Description of Meteorological Indices Presented Based on Long-Term Yields of Winter Wheat in Southern Germany

This study had three main objectives. First, weather indices were listed and their derivations were described to show which weather parameters could be used to describe the influence on agricultural yields. Second, farmers and agricultural scientists should be given the opportunity to evaluate the weather of the observation years in the study region. Furthermore, significant fluctuations in winter wheat yields were compared with weather events. As weather variables, 45 meteorological indices were used, such as precipitation-, temperature-, precipitation-temperature-, growing-period-, and radiation-related indices. In the case of winter wheat, heat waves and dry periods were the most important factors that affected the yields. For the past 20 years, in particular, there have been recurrent spring and summer months with low precipitation and, in some cases, significantly too warm periods, such as in 2003 and 2018 (April to October 2003: +16% degrees C, 2018: +27% degrees C, 2003: -38% mm, 2018: -12% mm in relation to 1978 to 2020), which were associated with particularly high yield losses. The qualitative assessments illustrate that in the observation period, years with reduced yield compared with the multiannual trend were frequently well explainable by extreme weather events.