-annual course?

The individual contributions of increases in radiation and air temperature on the ETP was calculated using trend analysis over the period from 1980 to 2025. It shows that, according to the Turc-Wendling approach, 69% of the ETP increase at the site is radiation-driven, while air temperature only has an influence of 28%. Additionally, clear seasonal patterns are found in the individual contributions.  Overall, the results show that global radiation increases are a mayor driver for the increase in potential evapotranspiration at the site and future developments of potential trends in global radiation should be considered in projections of potential evapotranspiration.