Modelling radiation interception and radiation use efficiency for sugar beet under variable climatic stress

European sugar beet crops suffer drought stress, and climate change is likely to increase the frequency and severity of drought. Changes in rainfall pattern may also affect the radiation environment. Modelling is an appropriate tool to assess the scale of these problems. In a sugar beet model we included the effects of water stress on foliage dynamics (radiation interception) and of variable radiation use efficiency (RUE). Foliage expansion and senescence were related to the relative water content in the root zone and drought response factors were calibrated from rain-shelter experiments for early and late drought in England. Simulating variable drought response in different growth phases explained 70–96% of the foliage variation for rain-fed growth. The dependence of RUE on the fraction of radiation that is diffuse was described theoretically; measured variation of RUE (0.9 g DM MJ−1) matched well the empirical representation of this theoretical relationship. We evaluated these modifications using experimental total dry matter and sugar yields in the UK and in Germany (1965–1995). Including foliage dynamics and variable RUE improved the simulation of observed yields in the UK in some years. For continental sites mean sugar yields were well described when differences in the antecedent soil water balance were accounted for. The agreement between estimated and observed yields was close to 1:1, explaining between 30 and 45% of observed variation, though not appreciably different from when the original model was used. This was because the test data included only a few seasons with either severe water stress or abnormal fractions of diffuse sunlight. Ideally the model should be tested against crop growth data from more extreme environments. Nevertheless, we conclude that the expanded model is now more suitable for use across much of Europe and future climatic change.